A 10 g monofilament detects large-fibre loss, and by the time it is abnormal, protective sensation has gone. It is a good test for identifying the foot that needs offloading and podiatry today. It is a poor test for identifying the neuropathy that is starting.
Painful diabetic neuropathy is predominantly small-fibre disease, and small fibres go first. A patient with burning feet, normal monofilament and normal vibration is not exaggerating and is not somatising — they have a pattern that the bedside large-fibre tests are not designed to pick up. Ask about the quality of the symptom, whether it is worse at night, and whether light bedding is uncomfortable.
Practically: use the monofilament to grade ulcer risk, and use the history to identify neuropathic pain. They answer different questions, and treating a normal monofilament as a normal nerve examination is how painful neuropathy goes unnamed for years.
- Test temperature and pinprick alongside the monofilament — a crude small-fibre screen takes seconds.
- Ask specifically about nocturnal burning and bedsheet intolerance.
- A normal monofilament with neuropathic symptoms warrants a named diagnosis and a treatment plan, not reassurance.
- Document the site and grade of any loss so the next reviewer can see whether it has progressed.
Why it matters
The commonest reason painful diabetic neuropathy goes untreated is that a normal screening test was read as a normal nerve examination.
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