Discussion
As with other syndromic disorders, CTS is associated with several clinical features, the presence of which correlates with the severity of median nerve involvement. During the earliest stage of CTS, episodic hand tingling (a positive symptom) is commonly reported. This tingling typically is more pronounced at night and during relaxation. In addition, many patients come to recognize that their hand tingling is precipitated by activities that involve sustained upper extremity elevation (eg, driving with a limb resting on upper portion of steering wheel; reading with upper extremities maintained in forward abduction) and that lowering a symptomatic limb relieves its tingling.
With progression, negative symptoms appear (eg, numbness and then weakness and wasting). Unfortunately, as the negative symptoms replace the positive ones, affected individuals may become less symptomatic and mistakenly believe their condition is improving. Features of autonomic fiber involvement may also be present but are less reliably elicited. Isolated hand pain is an uncommon manifestation of CTS because pain more commonly occurs later in the course and for this reason tends to be accompanied by other features of CTS.
The clinical features of CTS correlate with its underlying pathology. As demyelination precedes axon disruption pathologically, the clinical features of demyelination (episodic paresthesias) precede those of axon loss (numbness, weakness, wasting). However, clinical features may go unrecognized or be dismissed by the patient. Moreover, there is substantial variation in type, intensity, and frequency of symptoms.16,17
The EDX features of CTS correlate with its underlying pathology and pathophysiology. As demyelination (loss of insulation) increases the capacitance of the membrane and increases internodal current leakage, conduction velocity is reduced. As severity worsens and pathology changes from predominantly demyelination to predominantly axon loss, the individual nerve fiber action potentials, which make up the compound responses being recorded, are lost. As a result the amplitude and negative area under the curve values decrease. Thus, the EDX features of demyelination (eg, prolonged latencies) precede those of axon loss (eg, amplitude, negative area under the curve reduction).
As with other focal mononeuropathies, the sensory responses tend to be affected earlier and to a greater degree than do the motor responses. Consequently, the EDX features of CTS typically follow a standard progression. The median palmar responses are involved sooner and to a greater degree than the median sensory responses recorded from the digits, which in turn tend to be involved earlier and to a greater degree than are the median motor responses.
Awareness of this relationship dictates the severity of the lesion and helps in the recognition of a cool limb and in the avoidance of a false-positive study interpretation. In a cool limb, the fingers are cooler than the wrists. Thus, the peak latency of the median digital sensory response is delayed to a greater extent than the ipsilateral median palmar response (the opposite of the CTS pattern). Accordingly, whenever this pattern is identified, the hand must be warmed or rewarmed and the studies repeated. The hand is also warmed or rewarmed whenever the median motor response is delayed out of proportion to that of the median palmar response.
Conclusion
Cases of CTS mainly in the nondominant limb provide an opportunity to identify particular limb usage patterns that might be associated with CTS. Of the present study’s 21 affected patients, 16 were using their nondominant limb to perform activities that required sustained gripping. Fourteen of the 16 activities were related to occupation. These findings strongly suggest an association between activities that require sustained gripping and development of CTS.
That the card dealers simultaneously used their nondominant hand for sustained gripping and the dominant hand for the repetitive activity of dealing suggests that sustained gripping is a stronger risk factor than repetitive activity for the development of CTS—an unanticipated finding. Interestingly, in a 2001 study that suggested repetitive activity might not be a CTS risk factor, there was a higher incidence of CTS among computer users working with a mouse—an activity that requires sustained gripping.12
Episodic hand tingling during mouse use likely reflects impaired blood flow to the median nerve, which occurs when carpal tunnel pressure approaches or exceeds 20 to 30 mm Hg.18 Placement of a hand on a mouse increases intracarpal pressure from 3 to 5 mm Hg (wrist in neutral position) to 16 to 21 mm Hg, whereas mouse use increases intracarpal pressure to 28 to 33 mm Hg.18-20
