Electrodiagnostic studies are currently the gold standard test for the evaluation of suspected CTS. Electrodiagnostic testing includes electromyography (EMG) and nerve conduction studies (NCS). These investigations are recommended for all patients who have a clinical diagnosis of CTS because results will confirm the clinical impression, provide prognostic information (i.e. determine the site and severity of nerve compression, as well as guide management.
The electrodiagnostic investigations are the most sensitive and specific tests available for peripheral neuropathy, making them the diagnostic test of choice. Electrodiagnostic testing has been found to have an 85% sensitivity and greater than 95% specificity for diagnosing CTS1. It is important to note that clinically symptomatic CTS may have normal nerve conduction findings and asymptomatic patients may have severe neuropathy1.
Nerve Conduction Studies
The NCS is the non-invasive portion of the EMG test. Sometimes a patient will only require the NCS study to accurately diagnose any neuropathy. The NCS study shows the level of severity of the patient's problem, and could suggest further studies, such as an EMG. These studies measure the speed and strength of an electrical impulse conducted along a peripheral nerve, i.e. how well and how fast the nerves can send electrical signals. Therefore, NCS help to confirm and localize the potential damage to the peripheral nerve that is causing the altered sensations.
Procedure
Recording electrodes are either placed over the median nerve sensory branches (index finger) for sensory NCS or over the median nerve innervated abductor pollicis brevis (motor endplate region) for motor NCS. Then a grounding electrode is placed on the dorsum of the hand. To begin the study a bipolar stimulator is placed over the median nerve at the wrist regardless of whether sensory or motor NCS is being conducted and a transcutaneous stimulus is delivered. This stimulus is increased until an action potential is generated within the nerve. The action potential passes under the recording electrodes and a waveform is recorded.
Interpretation
Conduction Velocity (CV) is the speed an action potential travels along a nerve. Normal values are 50 meters per second in the upper limbs and 40 meters per second in the lower limbs. It can be decreased with nerve injury. The electrical strength of the action potential is indicated by the waveform amplitude. The time difference between the stimulus delivery and the waveform onset is the onset latency and reflects the conduction speed along the fastest axons. The time difference between the stimulus delivery and the peak of the waveform is the peak latency and reflects the average conduction speed along the majority of axons.
Both latency and CV depend on the integrity of myelin sheath of a nerve. Whereas, the waveform amplitude depends on the number of functioning axons within the nerve. Therefore slowing of the CV or prolongation of the latency implies demyelination and decreased waveform amplitude implies the loss of axons, i.e. a conduction block.
To summarize, a patient with CTS will likely have a CV slower than 50 meters per second and/or a prolonged latency. If the condition is severe the waveforms produced by stimulation proximal to the flexor retinaculum will have smaller amplitude than those produced by distal stimulation, indicating a conduction block.
Sensory NCS
Sensory NCS are performed by stimulating the peripheral nerves and recording from a purely sensory portion of the nerve. The sensory nerve action potential (SNAP) that is generated after stimulation represents the conduction of an action potential along sensory nerve fibers. The latency is measured at several different stimulation locations along the same nerve. The sensory CV is calculated using the distance between the different stimulating and recording electrodes as well as the difference in latencies.
Above: Sensory NCS apparatus. Below: SNAP.
Image Source: Victoria Squissato
Motor NCS
Motor NCS are performed by stimulating peripheral nerves and recording from the muscle supplied by those nerves. The compound motor action potential (CMAP) that is generated after stimulation represents the conduction of an action potential along motor nerve fibers. Similar to the sensory CV, several latencies are recorded and the motor CV is calculated based upon the latency and the distance between the stimulating and recording electrodes.
NB: SNAPs are typically considered more sensitive than CMAPs in the detection of an incomplete peripheral nerve injury.
Above: Motor NCS apparatus. Below: CMAP.
Image Source: Victoria Squissato
EMG
In contrast to NCS where electrodes are placed on the surface of the skin, EMG involves the insertion of small needles into certain muscles to measure the electrical impulses being generated. Thus, an EMG is more invasive than an NCS. Typically, muscle activity is monitored with the muscle relaxed and while moving.
NB: EMG is the gold standard the WSIB uses in confirming a CTS diagnosis.
Findings
- Prolongation of the distal motor and/or sensory latencies of the median nerve
- Slowing of the median sensory CV across the wrist
- Denervation of abductor pollicis brevis
1Jablecki CK, Andary MT, Floeter MK. Practice parameter: Electrodiagnostic studies in carpal tunnel syndrome. Report of the American Association of Electrodiagnostic Medicine, American Academy of Neurology, and the American Academy of Physical Medicine and Rehabilitation. Neurology. Jun 11 2002;58(11):1589-92.
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