Two-Hour Glucose Test Still Useful in Idiopathic Nerve Pain

SCOTTSDALE, Ariz., June 13 - The two-hour oral glucose tolerance test should be part of the diagnostic work-up for patients with chronic idiopathic axonal neuropathy, researchers here contend.

The two-hour oral glucose tolerance test is "often avoided by busy health care professionals, perhaps because of the inconveniences posed by the testing procedure," according to Charlene Hoffman-Snyder, MSN, of Mayo Clinic Arizona here.

But the use of the test is "still of greater value" than the recently revised fasting plasma glucose values in detecting impaired glucose metabolism in patients whose chronic neuropathies have no known cause, Hoffman-Snyder and colleagues reported online today in the June Archives of Neurology.

In 2003, the American Diabetes Association revised its criteria for impaired fasting glucose - from between 110 and 125 mg/dL to between 100 and 126 ml/dL. The wider criteria were seen as "raising the question whether there was still value to using the two-hour oral glucose tolerance test when evaluating patients presenting with chronic idiopathic axonal neuropathy," the researchers noted.

Diabetes, genetic disorders, exposure to toxic substances, and primary amyloidosis can all cause neuropathy, the researchers noted, but many cases do not have an easily identifiable underlying cause and are classified as chronic idiopathic axonal neuropathy. A cause is eventually found in only 7% to 30% of chronic idiopathic axonal neuropathy cases.

However, recent studies have suggested that patients with chronic idiopathic axonal neuropathy have nearly twice the rate of undiagnosed diabetes and impaired fasting glucose than the general population, using the two-hour oral glucose tolerance test and the old ADA guidelines for impaired fasting glucose.

To estimate the prevalence of undiagnosed abnormal fasting glucose metabolism using the 2003 ADA criteria compared with the two-hour oral glucose tolerance test, the researchers conducted a retrospective database analysis of 100 consecutive patients - 60 women and 40 men -- with chronic idiopathic axonal neuropathy at the Mayo Clinic Arizona.

Using the 2003 ADA guidelines, 39 patients had abnormal fasting glucose metabolism - 36 with impaired fasting glucose and three with diabetes. Using the two-hour oral glucose tolerance test, the rate was considerably higher - 62 patients had abnormal fasting glucose metabolism, including 38 with impaired glucose tolerance and 24 with diabetes.

The difference between the rates was statistically significant at P<0.001, the researcher said.

The rate found using the two-hour oral glucose tolerance test is nearly twice that found in the general population, the researchers reported, suggesting that impaired glucose tolerance and impaired fasting glucose may be risk factors for chronic idiopathic axonal neuropathy.

"This investigation supports the hypothesis that distal axonal polyneuropathies may occur in much earlier stages of abnormal glucose metabolism than previously thought," the researchers said, adding it may be that the neuropathy associated with impaired glucose tolerance is milder than neuropathies usually associated with diabetes.

Indeed, such neuropathy "may be the earliest detectable sign of abnormal glucose metabolism," they said.

Using the test may have clinical benefits, since it would identify "opportunity for more aggressive management of the risks associated with hyperglycemia," the researchers argued.

They noted that lifestyle changes, such as diet control and exercise, have been shown to correct hyperglycemia and also to reduce neuropathic pain.

The authors pointed out, however, that causal relationships cannot be determined with this study design. "Additional studies with age-matched case-control subjects are warranted before definite causal relationships between peripheral nerve dysfunction and abnormal glucose metabolism can be fully accepted," they wrote.