The LDIflare: a novel test of C-fiber function demonstrates early neuropathy in type 2 diabetes.
OBJECTIVE
The aim of this study was to evaluate a novel method for assessing the axon reflex and to determine its value in detecting neuropathy in type 2 diabetes.


METHODS
The neurogenic flare response to nociceptive stimuli is mediated by an axon reflex involving small unmyelinated C-fibers. We developed a method to assess this reflex involving skin heating to 44 degrees C to evoke the flare followed by scanning the site using a laser Doppler imager (LDI) to measure the area; we termed this method LDIflare. To confirm its neurogenic nature, we examined the LDIflare in eight healthy subjects before and after topical administration of anesthesia. We used this technique to detect C-fiber neuropathy in people with type 2 diabetes. A total of 36 subjects were studied: 12 subjects with neuropathy (group DN), 12 subjects without neuropathy (group DC), and 12 age- and sex-matched control subjects (group NC). For comparison, small-fiber function was also assessed using the Computer Aided Sensory Evaluator-IV (CASE IV) (WR Medical Electronics, Stillwater, MN).


RESULTS
In the eight healthy control subjects, LDIflare was markedly reduced after topical administration of anesthesia (1.62 [1.45-1.72] vs. 5.2 cm2 [3.9-5.9], P <0.0001), confirming its neurogenic nature. Similarly, in neuropathic subjects, LDIflare was significantly smaller compared with normal and diabetic control subjects (LDIflare area: DN 1.3 cm2 [0.9-1.8], NC 5.5 cm2 [3.9-5.8], and DC 2.8 cm2 [2.5-3.8]; P <0.0001 and P=0.01, respectively). The group without neuropathy (DC) also demonstrated a reduced flare compared with the NC group (P=0.01). In contrast, C-fiber function assessed by evaluating the quantitative thermal thresholds (CASE IV) did not detect a difference between the latter two groups.


CONCLUSIONS
This study confirms the neurogenic nature of the LDIflare and clearly demonstrates loss of C-fiber function in neuropathic subjects with type 2 diabetes. Moreover, it demonstrates C-fiber dysfunction before its detection by other currently available methods, including CASE IV. The LDIflare seems to be a simple objective method to detect early neuropathy and may be of value in assessing therapeutic interventions aimed at preventing or reversing C-fiber dysfunction.
Krishnan
Singhan T M
ST
FRCP, The Ipswich Diabetes Centre, Ipswich Hospital NHS Trust, Heath Road, Ipswich, UK.
Rayman
Gerry
G
D015992:Body Mass Index; D001917:Brachial Plexus / Q000503:physiopathology; D003924:Diabetes Mellitus, Type 2 / Q000503:physiopathology*; D003929:Diabetic Neuropathies / Q000175:diagnosis* / Q000503:physiopathology*; D003943:Diagnostic Techniques, Neurological; D005260:Female; D006442:Glycated Hemoglobin / Q000032:analysis; D006801:Humans; D008297:Male; D008875:Middle Aged; D009412:Nerve Fibers / Q000502:physiology*; D011312:Pressure; D012016:Reference Values; D015203:Reproducibility of Results
D006442:Glycated Hemoglobin