Peripheral Neuropathy — Written Clinical Reasoning

Part A — Diagnostic approach and classification

The first discipline is to classify the neuropathy before generating a differential. This patient has two superimposed patterns — a chronic distal symmetric polyneuropathy (the 3-year burning numbness, stocking-distribution sensory loss, absent ankle reflexes, and painless foot ulcer) and a subacute mononeuritis multiplex (the 6-week history of painful, asymmetric, stepwise involvement of the right peroneal nerve and the right ulnar nerve, with weight loss and night sweats). ^[1]

The chronic distal symmetric pattern is, by far, most likely diabetic distal symmetric polyneuropathy (DSPN) in a man with a 15-year history of type 2 diabetes and an HbA1c of 78 mmol per mol — the poor glycaemic control, the length-dependent sensory loss, the absent ankle reflexes, and the painless foot ulcer (the classic triad of neuropathy, ischaemia, and deformity) are all consistent ^[1].

The superimposed subacute mononeuritis multiplex is the concerning new development. Mononeuritis multiplex — painful, asymmetric, stepwise involvement of multiple individual named nerves — is the signature of vasculitic neuropathy until proven otherwise ^[4], and the systemic symptoms of weight loss and night sweats raise the possibility of an underlying systemic vasculitis or a paraproteinaemic process. The small IgM kappa paraprotein adds a further layer — it may be an incidental MGUS, or it may be relevant (anti-MAG neuropathy is typically a chronic distal demyelinating sensory neuropathy, not a mononeuritis multiplex, but paraproteinaemic disease and vasculitis can coexist, and cryoglobulinaemic vasculitis is classically associated with a paraprotein and hepatitis C).

Key point: Never assume that all neuropathy in a diabetic patient is due to diabetes. Up to 10 to 50 per cent of neuropathy in diabetics has an alternative or additional cause. The new, subacute, asymmetric, painful, stepwise pattern in this patient demands investigation for vasculitis and paraproteinaemic disease. ^[1]