Limitations of quantitative sensory testing when patients are biased toward a bad outcome

To the Editor: Having conducted research on quantitative sensory tests (QSTs) for the past 20 years, I cannot argue with the conclusions of the Views and Reviews by Dyck et al.1 It is important to note, however, that these conclusions are particular to classic QSTs such as those implemented in the commercially available devices developed by Dyck et al. Those devices use physical stimuli such as vibration or temperature to assess nerve function. Contemporary research has yielded an electrodiagnostic QST that generates independent sensory nerve conduction threshold measures from the large and small myelinated and unmyelinated nerve fibers. The electrical stimulus used directly excites the nerve fibers instead of their specialized sensory nerve endings.2 Direct neuronal stimulation with a constant current permits reliable and painless current perception threshold (CPT) measures to be obtained. These measures are not affected by bone conduction, skin thickness, edema, or ambient temperature—factors that can be problematic for classic QSTs. CPT measures are also capable of quantifying both hyperesthetic and hypoesthetic conditions. Because hyperesthesia precedes hypoesthesia in progressive neuropathy, this test is sensitive to the earliest stages of neuropathy associated with conditions such as diabetes and carpal tunnel syndrome.3-5 The article also cites “mind-altering medications” and “psychophysical” factors that compromise the validity of the classic QSTs. CPT measures are not affected by medications such as ethanol and opiates, although they have been proven to provide a sensitive neuroselective measure of the anesthetic effects of intravenous, epidural, and …