DIAGNOSTIC WORKUP OF PATIENTS WITH NEUROPATHIC PAIN
The most significant measured parameters are maximum conduction veloc- ity (NCV) for the segment of nerve between the stimulating and recording electrodes, and amplitude and configuration of the resulting signalsdthe compound motor action potential (CMAP) evoked from motor fibers and the sensory nerve action potential (SNAP) evoked from sensory fibers. For central nervous system or proximal peripheral nerve disorders, somato- sensory and magnetic evoked potential studies can be helpful. Electromyog- raphy (EMG) is the needle evaluation of muscles and evaluates muscle and motor nerve fiber activities.
Unfortunately, Ad- and C-fiber activities cannot be tested with these techniques. Slowing in maximum NCVs or loss of CMAP or SNAP ampli- tudes, indicative of peripheral nerve disease either focally or generally, occur as a consequence of large fiber dysfunction. Abnormal EMG features such as acute and chronic denervation indicate involvement of large motor nerve fibers, also focally or generally, from the anterior horn cell distally. If pres- ent in a patient with neuropathic pain, these abnormalities can be used to corroborate the clinical impression of damage to a specific peripheral nerve or to peripheral nerves in general as in a polyneuropathy, eg, diabetic or al- coholic neuropathy. However, polyneuropathies or focal nerve lesions with only small-fiber involvement can have normal NCVs and EMG despite sig- nificant nerve damage and neuropathic pain.
Quantitative sensory testing
Quantitative sensory testing (QST) is used with increasing frequency, especially in clinical therapeutic trials, and measures sensory thresholds for pain, touch, vibration, and hot and cold temperature sensations. A num- ber of devices are commercially available and range from handheld tools to sophisticated computerized equipment with complicated testing algorithms, standardization of stimulation and recording procedures, and comparisons to age- and gender-matched control values. With this technology, specific fiber functions can be assessed: Ad-fibers with cold and cold-pain detection thresholds, C-fibers with heat and heat-pain detection thresholds, and large fiber (Aab-) functions with vibration detection thresholds. Elevated sensory thresholds correlate with sensory loss and lowered thresholds occur in allo- dynia and hyperalgesia . In a generalized polyneuropathy when all quan- titative sensory thresholds are elevated, it is inferred that all fiber types are affected, whereas if a dissociation exists wherein vibration thresholds are normal, but the other thresholds are elevated, the presence of a small-fiber neuropathy is suspected. In asymptomatic patients, abnormal QST thresh- olds suggest subclinical nerve damage.
The advantages of quantitation of sensory perception are that by enumer- ating an individual patient’s findings and comparing them with normative values a clearer distinction between normal and abnormal responses occurs, thereby allowing analyses across patient and disease groups and for baseline