WASHINGTON — Analysis of skin punch biopsy specimens has the potential to revolutionize the diagnosis of diabetic neuropathy.
Using this technique, neurologists may be able to predict its occurrence in at risk patients, diagnose diabetic neuropathy earlier in its course, and assess treatment response, Dr. Michael Polydefkis said at the annual scientific sessions of the American Diabetes Association.
Specimens obtained by skin punch biopsy contain small unmyelinated nerve fibers that are damaged early in the course of diabetes. Because the degree of involvement is related to the degree of glucose dysmetabolism, the fibers provide a direct measure of axonal loss. Normative data for them has been established already, said Dr. Polydefkis, codirector of the cutaneous nerve laboratory at Johns Hopkins University, Baltimore.
Skin biopsy provides a clinical marker for early disease that also can help exclude other potential causes of painful feet such as radiculopathy, Morton's neuroma, tarsal tunnel syndrome, and intrinsic foot disease, he said. “It gives you a good insight into nerve morphology, which is likely to be a predictive feature and potentially can be used to follow-up patients with more advanced neuropathy,” he said.
A 3-mm diameter, circular skin punch biopsy specimen (about half the size of a pencil eraser) is sliced into about 60 sections, 4 of which are randomly selected for analysis to reduce sample bias since there may be differences in nerve density in different parts of the specimen. There also is a normal gradient of nerve density in the skin that decreases from proximal to more distal sites, such that there are about 30% more nerve fibers in proximal sites near the hip than there are in distal sites near the ankle, Dr. Polydefkis said.
Skin punch biopsies should be taken at the ankle or other distal sites in patients with early neuropathic symptoms but at the thigh or other proximal sites in those with advanced neuropathy, he advised.
In a study conducted by Dr. Polydefkis and his associates on 73 patients with an unknown cause of peripheral neuropathy, epidermal nerve fiber density (ENFD) in skin punch biopsy specimens was a good marker of early neuropathy. Of those 73 patients, 25 were diagnosed with impaired glucose tolerance (IGT) and 16 with diabetes, based on the results of an oral glucose tolerance test.
Nerve conduction tests of amplitude and velocity tested in the normal range on average in both groups. But the ENFD in biopsy specimens of patients with diabetes or IGT was significantly reduced compared with healthy control patients. In specimens from the distal leg but not the distal or proximal thigh, diabetic patients had significantly lower ENFD than did patients with IGT. Based on these results, Dr. Polydefkis and his coinvestigators concluded that the measurement of ENFD in skin punch biopsies is a more sensitive marker for detecting neuropathy early than are nerve conduction studies that test the function of large myelinated nerve fibers (Neurology 2003;60:108–11).
He and his colleagues continued to follow 39 of these patients, including healthy control patients and those with idiopathic neuropathy or diabetes- or IGT-associated neuropathy, for an average of 4.4 years. At the end of the follow-up, repeat testing showed that patients with diabetes had the greatest decline in ENFD, followed by patients with IGT, those with idiopathic neuropathy, and healthy control patients (essentially stable). Such longitudinal data support the idea that IGT causes neuropathy, Dr. Polydefkis said.
Information on nerve morphology also can be extracted from skin punch biopsy specimens. In one study, investigators performed nerve conduction studies, quantitative sensory testing, and skin punch biopsies in the proximal thigh and distal leg at baseline and after a mean of 19 months of follow-up in 15 patients with foot pain but few or no symptoms of neuropathy (6 with diabetes, 1 with AIDS, 1 with paclitaxel toxicity, 7 with an idiopathic nature) and 15 age-matched, healthy control patients. At baseline, patients had significantly lower ENFD in the distal leg than did healthy control patients. At follow-up, the ENFD had declined in both biopsy locations but only by a significant amount in the distal leg, compared with the health control patients. The ratio of epidermal nerve fibers to the number of axonal swellings in the distal leg declined but remained significantly higher than in control patients (Neurology 2003;61:631–6).
Clinically meaningful changes in ENFD are on the order of a loss of 2–3 nerve fibers per millimeter. Diabetic patients who present with painful peripheral neuropathy appear to have ENFD losses in the range of 1 fiber/mm per year while losses for those with established diabetes and peripheral neuropathy may be slightly higher. Meaningful changes in nerve conduction velocity have been determined to be a loss of about 1.2 meters per second in a 2-year period. In a clinical trial assessing an intervention's effect on nerve regeneration, the sample size needed to detect these clinically meaningful changes would be comparable if skin punch biopsy (n=80) or nerve conduction velocity (n=70) were used, according to Dr. Polydefkis.
Skin punch biopsies also may help to understand how nerve regeneration occurs after an injury in diabetic patients, Dr. Polydefkis said. In a study of patients who applied capsaicin topically to their distal thighs—causing denervation of the epidermis—he and his colleagues found that the rate of regeneration depended in part on the patient's baseline ENFD. Diabetic patients had a significantly lower regeneration rate than did healthy control patients even after adjustment for baseline differences; the rate was even lower among diabetic patients with neuropathy than in those without it. After 100 days, neither group of diabetic patients regenerated their ENFD to baseline levels (Brain 2004;127:1606–17). (See images.)
These results suggest that an improvement in regeneration should be the first detectable sign in the course of examining the effectiveness of an intervention for neuropathy, Dr. Polydefkis said.
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