Clinical Symptoms

Symptomatic diabetic neuropathy might affect 30-40% of diabetic patients with neuropathy. The most commonly reported symptom is pain in the distal extremities, in the legs more than in the arms with nocturnal exacerbation. Patients report deep aching pain, a burning feeling, sharp "shock-like" pain, or a more constant squeezing sensation (pressure myalgia). These symptoms are called positive sensory symptoms because of apparent "hyperactivity" of nerves and perceived as a presence of something that is normally absent. Negative sensory symptoms include "numbness," "wooden, rubber, or dead feet" feeling and commonly used descriptors are "a wrapped feeling," "retained sock feeling," "cotton wool under soles," and so on. Hyperalgesia and allodynia are also prominent elements of the neuropathic sensory symptom complex and are defined as hypersensitivity to a normally mild painful stimulus and painful sensation evoked by a normally nonpainful stimulus, respectively. In the vast majority of patients both positive and negative sensory symptoms coexist but they are typically picked up only by systematic questioning, as spontaneous reporting tends to favor the positive symptoms.

Because current treatments of painful diabetic neuropathy display limited efficacy and a troublesome side effect profile it forms a major target for clinical trials of patients with diabetic neuropathy (1). However, many patients have difficulty in describing their symptoms accurately and consistently, and many of the symptom questionnaires do not necessarily capture all of the many attributes of symptomatic diabetic neuropathy. Thus, a range of symptom questionnaires are available to record symptom quality and severity, many of which have been imported from pain states in general, and are therefore not specific to diabetic neuropathy. Although the most common outcome measure of pain response is the 11-point Likert scale, many other measures are used and there is no gold standard (Table 1) (2-14).

Moreover, there is no accepted cutoff for a level of pain response, which might be deemed clinically significant, with most studies accepting responses ranging from 30 to 50%, knowing that there is about 20-30% placebo response. To assess and compare therapeutic response between different drugs, responder rates should be considered across a range of responses from 30 to 90%. Limited head-to-head studies make comparison of relative efficacy between different therapies impossible. This compels us to develop a uniform, validated, and internationally accepted tool to quantify painful diabetic neuropathy.

Many of the drugs for painful diabetic neuropathy can result in significant side effects, particularly at higher doses. Therefore, in any clinical trial, adverse effects, maximal tolerated doses, mood, and quality of life should be evaluated as secondary outcome measures. This is particularly important in a "real world" scenario as opposed to a clinical trial in which treatment is often stopped by the patient or switched by the physician as a result of adverse effects.

To try and standardize and compare treatment efficacy with safety, the number-needed-to-treat (NNT) (reciprocal of the absolute risk reduction) for one patient to achieve at least 50% pain relief should be calculated in addition to the relative risk (RR) and number-needed-to-harm for adverse effects and drug-related study withdrawal. Eventhough the proposed approach is more systematic it is not without its problems particularly when combining different studies. Variable durations and numbers of patients in different clinical trials limit the usefulness of a summated analysis and extrapolation

Table 1

Variety of Outcome Measures Used in Epidemiological and Interventional Studies of Painful Diabetic Neuropathy

Table 1

Variety of Outcome Measures Used in Epidemiological and Interventional Studies of Painful Diabetic Neuropathy

Outcome measure

Study

Neuropathy symptom score

2,3

Simple visual analog or verbal descriptive scales

4,5

Brief pain inventory

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