Peripheral neuropathies: Viva

Branch 1: Classifying the neuropathies of childhood

A strong candidate sorts peripheral neuropathies along two axes: hereditary versus acquired, and demyelinating versus axonal. The hereditary family is dominated by Charcot-Marie-Tooth disease, the commonest inherited neuromuscular disorder with a prevalence of around one in two thousand five hundred, and CMT1A from a PMP22 duplication accounts for around half of all cases. The acquired family is dominated by treatable immune neuropathies, the most important being chronic inflammatory demyelinating polyradiculoneuropathy. The second axis is read off the nerve conduction study: demyelinating disease slows the velocity, and axonal disease reduces the amplitude. [1]

Branch 2: Interpreting the nerve conduction studies

When the examiner asks how the candidate interprets the studies, the answer turns on the median motor conduction velocity. The threshold of thirty-eight metres per second separates the demyelinating CMT1 from the axonal CMT2. Uniform slowing to under thirty-eight across all nerves, proportional to nerve length and without conduction block, is the signature of CMT1 and points to a PMP22 duplication test. A velocity above thirty-eight with reduced amplitudes points to CMT2 and an MFN2 test. The candidate stresses the importance of uniform versus patchy slowing: non-uniform slowing with conduction block is acquired, most often CIDP, and it changes the management entirely because CIDP responds to immunotherapy while CMT does not. [1][8]

Branch 3: Distinguishing hereditary from acquired disease

The examiner then offers a child who has progressed over three months rather than years, with proximal and distal weakness and patchy non-uniform conduction block on nerve conduction studies. The strong candidate recognises chronic inflammatory demyelinating polyradiculoneuropathy, defined by progression over more than eight weeks, which separates it from the monophasic Guillain-Barre syndrome that peaks within four weeks. The candidate explains that CIDP responds to intravenous immunoglobulin two grams per kilogram, to corticosteroids, and to plasma exchange, and that the response itself is diagnostic, because the hereditary neuropathies do not respond to immunotherapy. The Fernandez-Garcia study showed that a significant fraction of children referred for genetic neuropathy testing have an acquired demyelinating process, and the guard against mislabelling is the nerve conduction study pattern. [8][9]

Branch 4: The management ladder for Charcot-Marie-Tooth disease

When asked how the candidate runs the ladder, the answer is supportive, multidisciplinary, and lifelong, guided by the Yiu 2022 clinical practice guideline for paediatric Charcot-Marie-Tooth disease. No disease-modifying therapy has proven effective for CMT1A, and the ascorbic acid trials showed no benefit. The management is built around ankle-foot orthoses to correct foot drop, physiotherapy for stretching and strengthening, podiatry for foot care, orthopaedic surgery for fixed deformity in the second decade, neuropathic pain management with gabapentin or pregabalin, and genetic counselling. The candidate stresses that the goal is to preserve mobility, prevent contractures, control pain, and support participation, and that the prognosis is slow progression over decades with preservation of ambulation and a normal life expectancy. [6]

Branch 5: The vincristine precaution

If the examiner changes the picture to a child about to receive vincristine for leukaemia who has pes cavus and a family history of neuropathy, the candidate recognises the vincristine precaution. Vincristine can unmask or severely worsen Charcot-Marie-Tooth disease and cause irreversible quadriparesis and occasionally fatal neuropathy, because the drug impairs the microtubule-dependent axonal transport that an already-compromised nerve cannot sustain. The candidate states that any child about to receive vincristine who has foot deformity, a family history of neuropathy, or unexplained weakness must be screened before the first dose, with nerve conduction studies if there is any doubt, and that an alternative agent or dose modification is used if CMT is confirmed. [11]

Branch 6: Hereditary neuropathy with liability to pressure palsies

If the examiner asks about recurrent focal palsies, the candidate identifies hereditary neuropathy with liability to pressure palsies, caused by a deletion of the PMP22 gene, the mirror image of the CMT1A duplication characterised by Chance in 1993. The disease presents with recurrent painless focal mononeuropathies at common compression sites, such as a peroneal palsy at the fibular head, an ulnar palsy at the elbow, or a radial palsy at the spiral groove, and the episodes resolve over days to weeks. The nerve conduction studies show focal slowing at compression sites with mild background demyelination. The management is compression avoidance and protective padding, and the anaesthetic precaution is careful positioning because the nerves are exquisitely sensitive to compression. [3]

The examiner rewards a candidate who sorts the neuropathies on the two axes, who quotes the thirty-eight metres per second threshold, who distinguishes uniform hereditary slowing from patchy acquired slowing, who runs the supportive management ladder with the Yiu guideline, and who names the vincristine precaution and the CIDP-versus-CMT fork. [1]