Intraepidermal Nerve Fiber Density in Friedreich's Ataxia

Friedreich's Ataxia (FRDA) is caused by a homozygous intronic GAA expansion in the FXN gene. FRDA affects primarily the peripheral nervous system (PNS) with cumulative evidence from postmortem studies and in vitro models suggesting a developmental component of its pathology. In the present study, we aimed at gaining further insight in the PNS involvement in FRDA by investigating small nerve fibers in vivo. For this purpose, we evaluated the intraepidermal nerve fiber (IENF) density in skin-biopsies of the lower leg and applied clinical assessments of small fiber function (painDETECT, quantitative sensory testing) in 17 FRDAs. Mean IENF density was significantly lower in FRDAs compared to controls (5.77 ± 4.68 vs 9.33 ± 1.41, p = 0.013). Clinically, cold detection threshold was decreased in FRDAs (FRDA = -3.47(-6.64; -3.14), controls = -1.71 (-3.43; -1.23), p = 0.001) while other measures of small fiber function such as warm and pain sensation thresholds did not differ from controls. Five patients had sensory complaints, but none was diagnosed with neuropathic pain at painDETECT. The degree of small fiber loss was markedly variable in our cohort and showed an inverse correlation with the GAA repeat length (R2 = 0.573, p = 0.001). Our findings support a genetically determined small fiber loss in FRDA.