A number of considerations must be taken into account in the design of clinical trials to assess potential therapies for chronic painful sensorimotor neuropathy:
1. Is this chronic neuropathy? As noted earlier, there are two main types of painful symmetrical diabetic neuropathy: acute painful neuropathy, which is relatively rare and typically presents after a period of poor glycemic control with severe symptoms and few signs. The natural history of this condition is one of improvement of symptoms during a period of months (9). In contrast, chronic painful neuropathy is of insidious onset and although the symptoms are similar in character to those of acute neuropathy, on examination there is usually a peripheral sensory loss to multiple modalities and absent ankle reflexes. The natural history of this condition is that whereas the symptoms may wax and wane and persist for several years, the disappearance of symptoms is not necessarily a sign of improvement but might represent progression to the insensitive foot (25). It is clearly important in view of the difference in natural history, that trials of any potential new treatments should only include patients with the chronic sensorimotor neuropathy. If patients with acute neuropathy were included, improvement might be because of the natural history of the disease and not necessarily to a therapeutic effect of the agent under investigation.
2. Is the symptomatic neuropathy secondary to diabetes? There are numerous causes of painful sensory neuropathy (26), and Dyck et al. (27) have estimated that more than 5% of neuropathy in patients with diabetes is of nondiabetic causation. Moreover, the symptomatology of diabetic neuropathy can be relatively nonspecific, and a recent community study demonstrated that almost 5% of patients without diabetes reported neuropathic pain (28). A further study reported that chronic foot pain is common in the community with more than 20% of men and women reporting foot pain in the month before the survey, and almost 10% of patients reporting disabling foot pain (29).
3. Other considerations. A number of other considerations need to be taken into account in the design of clinical trials. Important among these is of course the measures used for symptom assessment as discussed earlier. A further question is "should the agent under investigation be compared against a placebo or against another commonly used and proven drug for symptomatic neuropathy?" Unfortunately, the vast majority of studies have the new agent compared against placebo: this probably relates to the fact that most such trials are sponsored by the pharmaceutical industry. Finally, the trial design should account for potential confounding variables, the most important of which is the level of glycemic control. As blood glucose flux might be important in the genesis of neuropathic pain, the stability of daily glycemic control is important if the agent under investigation is to be properly assessed.
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