Peripheral Neuropathy — Viva Defence

Long case viva — diabetic neuropathy with superimposed vasculitis

Candidate's opening statement (SASPOP)

"Doctor, my patient is a 62-year-old man with a 15-year history of type 2 diabetes. His problems are: a chronic distal symmetric polyneuropathy from poorly controlled diabetes (3 years of burning foot numbness, stocking sensory loss, absent ankle reflexes, and a painless foot ulcer); a superimposed subacute mononeuritis multiplex affecting the right peroneal and ulnar nerves over 6 weeks — painful, asymmetric, and stepwise — which is the signature of vasculitic neuropathy and is a neurological emergency; systemic symptoms of weight loss and night sweats, suggesting an underlying systemic vasculitis or paraproteinaemic process; a small IgM kappa paraprotein that requires characterisation, including exclusion of cryoglobulinaemic vasculitis and hepatitis C; poor glycaemic control with an HbA1c of 78 mmol per mol; and a 20 pack-year smoking history contributing to his peripheral arterial disease and foot ulcer risk. My priorities are to confirm the vasculitic neuropathy with a sural nerve biopsy, initiate urgent immunosuppression to prevent further irreversible nerve infarction, manage the foot ulcer aggressively to prevent amputation, optimise his glycaemic and cardiovascular risk factor control, and characterise the paraprotein." ^[1]

Problem list

1. Chronic diabetic distal symmetric polyneuropathy with a painless foot ulcer (the triad of neuropathy, ischaemia, and deformity).
2. Subacute vasculitic mononeuritis multiplex (right peroneal and ulnar) — a neurological emergency.
3. Small IgM kappa paraprotein — possible cryoglobulinaemic vasculitis, anti-MAG, or incidental MGUS.
4. Systemic symptoms (weight loss, night sweats) — possible systemic vasculitis or malignancy.
5. Poor glycaemic control (HbA1c 78 mmol per mol).
6. Peripheral arterial disease and smoking — foot amputation risk. ^[1]

Integrated management plan

Confirm the diagnosis. The key investigation is a sural nerve biopsy to confirm vasculitic neuropathy, combined with a vasculitic and paraproteinaemic blood panel (ANA, ANCA, rheumatoid factor, cryoglobulins, complements, hepatitis B and C, HIV, anti-MAG antibody), and nerve conduction studies to document the background axonal DSPN and the superimposed axonal mononeuropathies. ^[1]

Treat the vasculitic neuropathy urgently. Once the biopsy is obtained, commence intravenous methylprednisolone 1 g daily for 3 to 5 days followed by oral prednisolone 1 mg per kg per day, with intravenous cyclophosphamide pulses for severe disease ^[4]. Add pneumocystis prophylaxis, gastric and bone protection, and monitor for cyclophosphamide toxicity (haemorrhagic cystitis, cytopenias, infection). Maintenance with azathioprine or methotrexate follows induction.

Manage the foot ulcer. Urgent podiatric and vascular assessment: debridement, offloading with a total contact cast, ankle-brachial index and vascular imaging for peripheral arterial disease, wound culture, and antibiotics for infection. The ulcer precedes 85 per cent of diabetes-related amputations and is preventable with early aggressive management ^[1].

Optimise glycaemic and cardiovascular risk. Intensify diabetes therapy, add a statin and antihypertensive, and counsel on smoking cessation. Glycaemic control is the only disease-modifying therapy for the DSPN, with the strongest effect in type 1 diabetes and a more modest effect in type 2. ^[1]

Control neuropathic pain. First-line pregabalin or duloxetine for the burning pain; short-term opioid analgesia for the acute vasculitic pain, which will improve with immunosuppression. ^[1]

Characterise the paraprotein. Anti-MAG antibody, cryoglobulins (sent and transported warm), hepatitis C serology, serum free light chains, and a bone marrow biopsy if the paraprotein is significant or haematology parameters are abnormal. ^[1]

Examiner probing questions

Examiner: "Why do you think this is two separate neuropathic processes rather than one?" ^[1]

Because the patterns are distinct and the time courses are different. The chronic (3-year), symmetric, length-dependent, painless neuropathy with absent ankle reflexes and a foot ulcer is the classic pattern of diabetic DSPN. The subacute (6-week), painful, asymmetric, stepwise involvement of two individual named nerves (peroneal and ulnar) is the classic pattern of mononeuritis multiplex, which is vasculitic until proven otherwise. The systemic symptoms and the paraprotein further support an independent vasculitic or paraproteinaemic process. A single process would not typically produce both patterns simultaneously. ^[1]

Examiner: "What would the nerve conduction studies show?" ^[1]

A background distal symmetric axonal sensorimotor neuropathy — reduced sural and peroneal amplitudes with relatively preserved conduction velocity — consistent with diabetic DSPN. Superimposed axonal mononeuropathies of the right peroneal and ulnar nerves — reduced amplitudes in those nerves with evidence of active denervation (fibrillation potentials) and chronic reinnervation (large polyphasic motor units) on needle EMG. The axonal pattern in the mononeuropathies, combined with the asymmetric stepwise evolution, is consistent with vasculitic nerve infarction rather than demyelination or entrapment ^[2].

Examiner: "How do you justify the nerve biopsy?" ^[1]

The sural nerve biopsy carries a risk of permanent sensory loss in the biopsied distribution and wound complications, so it is reserved for when the result will change management ^[2]. In this patient, the biopsy is justified because the suspected vasculitic neuropathy requires confirmation before committing to potentially toxic immunosuppression, and less invasive tests (bloods, NCS) may not be definitive. A combined nerve and muscle biopsy (sural and peroneus brevis) increases the yield for vasculitis.