Aim: Review of the results of electrodiagnostic investigations in 3 patients with facial-onset sensory motor neuropathy (FOSMN) referred to the Department of Clinical Neurophysiology at the Royal Victoria Infirmary, Newcastle-upon-Tyne.
Background: FOSMN is a very rare progressive neurodegenerative disease firstly described in 2006 (Vucic et al, 2006). Its clinical course resembles that of bulbar-onset motor neuron disease (MND) but with a prodromal phase (often lasting years) characterised by both negative (i.e. loss of facial sensation) and positive (e.g. ‘burning mouth’ syndrome) sensory symptoms. Individual case reports describing post-mortem histology have shown that TDP43 aggregation and mislocalisation is not only identified in bulbar and spinal motor neurons but also in the trigeminal nucleus of patients with FOSMN (Ziso et al, 2015), suggesting that FOSMN might be a phenotypic variant of MND and not a distinct nosological disorder, as initially proposed.
Methods: Three male patients (aged between 56 and 65 years) with a clinical diagnosis of FOSMN (TLW) were referred for electrodiagnostic investigations, including nerve conduction studies, concentric needle electromyography (EMG), blink reflexes, somatosensory evoked potentials(SEPs) and motor evoked potentials (MEPs). In two cases, post-mortem examination was performed in brain and spinal cord tissue, specifically assessing TDP43 histopathology.
Results: In Patient-1, upper limb sensory nerve action potentials (SNAPs) were within acceptable limits but EMG showed evidence of widespread denervation. MEPs and blink reflexes were normal. SEPs were not performed. Patient-2 had reduced upper limb SNAPs, delayed lower limbs and right upper limb SEPs and evidence of denervation on needle EMG in bulbar, cervical and thoracic regions bilaterally. Additionally abnormal blink reflexes and ipsilateral MEPs (iMEPs, Krampfl et al, 2004) were identified in the right upper limb. Patient-3 had normal upper and lower limb SNAPs and needle EMG only demonstrated stable chronic neurogenic change in facial muscles bilaterally and right upper limb muscles. Blink reflexes were abnormal. SEPs and MEPs were normal and iMEPs were not evident.
Conclusions: Neurophysiological examination, specifically the identification of iMEPs in Patient-2, suggests early upper motor neurone dysfunction in FOSMN, which has not been previously reported. This observation further supports the thesis that FOSMN is a phenotypical variant of MND in concordance with clinical and histopathological data also presented for 2 patients in this series.