The Indications and Importance of Obtaining Electrical Studies

Normal Findings and Values

EMG

For an EMG of a normal muscle at rest, the isoelectric line should be silent and no spontaneous electrical activity should be seen [6, 7, 9]. With insertion of the needle, the muscle is depolarized which produces a burst of positive and negative spikes that should cease after the needle stops moving [6]. The MUAPs are then evaluated with voluntary contraction to analyze their amplitude, duration, and phase; MUAPs as recorded by EMG are normally large (300–3000 μV), having three to four phases, and last less than 12 ms [5, 6, 9].

EMG for the median nerve—normal findings
At rest	Isoelectric
Insertion of needle	Burst of spikes that cease when needle stops moving
Voluntary contraction	Normal MUAPs

MUAPs for the median nerve—normal values
Amplitude (μV)	300–3000
Duration (ms)	<12
Phases (number)	3–4

NCS

Normal nerve conduction studies of the median nerve are easy to interpret as the velocity and the distal latency should be within their normal ranges—velocity (m/s) 49–70 and distal latency (ms) 2.4–4.4 [5].

NCS for the median nerve—normal values
Velocity (m/s)	49–70
Distal latency (ms)	2.4–4.4

Abnormal Findings

In the early stages of carpal tunnel syndrome, electrodiagnostic tests may be normal, but as the median nerve continues to be compressed and the loss of myelin progresses, abnormal findings begin to appear. As the conduction velocity slows, NCS become positive and over the course of time, axonal loss propagates leading to positive EMG findings [4–6]. Fibrillation potentials on EMG are regular, bi-, or triphasic waveforms that are associated with axonal denervation; they represent involuntary contraction and intrinsic muscle abnormalities that are pathognomonic for denervation [4–6]. Sharp waves represent an involuntary release of acetylcholine and are evidence of muscle fiber denervation; they are thought to have the same significance as fibrillations though they may appear a few days earlier [5, 6]. Both fibrillation and sharp waves exist where nerve stimulation has been lost due to trauma, inflammatory myopathies, and degenerative myopathies [5, 6]. As mentioned earlier, results may differ depending on the skill of the operator, the amount of blood flow, temperature, and age of the patient [4, 6–8].

NCS

Any change in NCS velocity or distal latency outside the normal range is significant for nerve pathology though nonspecific; segmental slowing suggest a focal compression or trauma while multiple sites of slowing suggest a systematic neuropathy [5].

EMG

Needle Insertion

A brief burst of activity followed by silence is normal. Any increased or decreased insertional activity is the earliest sign of denervation though not specific [5].

Muscle Rest

At rest the muscle is normally electrically silent, but abnormal findings would include fibrillations, sharp waves, or fasciculations [5, 6].

Muscle Contraction

With minimal contraction, the MUAPs will show normal triphasic waveforms with amplitude and duration within their normal range; abnormal findings would include polyphasic waveforms and a decrease in amplitude or duration [5]. Maximal contraction normally shows destruction of the electrical baseline and inability to isolate individual waveforms (full interference pattern) [6]. Abnormal results would show only a partial interference pattern (muscle weakness or noncompliance), isolated action potentials (severe degeneration), or lastly the presence of early interference pattern (end-stage myopathy or neuropathy) [5].

Diagnosis

A diagnosis of carpal tunnel syndrome is obtained if the median sensory NCS across the wrist with a conduction distance of 13–14 cm is abnormal compared to another adjacent nerve in the symptomatic limb [12–15]. If these are normal, then a diagnosis can still be obtained using any of these other methods: comparison of the median sensory or mixed nerve conduction across the wrist over a short conduction distance (7–8 cm) with the ulnar sensory nerve conduction across the wrist over the same short conduction distance, or comparison of the median sensory conduction across the wrist with radial or ulnar sensory conduction across the wrist in the same limb, or comparison of the median sensory or mixed nerve conduction through the carpal tunnel to the sensory or mixed NCS of the proximal or distal segments of the median nerve in the same limb [12–15]. Additionally, testing may involve comparing median motor recording from the thenar musculature to one other nerve in the symptomatic limb to include the distal latency as well [12–15]. Optional supplemental testing not included as standard may include comparison of the median motor distal latency in the second lumbrical to the ulnar motor distal latency in the second interossei, median motor terminal latency index, median motor nerve conduction between the wrist and palm, median motor compound muscle action potential amplitude ratio from the wrist to the palm, median sensory action potential amplitude ratio from the wrist to the palm, and 1 cm short incremental segments median sensory across the carpal tunnel [6–9].