Visual Impairment in Pre-Clinical Models of Mild Traumatic Brain Injury

Abstract

Impairment in visual function is common after traumatic brain injury (TBI) in the clinical setting, a phenomenon that translates to pre-clinical animal models as well. In Morris et al. (2021), we reported histological changes following weight-drop-induced TBI in a rodent model including retinal ganglion cell (RGC) loss, decreased electroretinogram (ERG) evoked potential, optic nerve diameter reduction, induced inflammation and gliosis, and loss of myelin accompanied by markedly impaired visual acuity. In this review, we will describe several pre-clinical TBI models that result in injuries to the visual system, indicating that visual function may be impaired following brain injury induced by a number of different injury modalities. This underscores the importance of understanding the role of the visual system and the potential detrimental sequelae to this sensory modality post-TBI. Given that most commonly employed behavioral tests such as the Elevated Plus Maze and Morris Water Maze rely on an intact visual system, interpretation of functional deficits in diffuse models may be confounded by off- target effects on the visual system.

Introduction

In 2010, the Centers for Disease Control and Prevention (CDC) estimated that traumatic brain injuries (TBIs) accounted for ∼2,500,000 emergency department (ED) visits, hospitalizations, and deaths in the United States. Although the vast majority of TBIs are deemed “mild,” many patients with mild TBI (mTBI) experience long-lasting effects including headaches, cognitive difficulties, and mood changes.¹ Numerous studies have demonstrated that as many as 60–65% of patients with TBI report visual problems after injury.^2

–6 TBI-induced visual dysfunction encompasses a diverse set of potential symptoms including blurred vision, visual field loss, and decreased visual acuity that can present acutely or chronically.⁷ However, the mechanisms underlying TBI-induced visual dysfunction are largely unknown. For this reason, pre-clinical mTBI models offer a unique opportunity to explore mechanisms related to TBI-induced visual dysfunction. Notably, visual dysfunction is a common, yet understudied, feature of pre-clinical TBI models.

Here, we have conducted a narrative review of pre-clinical TBI models with an aim to understand differential approaches and histological changes, as well as the functional behavioral repercussions of each as they relate to the visual system. This is an essential line of investigation because visual deficits are often overlooked in pre-clinical TBI models, despite the confounding effects that these visual deficits could have on other commonly used behavior assays. Moreover, pre-clinical TBI models offer a translational opportunity to study this common and troubling sequela of clinical TBI with a unique ability to investigate mechanistic underpinnings of visual dysfunction. We will demonstrate that a range of pre-clinical TBI models including controlled cortical impact (CCI), fluid percussion injury (FPI), closed head weight drop models, and primary blast brain injury, result in damage to anatomical visual structures. These consistent findings have significant implications for a field of study whose behavioral tests, such as Novel Object, Elevated Plus Maze, and Morris Water Maze (MWM) rely heavily on vision. Importantly, potential changes to the visual system are not commonly taken into consideration when conducting behavior tests with visual dependency. The histological changes to the visual system summarized in this review do not specify the presence or absence of excluded animals; rather, the reported alterations to the neurological structures that support vision are consistent throughout the entire surviving injury groups of the cohorts being studied. As such, although mild TBIs may not lead to complete blindness, some degree of vision loss is a potential negative impact of TBI on mice that must be accounted for. Moreover, the “mild” nature of many of the injuries described in this review may necessarily lead to an assumption that an animal's vision is completely intact in the absence of obvious blindness. However, the histological changes to visual system structures consistently reported following numerous injury models point to a need to assess visual function more carefully before implementing vision-dependent behavioral testing. In addition, the unexplored mechanisms behind TBI-induced visual dysfunction offer new opportunities for the field to study axonal injury. This review offers a summary of visual deficits reported in the pre-clinical literature.

Methods

We conducted a narrative review of articles published in the PubMed database by identifying TBI articles reporting visual deficit outcomes published over a 20-year time frame (2003–2023). Visual deficits were defined as any reported changes to the in vivo pre-clinical model of TBI including retinal ganglion cell (RGC) loss, decreased evoked potential in the electroretinogram (ERG), optic nerve diameter reduction, induced inflammation and astrogliosis of the anatomical visual pathway, and loss of myelin accompanied by markedly impaired visual acuity.

Related articles in PubMed were identified with the advanced search tool, using the following keywords for injury models: TBI rodent, TBI mouse model, TBI porcine model,” and “TBI rat model.” Keywords for visual outcomes included: “TBI” AND “visual deficits,” “TBI” AND “lateral geniculate nucleus,” “TBI” AND “lateral geniculate nucleus (LGN),” “TBI” AND “retina,” “TBI AND “optic nerve,” ”TBI AND “traumatic optic neuropathy (TON),” “TBI AND “visual system,” “TBI AND “superior colliculus,” “TBI AND “optic tract,” “rat CCI optic,” “rat TBI vision,” “rat blast vision,” “porcine CCI optic,” “porcine TBI vision,” and “porcine blast vision.” These keywords for visual outcomes allowed for the identification of specific articles that used in vivo pre-clinical models of TBI that presented visual deficits as an outcome after brain injury. The article's inclusion criteria were as follows: in vivo pre-clinical models (1) in which mild injury severity classification was assigned to cases of either a single or repetitive traumatic brain injury (TBI), wherein the affected animal does not exhibit obvious blindness; (2) of the visual system (pupillary, retinal, and optic nerve) and its brain regions (lateral geniculate nucleus, superior colliculus, optic tract, suprachiasmatic nucleus) as analyzed outcomes (Fig. 1), (3) primarily injuring the brain and indirectly injuring the visual system; therefore, any study that intentionally damaged the visual system (e.g. optic nerve crush/transaction models) and directly triggered TON were excluded.

FIG. 1.

Summary of the locations of the visual pathway pathologies reported in each of the 29 articles selected for this narrative review “Created with BioRender.com.”

The data extraction from the selected articles was performed by identifying the TBI model type, the species being used (mouse, rat, or porcine), the site of injury where the TBI was conducted, and the reported visual system pathology with the assay performed. Further, another critical point was to evaluate the mechanical insult (primary etiology) from each TBI model by identifying the presence or lack of rotational-acceleration forces, helping to further understand the mechanistic underpinning of in vivo pre-clinical mild TBI and visual system dysfunction.

Results

The search identified 393 articles of which 29 articles met inclusion criteria. Of these 29 articles, 6 represented in vivo models of controlled cortical impact (CCI), 3 represented fluid percussion injury (FPI), 8 represented closed-head weight drop, 9 represented primary blast-induced brain injury, and 3 represented closed-head Impact model of engineered rotational acceleration (CHIMERA). It is of note that no relevant porcine studies met our inclusion criteria. This highlights the need for future research to enhance translatability from pre-clinical to clinical settings.

Visual deficits from In Vivo models of TBI

CCI

CCI is a method of inducing brain trauma by delivering a blow using a pneumatically or electromagnetically controlled impactor shaft. The CCI method is implemented by inducing either an open-head injury via craniotomy (surgical opening into the skull), or a closed-head injury (intact skull).^8
–10 The impactor shaft can be stereotactically adjusted according to the brain region of interest, thereby allowing the researcher to choose between the right and left hemispheres (ipsilateral vs. contralateral analysis). Injury parameters such as diameter of the tip of the CCI device, striking velocity, striking depth, and dwell time can be controlled, allowing for a higher reproducibility of the injury. Even though the injury region varies among studies, the injury directly overlays the optic nerve in a rodent model, and as such, injuries to the visual system often result.

For example, in 2019 Das and colleagues used the CCI method to evaluate the role of the CC-Chemokine Ligand 20 (CCL20; pro-inflammatory chemokine) – CC-Chemokine Receptor 6 (CCR6; CCL20 sole receptor) axis in the pathogenesis of repeated-TBI induced visual dysfunction.¹¹ Overall, they report that CCR6 knockout mice and pharmacological blockage of CCL20 leads to reduced retinal degeneration, microgliosis, and inflammation following CCI-induced TBI in the retina and optic nerve. However, the CCR6 knockout gene did not reduce glial fibrillary acidic protein (GFAP) positive astrocyte activation in the optic nerve post-injury compared with in the retina, perhaps indicating that the CCL20-CCR6 axis has different mechanisms within the visual system post repetitive TBI (rTBI).¹¹ In 2021, Honig and colleagues reported decreased contrast sensitivity and the B wave of ERG, increased light aversion and resting pupil diameter, and axonal degeneration in the optic nerve. They reported that raloxifene, a drug that mitigates microglial activation, diminishes these CCI-induced injuries to the visual system.¹² In 2016, Bolton Hall and colleagues found that repeated closed head injury using CCI left mice with argyrophilic axons, microgliosis, and astrogliosis within the optic tract, as well as a loss of neurofilament protein 200 in the optic nerve.¹³ In 2014, Tzekov and colleagues found that mice who underwent CCI demonstrated decreased optic nerve diameter, demyelination of the optic nerve, decreased cellularity in and alterations to RGCs, thinning of the inner retina, and ERG changes.¹⁴ In 2016, Tzekov and colleagues again reported that rTBI showed increased cellularity and demyelination in the optic nerve, as well as reduction in RGCs.¹⁵ They also reported several proteomic changes to the optic nerve following CCI.¹⁵ Finally, in 2005, Hall and colleagues used a lateral CCI methodology to demonstrate widespread cortical damage in young adult CF-1 mice post-hit, including neurodegeneration of the visual cortex, which may suggest visual disturbances.¹⁶ Overall, CCI-induced TBI consistently causes damage to the retinas, optic tracts, and optic nerves of mouse subjects, which further demonstrates the vulnerability of the visual system post brain injury.

FPI

FPI is an open-head, surgical TBI methodology in which a craniectomy is performed and a pulse of fluid pressure created by a pendulum striking an impactor device is applied to the intact dura.⁹ Like previous models described, FPI has been cited several times as bringing about injuries to the visual system in mice. In 2017, Patel and colleagues utilized a central FPI (cFPI) model to demonstrate diffuse axonal injury (DAI) post-injury, which is known to contribute to visual deficits following TBI.¹⁷ They reported that DAI, as well as RGC axon terminal loss, is significant 4 days post-injury. Interestingly, axonal reorganization in the following days contributed to some recovery, indicating some degree of neuroplasticity. Despite some improvement, cFPI-injured mice never returned to their axonal baseline.¹⁷ In 2013, Wang and colleagues contrasted axonal findings in the optic nerve with FPI's impact on RGCs; they reported that although FPI induced traumatic axonal injury (TAI), the injury resulted in no loss of RGCs.¹⁸ Wang and colleagues also produced consistent TAI using the FPI model in 2011 and tracked the pathobiological progression of the injury over a 48 h period.¹⁹ They reported delayed axonal swelling, regression, and reorganization of proximal swellings with persistence of distal swelling and significant axonal dieback.¹⁹ Overall, although reports of RGC loss may be conflicting, FPI is reported to consistently contribute to optic nerve damage by inducing diffuse and traumatic axonal injury.

Closed-head weight drop

The weight drop model consists of dropping a projectile with specified characteristics through a tube at a designated height onto the head of the animal.²⁰ Weight drop models have been used to replicate diffuse brain injury characterized by axonal damage, sometimes associated with focal contusion. More recently, closed head injury models with free head rotation have been developed to mimic more “mild” TBIs (i.e., those without overt brain damage) such as those sustained in the context of sports. Some laboratories utilizing the weight drop method keep the rodent stationary on a hard surface during the injury, whereas others incorporate some element of rotational forces along with acceleration-deceleration to enhance the translatability of their TBI. It is of note that reports of vision loss from laboratories that incorporate acceleration-deceleration with rotational forces and from those that do not are very similar.^19
–21 For example, Morriss and colleagues, Qiu and colleagues, and Pilipović and colleagues all utilized models in which the head of the rodent rested on a penetrable surface, through which the head rotated when the weight drop injury was delivered.^21
–23 Morriss and Qiu demonstrated reduction in optic nerve diameter, RGC loss, decreased performance on visual acuity tasks, increased inflammatory markers and formation of an astrocytic scar, and long-term axonal degeneration.^21,22 Similarly, Pilipović reported neurodegeneration, axonal injury, and gliosis in the optic tract post-injury. By comparison, they did not appreciate changes in the lateral geniculate nucleus (LGN) or superius colliculi (SC) and observed no behavioral deficit in a Barnes Maze task in injured mice.²³ Both groups demonstrated a clear link between TBI induced by a weight drop with rotational-acceleration-deceleration forces and alterations to visual system structures.

Importantly, anatomical changes to the visual system are also reported from several groups who use the weight drop method with no component of rotational acceleration-deceleration. In 2018, Evanson and colleagues reported axonal degeneration in the optic tract following weight drop-induced TBI, during which the mouse subject was held stationary.²⁴ Similarly, to compare novel repetitive mild TBI (rmTBI) paradigms to single hit injuries, Xu and colleagues used the traditional weight drop method to observe RGC loss and optic nerve degeneration that worsens as number of hits increases.²⁵ In 2021, Hetzer and colleagues used the weight drop method without rotational acceleration to analyze chronic TON outside the retina post-injury.²⁶ In the optic tract, they found significantly increased neuronal and axonal degeneration by observing an increase in FluoroJade-C (FJ-C) staining at 7, 14, 30, 90, and 150 days post-injury.²⁶ They also reported morphological microglia changes (an indication of microglial activation) via ionized calcium-binding adapter molecule 1 (IBA-1) marker, as well as astrogliosis via increased GFAP marker.²⁶ However, increased GFAP was only observed at 7, 14, 90, and 150 days post-injury.²⁶ Further, significantly increased axonal degeneration in the dorsal LGN, microglia activation in the suprachiasmatic nucleus (SCN), and degeneration of the superior colliculi were reported.²⁶ It is noteworthy that neurodegeneration in the visual cortex was not observed (lack of positive FJ-C staining); on the other hand, a delay in gliosis pattern was reported, with signals of astrogliosis starting at 30 and 150 days post-injury, and with microglia activation also arising at 150 days post-injury.²⁶ In 2023, Alexandris and colleagues found that the severity of optic nerve degeneration following a weight drop-induced TBI varied by genotype.²⁷ SARM1 knockout mice showed profound suppression of distal optic nerve axonal degeneration, while the severity of injury to the proximal optic nerve remained comparable to those with SARM1 intact.²⁷ Finally, Hetzer and colleagues utilized a weight drop without rotational acceleration methodology to demonstrate axonal degeneration of optic nerve fibers, RGC cell death, astrogliosis and microglia activation, elevated expression of endoplasmic reticulum (ER) stress markers in retinas, and decreased optomotor responses in injured mice.²⁸ Overall, reports of anatomical changes to the visual system in the diffuse weight drop TBI model are consistent, with or without rotational forces being applied along with acceleration-deceleration.

CHIMERA

CHIMERA is an injury apparatus that induces TBI using a high velocity metal piston. The positioning of the piston relative to the animal's head induces rotational acceleration of the brain while the body is held stationary.²⁹

In a 2020 narrative review of the CHIMERA model, McNamara and colleagues summarized a number of neurological deficits in CHIMERA-induced TBI mice. These findings included impacts to the visual system directly such as increased microglia (by IBA-1) and GFAP expression in the brachium of the superior colliculus and the optic tracts in injured animals.³⁰ Similarly, a 2022 study by McNamara and colleagues showed increases in dynamic contrast enhancement (DCE) MRI in the LGN and SC, as well as T2 increases in the optic tract in the 1st week after injury.³¹ Additionally, they report increased GFAP staining in the optic tract and SC 7 days following CHIMERA-induced TBI.³¹ Finally, in 2020, Desai and colleagues utilized CHIMERA in single- and multiple-hit TBI cohorts and reported impaired performance on the MWM and visual cliff tests and significant decrease in visual evoked potential (VEP), as well as increases in glial cell immunostaining in mice with multiple injuries.³² CHIMERA repeatedly induces visual deficits in TBI mice, with behavioral and histological findings similar to that of animals from other injury models.

Primary blast-induced TBI

A primary blast model consists of an enclosed shock chamber or advanced blast simulator (ABS) partitioned into three sections. The first section (driver) of the ABS is pressurized with compressed gas, which leads to a passive membrane rupture creating a shock wave. The shock wave propagates through the transition chamber section (driven) exposing the rodent, who is held in a restraint, to a primary blast-induced TBI (bTBI).³³ Similar to the presence or lack of rotational acceleration forces in the weight drop model, the apparatus in a blast injury can vary in the amount of acceleration the rodent's head is subjected to. In some blast models, the chamber is considered “open,” so that while the rodent's body is restrained, their head is able to move freely and is not fixed in a specific position.³⁴ By contrast, in a “closed” blast apparatus, the entirety of the rodent, including the head, is restrained during injury.^33,35 Importantly, alterations to the visual system are reported as a result in both forms of the blast model.

In 2013, Mohan and colleagues utilized an open apparatus to demonstrate an acute and chronic decrease in pattern electroretinogram (pERG, a measure of retinal function) following blast induced TBI, as well as decreased maximum pupil constriction diameter acutely post- injury.³⁶ They also reported a decrease in retinal nerve fiber layers observed with optical coherence tomography (OCT) in addition to RGC and optic nerve damage.³⁶ In 2018, Allen and colleagues used an open acoustic blast model to analyze the long term (> 1 month) visual sequalae to injured rats. Following sonic shockwave delivery to the right side of their animal subjects, they reported greater ERG amplitudes and delayed ERG implicit times 8 months post- injury, as well as an increase in retinal thickness at all time points measured. They also reported decreased spatial frequency in the ipsilateral eye and decreased contrast sensitivity bilaterally beginning 2 months and persisting through 8 months following blast exposure.³⁷ Following this work, in 2021, Allen and colleagues compared injuries sustained to the visual system in rats that underwent blast injuries in open and closed models. In both types of chambers, the blast wave was induced perpendicularly to the body; one eye (ipsilateral) directly faced exposure from the blast, while the other eye (contralateral) was shielded from direct impact. Spatial frequency and contrast sensitivity were reduced in the ipsilateral eyes of mice in both types of chambers, whereas contralateral eyes in the enclosed apparatus demonstrated greater deficits. Further, the authors reported retinal thinning and reduced ERG in both eyes in mice who were injured in the enclosed chamber, but not in the open one.³⁴ This work highlights important mechanical variables in the blast model – rapid head movement and concussive impact – and demonstrates the varied injuries to the visual system resulting from both.

There are several other reports of visual system injuries in this “open” blast model. In 2016, Yin and colleagues used an open blast apparatus to demonstrate resistance of some forms of axonal degeneration in a specific strain of mice called Wallerian degeneration slow strain (WIdS).³⁸ They compared the pERG results between wild type and WldS strain mice, and found that 4 weeks following blast injury, wild type mice demonstrated significantly decreased pERG amplitude compared with their WldS counterparts exposed to the same injury.³⁸ In other words, the blast injury model induced visual system deficits in the wild type mice. In 2020, Evans and colleagues also use an open blast apparatus to demonstrate preserved RGC function and optic nerve integrity in mice treated with an interleukin-1 (IL-1) inhibitor following three repetitive blast induced hits when compared with shams treated with saline.³⁹ They thus suggested that modulation of inflammatory pathways could potentially preserve visual function in TBI patients; without IL-1 blockade, mice experienced acute retinal inflammation, glial cell activation, and chronic neuronal dysfunction.³⁹

By contrast, an enclosed blast chamber was utilized in Boehme and colleagues' 2021 work, in which RGC loss and axon degeneration, and the finding that axonopathy precedes soma loss, are reported.⁴⁰ In 2020, Arun and colleagues' closed blast chamber resulted in decreased visual acuity 2 days following blast exposure (resolved by day 6), decreased ERG amplitude, increased retinal degeneration, and increased glial cell activation in the retina. Each of these visual system alterations were reportedly ameliorated with administration of an anti-lysophosphatidic acid (LPA) antibody. LPA is a pro-inflammatory phospholipid that is reported to increase in patients who sustain a TBI; by preventing the expected inflammatory cascade following blast exposure, alterations to the visual system were avoided in anti-LPA antibody rats compared with control rats injured with the same mechanism.⁴¹ In 2018, Shedd and colleagues reported numerous alterations to the visual system of rat subjects exposed to closed blast injury, including alterations to the intraocular pressure (IOP) in eyes directly and indirectly exposed to the blast, and decreased contrast sensitivity that began immediately after injury and persisted 8 weeks following blast exposure. They also reported stromal swelling and scarring of the cornea of injured animals; by the end of the 8-week study, half of the surviving animals had persistent stromal scars. Similar thickening is reported in the retinas of the directly exposed eye of injured animals at 7 weeks post-injury. On histology, although no microscopic retinal injuries were detected, a significant increase in endothelial cell density in the right and left eyes was discovered at 1 and 4 weeks post-injury compared with the 1 day and 8 week marks following blast exposure. Cataract development was indicated following discovery of damage to lenses of blast exposed eyes.⁴² Lastly, in 2014, Wang and colleagues reported increased expression of activated caspase 3, a stain identifying apoptotic cells, in the retinas and optic nerves of rats exposed to primary blast injury. Importantly, although the blast was delivered to the right side of the animals, indications of cell death with the activated caspase 3 stain was displayed bilaterally (with increased expression closer to the site of injury) in the tissues of blast subjects.⁴³ Overall, visual system injuries are consistently reported in blast models utilizing both “open” and “closed” restraints.

Discussion

Pre-clinical models of TBI are correlated with injury to the retina, optic nerve, optic tracts, LGN, SC, cornea, and lens in mouse and rat subjects (Table 1). The discoveries from each injury model are critical to efforts to understand the pathogenesis behind visual loss in TBI patients. These findings are also crucial to consider when creating future behavioral experimental paradigms to study TBI. Although we can reliably study the molecular and histological mechanisms of visual system injury in TBI, behavioral experiments that rely on a subject's vision may be compromised. To this point, using these models of TBI to assess learning and memory in injured rodents via behavioral tests that rely on intact vision is common practice.^20,44

Table 1.

Summary of Relevant Variables in Selected Studies That Identified Visual Pathology After mTBI

Author	TBI model	Strain	Rotational acceleration?	Site of injury	Reported visual system pathology
Das, 2019¹¹	CCI	C57BL/6 or CCR6-/-	No	Anterior-posterior coordinate of -0.8 mm from bregma and on the midline of the skull	Retinal pathology
Honig, 2021¹²	CCI	C57BL/6	No	Impactor tip was positioned at 1.5 mm caudal to bregma	Optic nerve axon loss, deceased ERG, increased resting pupillary diameter
Bolton Hall, 2016¹³	CCI	C57BL/6	No	The posterior edge of the tip was aligned at the lambda suture. The anterior edge of the tip meets the bregma suture (0 mm bregma level)	Argyrophilic axons, astrogliosis, and microgliosis of the optic tract, loss of neurofilament protein 200 in optic nerve
Tzekov, 2014¹⁴	CCI	C57BL/6	No	The location of the impact on the skull was central, equally distant from both eyes	Reduction in optic nerve diameter, decreased ERG, RGC loss, retinal thinning
Tzekov, 2016¹⁵	CCI	C57BL/6	No	The location of the impact on the skull was central, equally distant from both eyes	Optic nerve demyelination, RGC loss, proteomic changes of the optic nerve
Hall, 2005¹⁶	CCI	CF-1	No	A 4 mm craniotomy was made lateral to the sagittal suture, centered between bregma and lambda.	Widespread neurodegeneration, including visual cortex damage
Patel, 2017¹⁷	FPI	C57BL/6J	No	3 mm wide craniotomy midway between lambdoid and bregma cranial sutures. Brief deformation of the brain through intact dura ∼1 mm proximal to the chiasm	Diffuse axonal injury, RGC axon terminal loss
Wang, 2013¹⁸	FPI	Thy1-YFP-16 transgenic	No	Fluid pressure induced ∼1 mm proximal to the chiasm	Traumatic axonal injury with no RGC loss
Wang, 2011¹⁹	FPI	C57BL/6 wild type and Thy1-YFP-16 transgenic	No	Midline 3 mm craniotomy over the sagittal suture halfway between bregma and lambda with a syringe hub affixed to the craniotomy site, where the FPI was delivered	Traumatic axonal injury, axonal swelling, axonal dieback
Morriss, 2021²¹	Weight drop	C57BL/6	Yes	Impact delivered to the dorsal skull centered over bregma	Reduction in optic nerve diameter, RGC loss, long- term axonal degeneration, decreased visual acuity
Qiu, 2022²²	Weight drop	C57BL/6	Yes	Impact delivered on the midline and between the two ears, positioned over bregma	Reduction in optic nerve diameter, RGC loss, long- term axonal degeneration, decreased visual acuity
Pilipović, 2021²³	Weight drop	C57BL/6 wild type and TDP-43^G348C transgenic	Yes	Impact was delivered to the mouse head centrally between the ears	Neurodegeneration, axonal injury, and gliosis in the optic tract
Evanson, 2018²⁴	Weight drop	C57BL/6J	No	Impact centered approximately over bregma	Axonal degeneration in the optic tract
Xu, 2016²⁵	Weight drop	C57BL/6J	No	A steel disk fixed to the mouse skull was set at 3.2 mm, designed to ride over bregma and lambda sutures when affixed with cyanoacrylate to the skull, on top of which the impact was delivered	RGC loss, optic nerve degeneration
Hetzer, 2021²⁶	Weight drop	C57BL/6	No	Impact was delivered over the approximate location of bregma	Axonal degeneration in dorsal LGN, microglial activation in SCN, degeneration of superior colliculi
Alexandris, 2023²⁷	Weight drop	C57BL/6 wild type and Sarm1 KO transgenic	No	A 5 mm disc was glued onto the skull midway between the bregma and lambda sutures. The impact was delivered at the site of the disc	Optic nerve degeneration
Hetzer, 2021²⁸	Weight drop	C57BL/6J	No	Impact delivered onto the calvarium with the scalp intact, approximately above bregma	Degeneration of optic nerve fibers. RGC death, elevated expression of ER stress markers in retina, decreased optomotor response
McNamara, 2020³⁰	CHIMERA	n/a (review)	Yes	n/a (review)	Microglia and GFAP expression in the superior colliculus and in the optic tracts
McNamara, 2022³¹	CHIMERA	C57BL/6J	Yes	Impact was delivered at the region surrounding bregma	DCE MRI enhancement seen in the LGN and SC, as well as T2 increases in the optic tract. Increased GFAP staining in the optic tract and SC
Desai, 2020³²	CHIMERA	C57BL/6NCr	Yes	Not specified	Decreased performance on MWM and visual cliff tests, decrease in visual evoked potential, increases in glial cell immunostaining
Mohan, 2013³⁶	Blast	C57BL/6J	No	Injured mice were oriented with the left side of the head toward the blast membrane with the left eye facing the blast wave and the right eye facing away from the direction of the blast wave	Decrease in ERG, decreased maximum pupil constriction diameter, decrease in retinal nerve fiber layers, RGC and optic nerve damage
^*Allen, 2018³⁷	Blast	Long-Evans outbred rats	No	The acoustic blast was delivered directly to the right eye. The head was unrestrained while the subject's body was held in a chamber. A pillow was placed on the contralateral side of the blast to reduce the extent of impact and recoil	ERG alterations, decreased contrast sensitivity, increase in total retinal thickness, increased GFAP that persisted up to 8 months post-injury
^*Allen, 2021³⁴	Blast	Long-Evans outbred rats	No	Either the left or right eye was positioned in front of the blast tube	Retinal thinning, reduced ERG, decreased spatial frequency and contrast sensitivity
Yin, 2016³⁸	Blast	WldS-positive and wild-type littermates bred from heterozygous Wallerian degeneration slow strain mice	No	The left side of the head was closest to the origin of the blast wave	Decreased pERG
Evans, 2020³⁹	Blast	C57BL/6J	No	Mice were oriented within the chamber with the left side of the head positioned toward the blast wave and the right side facing away from the blast wave	Acute retinal inflammation, glial cell activation, and chronic neuronal dysfunction
Boehme, 2021⁴⁰	Blast	C57BL/6J	No	The left side of the unrestrained head was exposed directly to the overpressure wave	RGC loss, axonal degeneration, axonopathy precedes soma loss
^*Arun, 2020⁴¹	Blast	Sprague Dawley rats	No	Rats directly faced the oncoming shockwave in a prone orientation in this blast model	Decreased visual acuity, decreased ERG amplitude, cellular voids in the outer nuclear layer of the retina and retinal degeneration, increased GFAP in the retina
^*Shedd, 2018⁴²	Blast	Long-Evans rats	No	The blast was delivered to the right side of the subject. The right eye was exposed directly to the blast wave while the left side was exposed indirectly	Alterations in IOP, decreased contrast sensitivity, swelling of and damage to cornea and lens
^*Wang, 2014⁴³	Blast	Sprague-Dawley rats	No	Blast was delivered to the right side of the animal while the subject was positioned in a holder to prevent secondary and tertiary injuries	Increased expression of activated caspase 3 (a stain identifying apoptotic cells) in retinas and optic nerves of injured animals

All studies except those marked with ^* utilized a mouse model.

mTBI, mild traumatic brain injury; CCI, controlled cortical impact; FPI, fluid percussion injury; CHIMERA, closed-head Impact model of engineered rotational acceleration; ERG, electroretinogram; RCG, retinal ganglion cell; LGN, lateral geniculate nucleus; SCN, suprachiasmatic nucleus; ER, endoplasmic reticulum; GFAP, glial fibrillary acidic protein; DCE, dynamic contrast enhancement; MRI, magnetic resonance imaging; SC, superius colliculi; MWM, Morris Water Maze; pERG, pattern electroretinogram; IOP, intraocular pressure

Although mTBIs may not lead to complete vision loss in mice, the consistent histological changes to anatomical areas responsible for vision in the brain following injury could lead to a detrimental interpretation of sight-driven behavioral tests. For example, to evaluate the enduring behavioral impacts of repetitive mild traumatic brain injury (rmTBI) in a CCI model (with craniotomy) using Sprague-Dawley rats, Graham and colleagues⁴⁵ employed various behavioral assessments, including the elevated plus maze and open field tests for anxiety, forced swim test for depression, rotarod for locomotor function, and MWM for spatial learning and memory. They found a significant impairment specifically in the MWM test, indicating compromised spatial learning and memory in injured animals compared with controls. The MWM was divided into four zones (northwest [NW], northeast [NE], southwest [SW], and southeast [SE]), and according to the authors, a directional/visual aspect may further contribute to these deficits.⁴⁵ They noted that depending on the starting zone of the trial, injured animals took significantly longer to reach the platform. This suggested a potential visual acuity imbalance as a consequence of the injury, thereby further influencing the observed deficits in spatial memory and learning.⁴⁵ However, this study lacked histological analysis of the brain regions responsible for vision following injury. Additionally, Morriss and colleagues²¹ showed that rmTBI induced by a rotational-acceleration closed-head weight drop model in C57BL/6 mice resulted in impaired performance in the MWM task, primarily because of diminished visual acuity and reduced optic nerve diameter compared with control animals.²¹ Further, Pinkowski and colleagues⁴⁶ used a closed-head CCI model with C57BL/6 mice to demonstrate that rmTBI groups exhibited impaired visual discrimination learning in a touchscreen task compared with controls.⁴⁶ Importantly, the injured animal displayed no motor alterations. Hence, these outcomes cannot be attributed to a deficiency in motor function or locomotion, further confirming the effects of mild TBI.⁴⁶ Coupled with the histological observations detailed throughout the manuscript, these results emphasize the significance of comprehending how mTBI influences visual-memory tasks and visual-spatial learning, potentially reshaping interpretations of behavioral tests reliant on preserved visual acuity.

It is of note that there are several strategies currently used to evaluate potential confounds of visual deficits during performance of specific behavioral tests. For example, performance on the MWM task can be compared using hidden and visible platforms. Further, as many of the authors in this review demonstrate, electrophysiological techniques such as visual evoked potential and electroretinogram can estimate visual function of rodents.^{12,14,33,35,36} Looking forward, a combination of established electrophysiological procedures, tests of visual acuity or of novel objects, and behavioral techniques such as modified MWM platforms should be used consistently to evaluate visual function and mitigate potential confounders on behavioral tests. Alternatively, with our current collection of reports of visual deficits in a wide array of diffuse injury models, the utilization of behavioral tests that rely on a sensory modality other than vision should be considered. Finally, elucidating the mechanism behind varied visual system impact based on position of hit (side-alternating vs. center) is an exciting line of future inquiry; better understanding the biomechanical outcomes of diffuse pre-clinical TBI could translate to clinical practice.

Equally important for consideration is why vision loss is so common in these injury models (it if of note that techniques that are not consistent with TBI mechanisms, such as crush or transection, were excluded from this narrative review). One possible route of study is in the bidirectional spread of trans-synaptic degeneration in the visual system.⁴⁷ Post-TBI, a leading cause of vision loss is injury to the optic nerve, which subsequently disrupts brain regions involved with visual processing and/or perception. Indeed, previous work has demonstrated the spread of neurodegeneration from interconnected neurons between the eye and brain and has suggested that neuronal damage of the brain can be among the potential sequalae to retinal pathology and vice versa.⁴⁷ Although this work⁴⁷ focused primarily on neurodegenerative disorders such as Alzheimer's and Parkinson's disease, a similar pathophysiological mechanism could explain the relationship between TBI and vision loss. Other possible explanations for the visual deficits reported in diffuse injury models could be the physical positioning of the optic nerve in the rodent skull. The relatively shallow placement of the optic nerve in the mouse brain, as well as position of hit in a specific experimental apparatus, could be critical to the development of visual dysfunction. This theory is supported by the reports of Izzy and colleagues, whose weight drop study demonstrated no deficits in trials of the MWM task after repetitive lateral head impact.⁴⁸ Further work is necessary to elucidate the mechanisms behind vision loss in diffuse injury models in the laboratory as well as in the clinical setting.

The histological and biomechanical changes induced in the visual system following pre-clinical and clinical TBI are well documented. To this point, however, the pre-clinical, laboratory-based visual deficit findings were limited to individual studies and injury modalities. This review brings discoveries from different model types together and considers them as a unit. Diffuse pre-clinical models of TBI improve understanding the pathogenesis of brain injury in the clinical setting, including the impact on the visual system specifically. Although focal models of injury offer an opportunity to study the histological and behavioral sequelae of a highly precise lesion to the brain, they may lack the translatability that more diffuse modalities offer. TBIs in the clinical setting inherently lack precision; concussions are diffuse in nature and, as such, models that accurately mimic a more global head impact are needed to enhance the translatability of TBI models in the pre-clinical setting. These relative advantages of diffuse models, in conjunction with the widespread visual deficit findings reported in this review, should be considered carefully when pursuing investigations of pre-clinical TBI. Reconciling these opposing factors will be an essential component of future work.

Conclusion

Disruptions to the visual system such as blurred vision, visual field loss, and loss of visual acuity are common sequalae following TBI seen clinically, and yet the pathophysiological mechanism behind these widespread symptoms is not well understood. Although the occipital lobe is the primary cortical region responsible for the collection and parsing of raw visual data, the parietal lobe, temporal lobe, and other cortices play important roles in visual processing and in interpreting our visual environments. As such, regardless of the exact location of a given TBI, any mild injury to the brain has a high probability of affecting visual intake or processing. It is therefore critical that researchers utilizing pre-clinical models of TBI focus on these visual sequelae and that the models chosen are translatable to the injuries encountered in the clinical setting.

In this vein, enhancing our understanding of TBI-related vision loss by improving the pre-clinical models in use, as well as the behavioral tests used in these models, are key efforts. The widespread reports of visual deficits in an array of pre-clinical diffuse TBI models described in this review have important implications for future scientific inquiry, specifically regarding choice of injury modality and behavioral tests when evaluating learning and memory. Future exploration into the titration of each injury model in order to assess severity and impact on visual acuity is needed; perhaps a milder injury could preserve the visual function of animals in some models and could continue to be assessed using behavioral tests that rely on vision, following careful evaluation of visual function. Indeed, many of the histological changes to the visual system reported in this review may not correlate with loss of visual function: a decline in vision that is clinically observable may only occur at an injury severity beyond a certain threshold. To this point, the exact severity at which each model induces visual deficits is unknown. Further, this review reveals a gap in the available behavioral testing paradigms that use sensory systems other than vision to assess learning and memory following TBI. The development of novel behavioral tests that rely on auditory or olfactory cues is an exciting line of potential future inquiry.

Footnotes

Authors' Contributions

Gabriella Orbach: idea, conceptualization, methodology, writing – original draft, review and editing; Eva J. Melendes: writing – review and editing; Kaitlyn Warren: writing – review and editing; Jianhua Qiu: conceptualization, supervision, writing – review and editing; William P Meehan: conceptualization, supervision, writing – review and editing; Rebekah Mannix: idea, conceptualization, methodology, supervision, writing – original draft, review and editing; Fernanda Guilhaume-Correa: idea, conceptualization, methodology, supervision, writing –original draft, review and editing.

Funding Information

This work was supported by grant T32HD040128 from the National Institute of Childhood Diseases and Human Development (National Institutes of Health [NIH]).

Author Disclosure Statement

No competing financial interests exist.

References

Frieden

, Houry

, Baldwin

Traumatic Brain Injury in the United States: Epidemiology and Rehabilitation. New York, NY: NewYork University School of Medicine; 2015.

Magone

, Kwon

, Shin

. Chronic visual dysfunction after blast-induced mild traumatic brain injury. J Rehabil Res Dev, 2014; 51(1):71–80; doi: 10.1682/JRRD.2013.01.0008

Olver

, Ponsford

, Curran

. Outcome following traumatic brain injury: a comparison between 2 and 5 years after injury. Brain Inj, 1996; 10(11):841–848; doi: 10.1080/026990596123945

Master

, Scheiman

, Gallaway

, et al. Vision diagnoses are common after concussion in adolescents. Clin Pediatr (Phila), 2016; 55(3):260–267; doi: 10.1177/0009922815594367

Schlageter

, Gray

, Hall

, et al. Incidence and treatment of visual dysfunction in traumatic brain injury. Brain Inj, 1993; 7(5):439–448; doi: 10.3109/02699059309029687

Goodrich

, Flyg

, Kirby

, et al. Mechanisms of TBI and visual consequences in military and veteran populations. Optom Vis Sci, 2013; 90(2):105–112; doi: 10.1097/OPX.0b013e31827f15a1

Armstrong

RA.

Visual problems associated with traumatic brain injury. Clin Exp Optom, 2018; 101(6):716–726; doi: 10.1111/cxo.12670

Jamnia

, Urban

, Stutzmann

, et al. A clinically relevant closed-head model of single and repeat concussive injury in the adult rat using a controlled cortical impact device. J Neurotrauma, 2017; 34(7):1351–1363; doi: 10.1089/neu.2016.4517

Prins

, Alexander

, Giza

, et al. Repeated mild traumatic brain injury: mechanisms of cerebral vulnerability. J Neurotrauma, 2013; 30(1):30–38; doi: 10.1089/neu.2012.2399

10.

White

, VandeVord

. Regional variances depict a unique glial-specific inflammatory response following closed-head injury. Front Cell Neurosci, 2023; 17:1076851; doi: 10.3389/fncel.2023.1076851.

11.

Das

, Tang

, Han

, et al. CCL20-CCR6 axis modulated traumatic brain injury-induced visual pathologies. J Neuroinflammation, 2019; 16(1):115; doi: 10.1186/s12974-019-1499-z

12.

Honig

, Del Mar

, Henderson

, et al. Raloxifene modulates microglia and rescues visual deficits and pathology after impact traumatic brain injury. Front Neurosci, 2021; 15:701317; doi: 10.3389/fnins.2021.701317

13.

Bolton Hall

, Joseph

, Brelsfoard

, et al. Repeated closed head injury in mice results in sustained motor and memory deficits and chronic cellular changes. PloS One, 2016; 11(7):e0159442; doi: 10.1371/journal.pone.0159442

14.

Tzekov

, Quezada

, Gautier

, et al. Repetitive mild traumatic brain injury causes optic nerve and retinal damage in a mouse model. J Neuropathol Exp Neurol, 2014; 73(4):345–361; doi: 10.1097/NEN.0000000000000059

15.

Tzekov

, Dawson

, Orlando

, et al. Sub-chronic neuropathological and biochemical changes in mouse visual system after repetitive mild traumatic brain injury. PloS One, 2016; 11(4):e0153608; doi: 10.1371/journal.pone.0153608

16.

Hall

, Sullivan

, Gibson

, et al. Spatial and temporal characteristics of neurodegeneration after controlled cortical impact in mice: more than a focal brain injury. J Neurotrauma, 2005; 22(2):252–265; doi: 10.1089/neu.2005.22.252

17.

Patel

, Jurgens

CWD

, Krahe

, et al. Adaptive reorganization of retinogeniculate axon terminals in dorsal lateral geniculate nucleus following experimental mild traumatic brain injury. Exp Neurol, 2017; 289:85–95; doi: 10.1016/j.expneurol.2016.12.012

18.

Wang

, Fox

, Povlishock

. Diffuse traumatic axonal injury in the optic nerve does not elicit retinal ganglion cell loss. J Neuropathol Exp Neurol, 2013; 72(8):768–781; doi: 10.1097/NEN.0b013e31829d8d9d

19.

Wang

, Hamm

, Povlishock

. Traumatic axonal injury in the optic nerve: evidence for axonal swelling, disconnection, dieback, and reorganization. J Neurotrauma, 2011; 28(7):1185–1198; doi: 10.1089/neu.2011.1756

20.

Bodnar

, Roberts

, Higgins

, et al. A systematic review of closed head injury models of mild traumatic brain injury in mice and rats. J Neurotrauma, 2019; 36(11):1683–1706; doi: 10.1089/neu.2018.6127

21.

Morriss

, Conley

, Hodgson

, et al. Visual dysfunction after repetitive mild traumatic brain injury in a mouse model and ramifications on behavioral metrics. J Neurotrauma, 2021; 38(20):2881–2895; doi: 10.1089/neu.2021.0165

22.

Qiu

, Boucher

, Conley

, et al. Traumatic brain injury-related optic nerve damage. J Neuropathol Exp Neurol, 2022; 81(5):344–355; doi: 10.1093/jnen/nlac018

23.

Pilipović

, Rajič Bumber

, Dolenec

, et al. Long-term effects of repetitive mild traumatic injury on the visual system in wild-type and TDP-43 transgenic mice. Int J Mol Sci, 2021; 22(12):6584; doi: 10.3390/ijms22126584

24.

Evanson

, Guilhaume-Correa

, Herman

, et al. Optic tract injury after closed head traumatic brain injury in mice: a model of indirect traumatic optic neuropathy. PloS One, 2018; 13(5):e0197346; doi: 10.1371/journal.pone.0197346

25.

, Nguyen

, Lehar

, et al. Repetitive mild traumatic brain injury with impact acceleration in the mouse: multifocal axonopathy, neuroinflammation, and neurodegeneration in the visual system. Exp Neurol, 2016; 275 Pt 3:436–449; doi: 10.1016/j.expneurol.2014.11.004

26.

Hetzer

, Shalosky

, Torrens

, et al. Chronic histological outcomes of indirect traumatic optic neuropathy in adolescent mice: persistent degeneration and temporally regulated glial responses. Cells, 2021; 10(12):3343; doi: 10.3390/cells10123343

27.

Alexandris

, Lee

, Lehar

, et al. Traumatic axonal injury in the optic nerve: the selective role of sarm1 in the evolution of distal axonopathy. J Neurotrauma, 2023;neu.2022.0416; doi: 10.1089/neu.2022.0416

28.

Hetzer

, Guilhaume-Correa

, Day

, et al. Traumatic optic neuropathy is associated with visual impairment, neurodegeneration, and endoplasmic reticulum stress in adolescent mice. Cells, 2021; 10(5):996; doi: 10.3390/cells10050996

29.

Namjoshi

, Cheng

, McInnes

, et al. Merging pathology with biomechanics using CHIMERA (Closed-Head Impact Model of Engineered Rotational Acceleration): a novel, surgery-free model of traumatic brain injury. Mol Neurodegener, 2014; 9:55; doi: 10.1186/1750-1326-9-55

30.

McNamara

, Grillakis

, Tucker

, et al. The closed-head impact model of engineered rotational acceleration (CHIMERA) as an application for traumatic brain injury pre-clinical research: a status report. Exp Neurol, 2020; 333:113409; doi: 10.1016/j.expneurol.2020.113409

31.

McNamara

, Knutsen

, Korotcov

, et al. Meningeal and visual pathway magnetic resonance imaging analysis after single and repetitive closed-head impact model of engineered rotational acceleration (CHIMERA)-induced disruption in male and female mice. J Neurotrauma, 2022; 39(11–12):784–799; doi: 10.1089/neu.2021.0494

32.

Desai

, Chen

, Kim

H-Y

. Multiple mild traumatic brain injuries lead to visual dysfunction in a mouse model. J Neurotrauma, 2020; 37(2):286–294; doi: 10.1089/neu.2019.6602

33.

Bryden

, Tilghman

, Hinds

. Blast-related traumatic brain injury: current concepts and research considerations. J Exp Neurosci, 2019; 13:1179069519872213; doi: 10.1177/1179069519872213

34.

Allen

, Motz

, Singh

, et al. Dependence of visual and cognitive outcomes on animal holder configuration in a rodent model of blast overpressure exposure. Vision Res, 2021; 188:162–173; doi: 10.1016/j.visres.2021.07.008

35.

Guilhaume-Correa

, Pickrell

, VandeVord

. The imbalance of astrocytic mitochondrial dynamics following blast-induced traumatic brain injury. Biomedicines, 2023; 11(2):329; doi: 10.3390/biomedicines11020329

36.

Mohan

, Kecova

, Hernandez-Merino

, et al. Retinal ganglion cell damage in an experimental rodent model of blast-mediated traumatic brain injury. Invest Ophthalmol Vis Sci, 2013; 54(5):3440–3450; doi: 10.1167/iovs.12-11522

37.

Allen

, Motz

, Feola

, et al. Long-term functional and structural consequences of primary blast overpressure to the eye. J Neurotrauma, 2018; 35(17):2104–2116; doi: 10.1089/neu.2017.5394

38.

Yin

, Voorhees

, Genova

, et al. Acute axonal degeneration drives development of cognitive, motor, and visual deficits after blast-mediated traumatic brain injury in mice. eNeuro, 2016; 3(5):ENEURO.0220-16.2016; doi: 10.1523/ENEURO.0220-16.2016

39.

Evans

, Woll

, Wu

, et al. Modulation of post-traumatic immune response using the il-1 receptor antagonist anakinra for improved visual outcomes. J Neurotrauma, 2020; 37(12):1463–1480; doi: 10.1089/neu.2019.6725

40.

Boehme

, Hedberg-Buenz

, Tatro

, et al. Axonopathy precedes cell death in ocular damage mediated by blast exposure. Sci Rep, 2021; 11(1):11774; doi: 10.1038/s41598-021-90412-2

41.

Arun

, Rossetti

, DeMar

, et al. Antibodies against lysophosphatidic acid protect against blast-induced ocular injuries. Front Neurol, 2020; 11:611816; doi: 10.3389/fneur.2020.611816

42.

Shedd

, Benko

, Jones

, et al. Long term temporal changes in structure and function of rat visual system after blast exposure. Invest Ophthalmol Vis Sci, 2018; 59(1):349–361; doi: 10.1167/iovs.17-21530

43.

Wang

H-CH

, Choi

J-H

, Greene

, et al. Pathophysiology of blast-induced ocular trauma with apoptosis in the retina and optic nerve. Mil Med, 2014; 179(8 Suppl):34–40; doi: 10.7205/MILMED-D-13-00504

44.

Mannix

, Berglass

, Berkner

, et al. Chronic gliosis and behavioral deficits in mice following repetitive mild traumatic brain injury. J Neurosurg, 2014; 121(6):1342–1350; doi: 10.3171/2014.7.JNS14272

45.

Graham

, Juzang

, White

. Effects of repetitive mild traumatic brain injury on weight gain and chronic behavioral outcomes in male rats. PLOS ONE, 2023; 18(7):e0287506; doi: 10.1371/journal.pone.0287506

46.

Pinkowski

, Guerin

, Zhang

, et al. Repeated mild traumatic brain injuries impair visual discrimination learning in adolescent mice. Neurobiol Learn Mem, 2020; 175:107315; doi: 10.1016/j.nlm.2020.107315

47.

Sharma

, Chitranshi

, Wall

, et al. Trans-synaptic degeneration in the visual pathway: Neural connectivity, pathophysiology, and clinical implications in neurodegenerative disorders. Surv Ophthalmol, 2022; 67(2):411–426; doi: 10.1016/j.survophthal.2021.06.001

48.

Izzy

, Brown-Whalen

, Yahya

, et al. Repetitive traumatic brain injury causes neuroinflammation before tau pathology in adolescent P301S mice. Int J Mol Sci, 2021; 22(2):907; doi: 10.3390/ijms22020907