Discussion

The diagnostic clue to an intraspinal lesion in this case was the pattern of weakness: the patient could heel-walk but not toe-walk, opposite to the usual presentation in CMT, where dorsiflexion is weaker than plantar flexion. Bourke and Dyck ^{Reference Bourque and Dyck1} first reported this pearl in 1990, describing four patients with distal weakness and hyporeflexia, presumed to have peripheral neuropathy, who were later found to have intraspinal lesions. Their unifying clinical feature was an ability to walk on their heels, but not on their toes (plantar flexion > dorsiflexion weakness). Further evaluation of 86 patients with HMSN-1 showed that without exception, ankle plantar flexion weakness never exceeded dorsiflexion weakness. Whilst any muscle compartments in the lower limb can be affected in CMT, it commonly first involves the intrinsic foot musculature and peroneal-innervated muscles, leading to anterolateral compartment weakness and pes cavus deformity. ^{Reference Stilwell, Kilcoyne and Sherman2} A more recent description of CMT due to SORD mutations noted that 41% of patients had prominent involvement of foot plantar flexion (equal or weaker than foot dorsiflexion). ^{Reference Cortese, Dohrn and Curro3}

Whilst pes cavus strongly suggests CMT – one study demonstrated a 78% probability of CMT in patients with bilateral cavovarus feet ^{Reference Qin, Wu and Zhang4} – pes cavus can have causes beyond CMT. These include spinal cord lesions, acquired peripheral nerve injuries or other inherited neurological diseases such as spinocerebellar ataxia. ^{Reference Piazza, Ricci and Caldarazzo Ienco5} Careful clinical examination is necessary to decide if the deformity is due to a neuropathy or another neurological condition. In intraspinal lesions, tibialis anterior and foot intrinsics are usually preserved compared to tibialis posterior and gastrocnemius-soleus, and cavus deformities are commonly unilateral, although bilateral cases have been reported. ^{Reference Bourque and Dyck1,Reference Qin, Wu and Zhang4,Reference Visser, Wolfe, Kouri and Aviles6} Recognizing this subtle difference in weakness should prompt spinal imaging.

SME is a slow-growing World Health Organization Grade I neoplasm, typically found in the lumbosacral spine, often affecting the conus medullaris and cauda equina. Symptoms vary depending on tumour location and size, with non-specific back pain being the most common initial symptom, then motor deficits, sensory and gait disturbances and possibly sphincter dysfunction. ^{Reference Pesce, Palmieri, Armocida, Frati, Miscusi and Raco7} Due to slow progression, SMEs often present with symptoms of several years’ duration. In adults, pain is most common (60–70%), with motor and gait issues more prominent than sensory involvement. Unilateral limb weakness is more typical. ^{Reference McCormick, Torres, Post and Stein8,Reference Shedid and Benzel9} This patient’s bilateral pes cavus and distal lower limb weakness, absent ankle reflexes and no back pain or sensory complaints made the diagnosis more challenging. The bladder neurostimulator was placed for mixed stress and urge incontinence, confirmed by urodynamics, before any motor symptoms appeared and without spinal imaging, and so was not an immediate clue to the final diagnosis.

The reported NEFL variant further complicated the presumptive diagnosis of a hereditary peripheral neuropathy. NEFL variants can cause both autosomal recessive (AR) and dominant CMT. The patient had one copy of a variant that has only been associated with the AR form of disease. Due to variable penetrance and expressivity, the presence of a genetic variant does not automatically imply causation. Pathogenic variants are often found incidentally in asymptomatic individuals, raising questions about their relevance and penetrance. ^{Reference Cassa, Tong and Jordan10} Genetics consultation was obtained, and they felt that this variant was unrelated to her clinical presentation.