A pediatric physiatrist’s approach to neuromuscular hip dysplasia in cerebral palsy

1 Introduction

Cerebral palsy (CP) is an umbrella term encompassing a group of disorders pertaining to abnormalities in movement, tone, and/or posture due to a nonprogressive lesion to an immature brain [1]. CP is the most common cause of motor disability in childhood [2].

Hip dysplasia is the second most common orthopedic deformity seen in CP, and its severity can range from a hip at risk for subluxation to full hip dislocation with degenerative changes [3]. The purpose of this article is to review the hip pathologies that occur in CP focusing on their pathogenesis, physical exam findings, impact on function, and conservative treatment.

2 Pathogenesis of neuromuscular hip dysplasia

Neuromuscular hip dysplasia results from imbalanced tension placed on the hip joint by spastic or dyskinetic muscles, typically the hip flexors and adductors [4]. The abnormal forces lead to osseous deformities of the growing femoral head and acetabulum, which are further accentuated by limited weight bearing. The increased flexion and adduction of the hip creates increased pressure on the posterolateral aspect of the malleable acetabulum, altering its shape and articulation with the femoral head [3]. The abnormal femoral head position against the posterolateral acetabular labrum causes focal epiphyseal deformation ranging from mild medial flattening to a wedge-shaped defect [5]. Of note, anterior hip dislocation is present in 1.5% of children with CP. These children typically have severe quadriplegia with hypotonia or extensor posturing leading to an anterior acetabular deficiency [3, 5].

Additionally, children with CP are born with normal femoral anteversion of 30 to 40 degrees that fails to decrease to 15 to 20 degrees as expected with typical development [6]. As shown on radiographs, the angle between the femoral neck and shaft is also increased, known as coxa valga; however, this could be a finding accentuated by the increased anteversion [3, 5]. These torsional bone deformities are thought to be most commonly due to attenuated femoral head remodeling in the setting of iliopsoas and hamstring spasticity in addition to decreased weight bearing through the hip [3, 7]. Not surprisingly, there is a linear relationship between Gross Motor Function Classification System (GMFCS) level and the incidence of hip displacement, occurring in 15% of those classified as GMFCS II and 90% of those considered GMFCS V [8, 9].

Hip subluxation can begin as early as 2 years old and, once it occurs, there is rarely improvement without intervention [3, 10]. The progression from subluxation to displacement is gradual, typically taking several years, with most dislocations occurring before age 7 across GMFCS levels [5, 10].

3 Essential components of the physiatric exam of the hip

Beginning with the patient supine, observe the resting position, noting the alignment of the pelvis and spine, abnormal movements and postures, or obvious contractures.

In non-ambulatory children, look for a “windswept deformity,” which is an adducted and internally-rotated position of one hip with abduction and external rotation of the opposite hip, due to asymmetric muscle activation resulting in pelvic obliquity [3, 12]. The adducted, elevated hip is more at risk for dislocation [5]. Pelvic obliquity with hip dislocation is often associated with scoliosis. However, studies have found them to be secondary to the underlying neuromuscular condition, not the result of a causal relationship [13, 14].

Prior to evaluating the range of motion, the severity of hypertonia should then be assessed if present, with the head in a neutral, midline position as some children with CP exhibit a persistent asymmetric tonic neck reflex [15]. Commonly used measures to assess hypertonia include the Modified Ashworth Scale, the Modified Tardieu Scale, and the Hypertonia Assessment Tool [16].

Passive range of motion is essential to evaluating the available movement and the presence of pain. Hip flexor tightness can be assessed supine with the Thomas test and/or prone with the Staheli test. The Staheli test was found to be more valid to detect hip flexion contracture in patients with CP [17]. Hip abduction should be assessed with the hips extended and the knees both flexed and extended [15]. Passive hip abduction of less than 45 degrees and abduction asymmetry are associated with increased risk of hip subluxation [18]. It is important to note the presence of the pseudo-Galeazzi sign, or asymmetry in femoral height, when evaluating hip abduction. The asymmetry could be due to pelvic obliquity alone or could be exaggerated by subluxation or dislocation of the hip with the adduction tendency [19].

Internal rotation of greater than 70 degrees can be seen with increased femoral anteversion. Craig’s test, also known as the trochanteric prominence angle test, can also be used to evaluate the degree of femoral anteversion [20]. Hamstring tightness can be assessed with the popliteal angle test, either unilateral or bilateral. In the unilateral test, the contralateral leg is fully extended, while in the bilateral test, the contralateral hip and knee are flexed. This results in a more posteriorly tilted pelvis with relaxation of the contralateral hip flexors, resulting in a smaller popliteal angle, measured from the vertical. Both tests have been found to be reliable methods of measuring the popliteal angle [21]. These authors have observed that, if on exam the more involved leg is unexpectedly found to have a smaller popliteal angle, it could indicate hip displacement.

Although essential, the physical exam alone is insufficient to diagnose hip dysplasia, and hip surveillance through radiologic evaluation is critical. There is moderate-quality evidence and a strong recommendation to use comprehensive hip surveillance practices to facilitate early detection and management of hip displacement [22]. This is further discussed in “Hip Surveillance for Patients with Cerebral Palsy in the United States” by Shrader and Whitaker.

4 Impact of hip dysplasia on function

The progression of hip dysplasia can negatively impact multiple aspects of function, particularly hygiene, sitting, standing, and ambulation, and eventually lead to chronic pain [1]. As hip dysplasia progresses it can lead to pelvic obliquity, which can impair sitting balance and necessitate custom seating accommodations to avoid the development of pressure ulcers [23]. In non-ambulatory children, located hips provide a stable base for participating in standing programs and, if able, maintaining the ability to comfortably transfer as they age.

In ambulatory children, this stability is essential for continued ambulation [1]. In children functioning at GMFCS levels I and II, motor function remains stable into young adulthood, while those at levels III-V experience a decline beginning in their teenage years [24]. The distribution of this change in function among GMFCS levels closely mirrors the incidence of hip dysplasia among the levels, raising the possibility that hip dysplasia contributes to motor function decline [24].

Regarding pain, one study reviewing 64 hips in 45 patients with CP over 19 years found patients with dislocated hips experienced the most severe degenerative arthritis, limited range of motion, and pain [25]. Chronic pain is reported in more than 50% of individuals with dislocated hips as early as young adulthood [3, 5].

5 Conservative management techniques

Non-invasive techniques used in the management or prevention of hip dysplasia include physical therapy (PT), bracing, oral medications, and chemoneurolysis.

As discussed in “Postural Management for Hip Health in Children and Adults with Severe Cerebral Palsy” by Paleg and Livingstone, therapy program goals should include maintaining hip range of motion, preventing contractures, and promoting weight bearing with the use of orthotics and/or equipment as needed [5]. Given the significant increase in hip dislocation among non-ambulatory children with CP and knowing that weight-bearing facilitates the development of stable hips, promoting weight-bearing and ambulation via a stander or gait trainer is essential [26]. One study looked at the effect of straddled weight bearing for 1.5 hours per day on migration percentage (MP) in 14 non-ambulatory children with CP and found a reduction in MP and preservation of passive range of motion compared to controls [26].

Oral medications are useful in the treatment of global hypertonia and pain. In selecting an appropriate medication, one important factor to consider is the subtype(s) of hypertonia present, with the most common forms being spasticity, dystonia, and rigidity [27]. Other factors to consider include the child’s function, the goals of the child and caregiver, and the involved muscles [27]. Medications used in the management of spasticity include baclofen, benzodiazepines such as diazepam, alpha-2 adrenergic agonists such as tizanidine, and direct muscle relaxants such as dantrolene. Oral medications for dystonia are considered less effective, but medications used include levodopa, trihexyphenidyl, and gabapentin [27]. Medication selection and dosing is frequently a trial-and-error process; however, a medication’s site of action, mechanism, and side effect profile can help guide treatment.

Botulinum neurotoxins (BoNT), chemoneurolysis with ethanol, and/or phenol are considered a more focal treatment option for hypertonia. The use of these agents in combination allows for a more appropriate and complete treatment plan without the concern of systemic side effects, as BoNT has weight-based dosing limitations [28]. These approaches are preferable to oral medication as to avoid systemic side effects such as sedation or diffuse weakness. The peak age for the use of BoNT is typically between 2 and 6 years [29]. Clinically, injectors tend to target the iliopsoas, hip adductors, and medial hamstrings to decrease pressure on the posterolateral aspect of the malleable acetabulum to improve its articulation and relation with the femoral head based on pathophysiology.

One randomized controlled trial that evaluated the use of serial BoNT to the hip adductor and hamstring muscles in combination with a hip abduction brace for management of early hip dysplasia in children with bilateral spastic CP found a small effect on the rate of progression of hip displacement, which may in turn lead to delay for some children in the timing of surgery [30]. An earlier study examined MP in 16 children with bilateral CP (32 hips) aged 9 to 43 months who received BoNT type A (BoNT-A) to the hip adductors. They found significant benefit to children younger than 24 months with a greater initial hip subluxation of > 30%, as 18 out of 32 hips appeared stable over the study period with a change of 10% or less in the MP [31, 32]. Another study compared BoNT-A injections and hip bracing to PT alone in 39 children with bilateral CP aged 1 to 4 years. At follow-up 3 years later, surgery was performed in 47% of children who did not receive BoNT-A and in only 27% of those who did [31, 33].

There is also a role for BoNT as an analgesic agent, with a recent review concluding Level A evidence for its use in various neuralgias [34]. Children with CP commonly experience musculoskeletal pain in the setting of hypertonia, which can provoke muscle spasm and lead to a vicious pain-spasm cycle. Injection of BoNT could interrupt this cycle. One randomized controlled trial demonstrated a decrease in opioid use and length of hospital stay in children that underwent adductor tenotomy and received pre-operative BoNT [35]. Multiple other studies have also demonstrated reduced pain levels in non-ambulatory children with spastic CP who received BoNT [36, 37].

6 Surgical management techniques

If conservative management techniques are not sufficient to control hypertonia, surgical procedures including intrathecal baclofen (ITB) pump placement, selective dorsal rhizotomy (SDR), and deep brain stimulation can be considered. There are also numerous orthopedic procedures used in the treatment of musculoskeletal deformities that result from hypertonia, including hip displacement, which are discussed in “Operative Treatment of the Young Cerebral Palsy Hip” by Hyman and Judd.

ITB can be useful for both generalized spasticity and dystonia and has been linked to improvements in function, ease of care, pain, and quality of life in patients with CP [38, 39]. One case series published by Krach et al. in 2004 reviewed hip MP in patients with CP one year after pump implantation. They found 66% of hips were improved or unchanged in absolute MP and 90.9% of hips were improved or unchanged in MP class [40].

SDR, a neurosurgical procedure designed to reduce lower extremity spasticity, can influence the balance of tone in the muscles of the trunk and hip and thus development of hip joint deformities [41]. Multiple studies have compared hip MP before and after SDR and found the majority of hips, ranging from 82-93%, remained either radiographically stable or improved post-operatively [41–43].

7 Conclusion

Although there is not conclusive evidence in support of many of the above interventions, longitudinal population-based studies in Sweden have shown that comprehensive multidisciplinary intervention can prevent hip dislocation [22, 44]. This early, aggressive, and comprehensive care led by a pediatric physiatrist is essential to mitigate progression to complete hip dislocation and preserve range of motion, prevent contracture, and promote maximum functional ability in all children with CP. The authors have no acknowledgments.