1. Neurogenic inflammation, mediated by nociceptor C fibres, is part of the acute neurovascular response to injury producing the axon reflex flare. Laser Doppler flowmetry was used to measure the flare response induced by the electrophoresis, at various current strengths, of a ring of acetylcholine solution into dorsal foot skin.
2. Nineteen control subjects and 52 long-duration insulin-dependent (Type 1) diabetic patients of similar age (20 without complications; 19 with laser-treated retinopathy; 13 with reduced vibration perception and retinopathy) were studied in order to investigate the possible attenuation of this defence mechanism in diabetes.
3. The maximal (1 mA) flare response [control median (interquartile range): 1.55 (1.16–2.06) arbitrary units] was reduced greatly in neuropathic patients [0.37 (0.24–0.66) arbitrary units; P ≤ 0.001 with respect to all other groups], especially those with a previous history of foot ulceration. The flare was also reduced in some patients with retinopathy alone [1.06 (0.56–1.27) arbitrary units; P < 0.005 with respect to control subjects].
4. No rightward shift of the curve of hyperaemic response plotted against current strength was found, suggesting that the abnormal response was due to axonal loss rather than to dysfunction.
5. Neurogenic inflammation, mediated by small pain fibres, was markedly impaired in a group of diabetic patients at risk of foot ulceration. Furthermore, impairment of this nociceptor C fibre response can develop before clinical large-fibre neuropathy and could itself predispose to foot complications.