Ultrasound Measurements of Nerve Size Could Help Diagnose FAP, Study Says

Using ultrasound to measure the size of the median nerve — which runs down the upper arm and continues into the forearm and hand — could be useful for the early diagnosis of familial amyloid polyneuropathy (FAP), a new study indicates.

The study, “Nerve ultrasound in hereditary transthyretin amyloidosis: red flags and possible progression biomarkers,” was published in the Journal of Neurology.

Also called hereditary transthyretin amyloidosis (hATTR) with polyneuropathy, FAP is caused by mutations in the TTR gene, which provides instructions for producing a protein called transthyretin. This protein is mainly produced in the liver and transports vitamin A and a hormone called thyroxine throughout the body.

With the advent of new therapies that can slow down disease progression, early diagnosis has become of great importance for providing the best possible care to people with FAP.

A common symptom of hATTR is carpal tunnel syndrome (CTS) — a sensation of numbness and tingling in the hand and arm, as a result of pressure being put on the median nerve. CTS has been reported to occur years before other symptoms, making it potentially useful for identifying people who might have hATTR. However, carpal tunnel syndrome is fairly common in the overall population (referred to as idiopathic CTS), which complicates its use as a tool for specifically diagnosing hATTR and FAP.

In the new study, researchers in Italy tested whether measuring nerve size via ultrasound — which has helped establish the severity of CTS generally — would be suited for differentiating idiopathic CTS from hATTR-associated CTS.

A total of 62 people, comprised of 34 men and 28 women, with a mean age of 59.8 years, were evaluated. All had confirmed hATTR-causing mutations.

The researchers analyzed 21 CTS-affected hands from 16 participants without polyneuropathy. Specifically, they used ultrasound to measure the cross-sectional area (CSA) of the median nerve at the wrist. This group was compared with 1,669 hands from 1,196 people with idiopathic CTS.

While median nerve CSA correlated with CTS severity in the idiopathic group, no such link was found in pre-symptomatic hATTR carriers with CTS.

Importantly, individuals with FAP had significantly higher mean CSA than hATTR carriers (9.7 vs. 8.8 mm² at the wrist). Comparisons at different sites of the median nerve and other nerves of the arms and legs showed the same general trend.

Overall, these findings indicate that measuring the median nerve CSA could help differentiate between idiopathic carpal tunnel syndrome and CTS associated with FAP. Also, they suggest that CSA may have prognostic value in FAP.

“In conclusion, although the present study missed to identify diagnostic cut-off values, the results may be of high clinical impact,” the researchers wrote. “If these [ultrasound] findings may become a marker of disease progression and/or the response to therapy should be a matter of further studies.”