Neuroselective Sensory Nerve Conduction Threshold (sNCT)™
Evaluation of Diabetic Neuropathy

The following is a brief discussion about the use of the Neurometer^® CPT electrodiagnostic sNCT™ evaluation for the evaluation of diabetic neuropathy.

The sNCT procedure had its origins in the evaluation of diabetic neuropathy patients at Johns Hopkins University School of Medicine, Department of Neurology in 1980. That clinical research led the FDA to grant permission to market the Neurometer® CPT device in 1986. Presently, there are more than 275 peer reviewed articles demonstrating clinical utility of the Neurometer® CPT evaluation. Approximately 20% of the publications involve the characterization of diabetic neuropathy, including a recent publication appearing in The Lancet(1).

The sNCT is the only electrodiagnostic sensory evaluation procedure able to quantify hyperesthesia as well as hypoesthesia. Hyperesthetic conditions reflect inflamed or irritated sensory nerve fibers that have not yet lost their functioning (i.e. become hyperesthetic). Hypoesthesia is associated with a more advanced stage of neuropathy and represents a loss of function. Hyperesthesia occurs before hypoesthesia as peripheral nerve damage progresses. Detection of hyperesthesia permits earlier therapeutic intervention, thereby improving the prognosis, with the potential of limiting more severe damage and reducing the cost of care. A recent publication from the Japanese Journal of Diabetes(2) (1998) demonstrated how the sNCT evaluation was able to detect and evaluate hyperesthesia in 50% of the neurologically asymptomatic diabetic subjects in the study. The same study shows that in those diabetics with clinical signs of neuropathy about 75% of the abnormalities were hyperesthetic. Diabetics with foot ulcerations had more than 98% hypoesthetic measures.

The advantage of being able to detect even sub-clinical levels of hyperesthesia in diabetic subjects should not be underestimated. The authors of the Japanese Journal of Diabetes study later reported that after following their patients for a year, they were able to demonstrate a reversal of hyperesthetic conditions in response to more stringent control of blood glucose levels. To our knowledge, this is the fastest objective documentation of the reversal of diabetic neuropathy of any study.

The sNCT evaluation is also the only electrodiagnostic procedure capable of selectively evaluating the functional integrity of the large myelinated, small myelinated and unmyelinated sensory nerve fibers. This is particularly important for diabetic subjects because diabetic neuropathy is not selective for a specific sub-population of sensory nerve fibers. For instance, the sNCT evaluation can detect small unmyelinated fiber neuropathy in diabetic subjects who have normal myelinated fiber function. Electron microscopic evaluation of sural nerve biopsies, obtained from diabetic subjects and healthy controls, established that the sNCT evaluation was able to selectively evaluate small fiber function while the other neurodiagnostic tests in the study were not.(3) Another recent publication in the Journal of Neurology, compared the sNCT evaluation quantitative vibratory and monofilament measures from diabetic subjects.(4) The authors reported the sNCT evaluation to be three times more sensitive than the other two tests, even when using a relatively insensitive averaging analysis of the CPT data instead of the far more sensitive standardized range and ratio analyses.

Small fiber functioning represents protective sensation. Protective sensation is extremely important because it is the loss of protective sensation that can lead to amputation. Small unmyelinated fiber function is also correlated with autonomic nerve integrity. Autonomic neuropathy is responsible for the majority of deaths occurring among diabetic patients and the ability to easily assess unmyelinated nerve function with the sNCT evaluation allows early identification of those patients at risk for related cardiac failure.

The sNCT evaluation is the only electrodiagnostic procedure which can test at the tips of the toes and fingers where the distal axonal neuropathies common in diabetics begin. This allows the sNCT evaluation to detect diabetic neuropathy months or years before it would be detectable by other procedures such as the sensory Nerve Conduction Velocity (NCV) exam. The tuning fork evaluation is not capable of selectively assessing large fiber function in the tip of the great toe because vibration is conducted by bone conduction proximally and the loss of sensation can not be isolated to a specific location. Also, unmyelinated fibers can be polymodal and respond to a mechanical stimulus such as vibration. As a consequence, patients with a selective loss of large fiber function may show up normal with the tuning fork evaluation.

Another type of polyneuropathy that occurs in diabetes is immune mediated. This type of polyneuropathy presents with an asymmetric distribution of sensory dysfunction which can occur anywhere. The sensory NCV evaluation is limited to testing the physiological integrity of the peripheral distal segment of nerves, which is why diabetic patients with proximal immune mediated lesions of peripheral sensory nerves have normal peripheral sensory NCVs. Recent studies from Columbia University, New York Medical College and St. Vincents Medical Center in New York, University of Louisville School of Medicine and the United States Air Force, however, documented the ability of the sNCT evaluation to detect and quantify these proximal demyelinating polyneuropathies.(5,6,7) These types of neuropathies can be treated with IvIg and other types of therapies but if left undetected they can ultimately condemn a diabetic patient to a wheelchair.

The third major type of peripheral neuropathy that diabetics suffer from is vasculitis related. These neuropathies are focal and can effect sensory function anywhere on the body. The sNCT evaluation is unique because it can be used to test any cutaneous body site which allows the clinician to objectively quantify this condition.

One additional and important consideration about the sNCT evaluation is that it is painless. This feature encourages far better patient compliance for repeated evaluation than the painful NCV evaluation. The painless nature of the sNCT procedure is also of particular value for the electrodiagnostic analysis of juvenile diabetic patients.

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