Early FAP Diagnosis Possible with Specific Sensory, Heart Rate and Conductance Tests, Study Says

Measuring thresholds of cold detection, heart rate variability, and the skin’s electrical conductance may allow for earlier diagnosis of familial amyloid polyneuropathy (FAP), according to new research.

The study, “The diagnostic utility of neurophysiologic tests for early diagnostic of transthyretin familial amyloid polyneuropathy,” appeared in Amyloid: the Journal of Protein Folding Disorders.

Diagnosing FAP in its early stages is often difficult due to the variability of its subtle and non-specific symptoms. A team at the Neurophysiology Service of the Centro Hospitalar do Porto, Portugal, assessed whether sensory and nerve conduction tests, along with analysis of heart rate changes, may address this gap.

A total of 110 FAP patients — age range, 21 to 77 — were divided into three groups: a first with 45 asymptomatic patients (28 women, mean age 35), a second with 37 people with doubtful neurological symptoms (27 women, mean age 44), and the third with 28 symptomatic patients in stage 1 and with less than 3 years of disease duration (15 women, mean age 40).

All underwent foot sensory testing to determine cold detection thresholds (CDT), assessments of heart rate variability to deep breathing (HRdb), and foot sympathetic skin responses (SSR), which measure changes in the skin’s electrical potential upon stimulation. Analysis was also taken of action potentials (electrical impulses) from the sural nerve in the calf (SNAP) and of the skin’s electrical conductance in the feet, measured with a device called Sudoscan (by Impeto Medical).

Results found that only SSR did not significantly differ across the three groups.

As for the other measures, CDT was highest in group 3 and lowest in group 1, with HRdb and skin conductance showing the opposite. These findings demonstrate that these three assessments “have similar diagnostic performance in detecting early dysfunction in [FAP],” the researchers wrote.

Measures of electrical impulses from the sural nerve (SNAP) showed very low sensibility. This was an expected result, the team wrote, as this test assesses large myelinated fibers that typically are not affected in the early stages of FAP. (Myelin is the protective layer of nerve fibers.)

The researchers added that future work will analyze “if and which of these tests combinations is the best approach for the early diagnosis.”