One drawback of this approach is that the group with “newly diagnosed” diabetes (i.e., diagnosed as part of the study by blood glucose testing) is usually identified by an abnormal glucose value measured on a single day. Since the diagnosis of diabetes requires verification of this on another day [13], it is likely that the group identified includes a number of individuals who would have a nondiabetic glucose value on repeat testing, and therefore do not actually have diabetes. Assuming that these people do not have neuropathy (since they do not have diabetes), their inclusion would dilute the sample and lead to an underestimate of the prevalence of neuropathy in the newly diagnosed group. Nevertheless, neuropathy prevalence estimates derived from population-based samples that include all of those people who have diabetes (both diagnosed and undiagnosed) should be the most accurate and reproducible approach.
How Common Is Diabetic Neuropathy?
Prevalence in Diabetes
Taking into account the issues discussed above, it is difficult to determine with any precision the prevalence of diabetic neuropathy. Several large studies have examined the prevalence in hospital-based populations. A number of these are in fairly close agreement, and reveal prevalences of DSP at approximately 30%, amongst both European and African populations [8,14-16]. However, other hospital-based studies have produced figures closer to 20% [3], and a prevalence of 50 % was reported from a US veterans population [17].The higher figure in this last study may relate to the age of the participants, and to the fact that they were almost all male.
Studies which have used primary care registers of people with diabetes, or which report the prevalence in people with previously diagnosed diabetes derived from population-based surveys, have mainly reported lower prevalences than those given in hospital-based studies. Some studies report prevalences of around 20% [6,11,18-20], although a figure as high as 54% in type 1 and 45% in type 2 diabetic patients was reported from a population based sample from Rochester [21], and 42% of a sample of 811 type 2 diabetic subjects drawn from 37 UK general practices were found to be neuropathic [22]. The Rochester study used electrophysiology as part of the neurological assessment. This is frequently abnormal in diabetes, even within a short time of diagnosis of type 1 diabetes, and has not been validated as a predictor of foot ulceration. The prevalence rates of DSP in type 1 and type 2 diabetic patients are summarized in Table 3.2.
When general populations are screened by glucose tolerance testing, and those identified as having diabetes are assessed for peripheral neuropathy, the prevalence has been relatively low (Table 3.3), In our own study from the Indian Ocean island of Mauritius (in which 70% of the population originates from India) the data showed the lowest prevalence of DSP yet reported [5]. The overall prevalence in the total diabetic population was 8.3%, i.e., 12.7% in subjects with known diabetes and just 3.6% among those with newly diagnosed diabetes. Consistent with the low neuropathy prevalence was a low prevalence of lower limb amputation - a finding that has been reported elsewhere for people who originate from the Indian subcontinent [36]. Neuropathy was based on VPT measurements, using locally derived age specific normal ranges. In the only similar study, from Egypt [6], 14% of the newly diagnosed diabetic population were found to have DSP. However, the reference range used for VPT in that study related to healthy adults under the age of 45, and therefore probably did not account for the normal age related rise in VPT. Thus, since the majority of those with diabetes were over the age of 45, the neuropathy prevalence in that study may be an overestimate. In two Native American populations, which are regularly screened for diabetes with glucose tolerance testing, the prevalence of DSP in the whole diabetic population was 19% [4,9].
The prevalence of diabetic neuropathy in general populations was evaluated in two door-to-door surveys that were restricted to a questionnaire which represents the most crude and simple screening instrument [37,38]. However, since no screening for diabetes had been performed, the exact prevalence of diabetes in these populations was unknown.
It can be seen from Tables 3.b and 3.3 that diagnostic methodology is far from uniform. Indeed, there are hardly any two studies that have used identical methods. Nevertheless, a pattern can be discerned in these studies. The prevalence in hospital-based populations is probably around 30%, falling to about 20% when people with established diabetes are selected from a community base. Amongst the complete diabetic population, including both diagnosed and undiagnosed diabetes, the prevalence of DSP is probably less than 20%.
Prevalence in Impaired Glucose Tolerance
Although the development of diabetic polyneuropathy is typically insidious over several years, it may occasionally be the presenting feature in type 2 diabetic patients [39]. It is not known whether this “early” nerve alteration has developed during a period of unrecognized diabetes or evolves gradually during a state of impaired glucose tolerance (IGT) prior to the transition to overt diabetes. In this context, it is unclear whether there is a glycemic threshold beyond which nerve dysfunction develops. If IGT constitutes such a threshold which needs to be passed, subjects with IGT should not have a degree of neuropathy higher than that of nondiabetic subjects [40] The results of several studies comparing the prevalence of nerve dysfunction in patientswith IGT and normal glucose tolerance (NGT) are summarized in Table 3.4. The hospital-based studies could not demonstrate a difference in peripheral nerve function and structure between patients with IGT and NGT, except for an abnormal expiration/inhalation ratio, a marker of cardiovascular autonomic neuropathy