Axon reflex-mediated vasodilation is reduced in proportion to disease severity in TTR-FAP

To evaluate the area of the vascular flare in familial amyloid polyneuropathy (FAP).

Healthy controls and patients with genetically confirmed FAP were prospectively examined, on the upper and lower limbs, for thermal sensitivity (Medoc TSA-II thermal analyzer) and for axon reflex-mediated flare. The latter was induced by iontophoresis of histamine on the forearm and leg on 2 different visits. We used laser Doppler imaging (LDI) to measure the flare area (LDIflare).

Six patients had FAP of variable severity; 1 had generalized analgesia secondary to leprosy (used as a positive control). The median Neurological Impairment Score-Lower Limbs (NIS-LLs) was 6 (0-27). The warmth detection thresholds in the feet were higher in patients (median 43°, interquartile range 39.0°-47.6°) compared with controls (37.4°, 35.3°-39.2°), indicating small fiber impairment. On the leg, LDIflare was smaller in the patients on 2 consecutive visits (controls: median 13.0 and 13.3 cm², IQR 9.7-22.8 and 8.3-16.9; patients 6.9 and 8.0 cm², 2.6-10.8 and 6.4-12.1; p = 0.011). LDIflare on the leg was correlated with NIS-LL (Spearman rank correlation 0.73, p = 0.09 on the first visit; Spearman rank correlation 0.85, p = 0.03 on the second visit).

Our study underscores that histamine-induced axon reflex-mediated vascular flare on the leg is reduced in proportion to disease severity in patients with FAP.