Abstract
Dear Editor,
In today's age of smaller, faster, and more portable technology, it is tempting to apply our fascination with high-tech devices such as hand-held ultrasound to low-tech wilderness environments. While the potential exists for the role of ultrasonography in a resource-limited setting, we are concerned that the utility of ultrasound for acute mountain sickness (AMS) management was overstated in the recent review article published in this journal (Canepa and Harris, 2019). We disagree that available high-altitude ultrasound studies are “proof of concept,” as our research group has published the largest prospective studies to date investigating ocular, pulmonary, and vascular ultrasounds with AMS. These studies demonstrated dynamic changes in sonographic findings at high altitude compared to asymptomatic baselines, and concluded that ultrasound was inadequate as a diagnostic or prognostic tool for AMS, as it may lead to false diagnoses and could place inadvertent limitations on high-altitude travelers.
For example, 86 hikers all had optic-nerve sheath diameter increase on ascent to 3,788 m (12,500 ft) regardless of the diagnosis of AMS in 56% of study participants, limiting its diagnostic utility (Kanaan et al., 2015). Vascular ultrasound examined total body fluid status at 4,242 m (14,000 ft) on Denali in 105 participants, and found that 57% of mountaineers were dehydrated (Ladd et al., 2016), which was not predictive of the 34–43% rate of AMS, likely prohibiting useful insight. Furthermore, we recently examined subclinical pulmonary edema with AMS on ascent to 3,788 m (12,500 ft) in 103 participants, with 73% total incidence of AMS (Alsup et al., 2019). While both the total and change in B-line scores were found with AMS, with extremely large confidence intervals encroaching on 0, any definitive diagnostic or prognostic conclusions are highly questionable. Likewise, this clinically silent pulmonary edema has been found in the majority of high-altitude trekkers (Strapazzon et al., 2015), making an ultrasound-based false-positive diagnosis likely, and potential prognostic utility for eventual clinical disease unreliable.
We agree with the authors that ultrasound should be an adjunct to, and not replace, a thorough history and physical. And while ultrasound may be useful in disaster settings and other resource-limited environments, awareness of its inherent limitations is important, as ultrasound is generally more specific than sensitive. This makes ultrasound a poor screening tool, and it is more useful to “rule in” disease, like the well-studied Focused Assessment of Sonography in Trauma (FAST). With its 98% specificity and sensitivity of only 42% (Miller et al., 2003), a positive FAST for intra-abdominal fluid in a concerning traumatic situation could expedite evacuation to definitive medical care, but the low sensitivity should not “rule out” traumatic injuries when clinically concerned and so could provide unwarranted reassurance. Likewise, at high altitude, the low sensitivity of ultrasound does not screen out high-altitude illness. Yet, the preponderance of many sonographic findings makes its ability to “rule in” disease inadequate to provide meaningful conclusions. Further studies could prove helpful in elucidating high-altitude illness pathophysiology, especially with more severe disease, but we recommend restraint in adopting ultrasound as a diagnostic or screening tool for high-altitude illness.
Footnotes
Author Disclosure Statement
No competing financial interests exist.