Sensory dysfunction in SMA type 2 and 3 - adaptive mechanism or concomitant target of damage?

The motor neuron survival protein performs numerous cellular functions; hence, SMA is considered to be a multi-organ disease with possible concomitant sensory system damage. Standard elec-trophysiological studies of the sensory nerves and assessments of the sensory thresholds for different modalities in adults with SMA types 2 and 3 were conducted. The study group consisted of 44 adult SMA patients. All patients underwent neurological examination using the Hammersmith Functional Motor Scale – Expanded (HFMSE). Standard sensory electrophysiological studies in the ulnar nerve and the estimation of vibratory, temperature, and warm-and cold-induced pain thresholds were performed using quantitative sensory testing (QST) with the assessment of high and low temperature dispersion. The most reproducible result was the high amplitude of the sensory nerve action potentials (SNAP) in SMA patients compared to controls. This was higher in type 2 patients compared to type 3a and 3b patients and patients with low HFSME scores. Patients with SMA, especially type 3b presented a longer sensory latency and slower conduction velocity than did controls. Cold pain threshold was higher and warm dispersion larger in SMA. The vibra-tory limit was higher in patients with high HFSME scores. A high SNAP amplitude suggests sensory fibre hyperactivity, which may be based on overactivation of metabolic pathways as an adap-tive mechanism in response to SMN protein deficiency with additionally coexisting small C-and A-delta fibre damage. SMA patients seem to have a concomitant, mild demyelinating process present at the early SMA stage.