Nerve conduction studies and EMG are often used as the gold standard in diagnosing CTS and differentiating CTS from cervical radiculopathy, polyneuropathy, fibromyalgia, rheumatoid and/or degenerative arthritis, local musculoskeletal trauma, peripheral vascular disease, complex regional pain syndrome, and others.
A diagnosis of CTS is made when there is evidence for conduction slowing in motor and/or sensory fibers across the wrist over standardized distances. Borderline conduction values can be further assessed by comparison of non-symptomatic nerve conduction values. The severity of CTS on EDX exam is based on the severity of conduction block and the presence or absence of reduced motor and/or sensory amplitudes. EMG can be useful in further assessment of axonal injury to motor fibers and most commonly tests the Abductor pollicis brevis muscle for abnormal insertional activity, abnormal motor units (indicative of denervation followed by reinnervation), early recruitment (increased motor unit firing rate to compensate for the loss of total number of motor units), and total motor unit recruitment.
In broad terms CTS can be quantified electrically as mild, moderate, moderately severe, and severe based on both the severity of conduction block, change in motor and/or sensory amplitudes compared to normal values, contralateral median, or ulnar and EMG evidence for axonal damage to median innervated thenar muscles.
Mild:
1. Slowing of Median sensory distal latencies with normal motor latencies.
2. Mild slowing of motor distal latencies with normal motor amplitudes.
3. Motor latencies of less than 4.5 msec
4. Sensory distal latencies of less than 4.0 msec
Moderate:
1. Slowing of motor latencies >4.5 and <5.0 msec
2. Slowing of sensory latencies of >4.0 and <5.0 msec
3. Motor amplitudes in normal range but may be reduced compared to opposite median if asymptomatic or significantly reduced compared to ulnar motor
4. EMG may show early recruitment and polyphasic motor units but no “denervation” potentials and typically no loss of total recruitment numbers
Severe:
1. Motor latencies >5.0 with loss of motor amplitudes
2. Sensory latencies >5.0 with/without loss of sensory amplitudes
3. EMG shows abnormal motor units, reduced motor unit recruitment, may show positive sharp waves and fibrillation potentials (rare)
In summary, evidence-based studies show that mild and mild to moderate CTS often respond favorably to cortisone injection, ultrasound, non-steroidal anti-inflammatory medications, and nighttime splinting. Once CTS has progressed to moderately severe or severe timely surgical decompression is indicated. If less severe CTS fails conservative care, repeat studies may be indicated to document progressive median nerve entrapment supporting surgical decompression even if the numbers suggest mild to moderate CTS. Intervention is based on first establishing that CTS is the cause of symptoms, determining whether conservative care is indicated, and since the primary cause is a mechanical compression of the nerve, determining if and when surgery decompression is indicated.
Typical recommendations are initial conservative care of mild to moderate CTS with repeat studies and surgical referral when unresponsive to conservative care and immediate surgical referral when CTS is moderate to severe with evidence for axonal damage to median thenar muscles. In the present of underlying peripheral nerve disease such as diabetic polyneuropathy, earlier referral for surgery may be indicated based on clinical experience that patients with ‘sick’ nerves are more susceptible to axonal damage due to entrapment neuropathy.
Patients treated conservatively should be re-tested within a year regardless of symptoms because of “accommodation”, a process of learning to ignore symptoms that often results in patients being unaware their CTS is progressing. Patients with underlying peripheral neuropathy such as diabetics may need to be followed more closely with frequent testing to determine the need for decompression due to an increased risk for axonal damage from focal entrapment of a diseased nerve.
In summary, carpal tunnel syndrome is the most common EMG/NCV diagnosis when testing for cervical or upper extremity pain syndromes with/without paresthesias, numbness, tingling, or weakness. CTS is also a common co-morbid finding in cervical radiculopathy and fibromyalgia. In my review of over 600 consecutive EMG referrals performed by me over a recent six-month period in a chronic pain practice CTS represented 26% of all diagnoses. CTS is one of the primary reasons my focus in educating referring physicians has put so much emphasis on co-morbidity. If precise treatment is our goal, then we must commit to more precise diagnosis.