Amyotrophic lateral sclerosis causes small fiber pathology.

BACKGROUND AND PURPOSE: Our aim was to address the correlation between small fiber loss and amyotrophic lateral sclerosis (ALS) for disease onset, phenotype, genotype, duration, severity and sensory findings.

METHODS: Consecutive patients referred for suspected ALS were screened. Exclusion criteria were possible ALS and previous diagnosis or known risk factors for small fiber neuropathies. A sural nerve conduction study (NCS) was bilaterally recorded. The ALS functional rating scale revised was administered and loss of functions were calculated using the Milano-Torino staging (MITOS) system. Sensory symptoms and signs were recorded. Genetic analysis was performed by the next-generation sequencing approach. Skin biopsy was performed at the distal leg and intraepidermal nerve fiber (IENF) density was quantified in three non-consecutive sections following published guidelines. Findings were referred to age- and sex-adjusted normative values.

RESULTS: Fifty-seven patients including six with facial onset sensory and motor neuronopathy (FOSMN) were enrolled. Eight (15.7%) pure ALS patients and five (83%) FOSMN patients complained of sensory disturbances with different distributions. Sural NCS was normal in all except two patients. IENF density was reduced in 75.4% of pure ALS and 50% of FOSMN patients, without correlation with any disease features. IENF density was similarly reduced in bulbar (78.5%), flail limb (87.5%), pyramidal (100%), and spinal (68.2%) onset, as well as in genetic (83.3%) and sporadic (82%) ALS. There was no correlation with genotype, disease duration and severity.

CONCLUSIONS: Intraepidermal nerve fiber loss is a feature of most ALS patients. It does not correlate with onset, phenotype, course and severity of the disease, and cannot be considered a clinical or prognostic biomarker.