Nerve Conduction Studies

What is NCS/EMG?

NCS examines the conduction properties of sensory and motor peripheral nerves. For both motor and sensory studies, the nerve is stimulated across the skin using surface (not needle) electrodes. The electrical stimulus depolarises the nerve underneath the electrode and then an action potential propagates along the nerve, much as it would during natural activation of the nerve.

NCS/EMG does not follow a fixed testing protocol that is applied to all patients (cf EEG, ECG etc). Instead, a series of tests are performed that will vary considerably depending on both the patient’s clinical problem and the neurophysiologist’s training.

Sensory nerve conduction studies.

These are performed in almost all patients as they tend to be most sensitive to many of the common conditions affecting peripheral nerves, such as Carpal Tunnel Syndrome (CTS) and peripheral neuropathy.

In the case of sensory studies, the action potential is recorded from a distant site on the nerve and is known as a SAP or SNAP (sensory nerve action potential). The action potential propagates equally in both directions away from the stimulating site. Thus sensory studies can be:

1. orthodromic – where the stimulation site is distal to the recording site, as occurs in natural sensory activation (e.g. stimulating digital nerves with ring electrodes);

2. antidromic – where the stimulation site is proximal to the recording site, opposite to the situation with natural sensory activation;

3. mixed – where both stimulation and recording is along a nerve that contains both sensory and motor fibres, but only the sensory response is recorded.

Both orthodromic or antidromic studies provide overall similar information, however antidromic responses are generally larger. Mixed studies are usually used for testing proximal sensory conduction, where standard sensory studies would produce responses too small to reliably record. The sensory studies performed by Dr Shaun Watson are antidromic unless otherwise stated.

Motor nerve conduction studies.

These are performed in most patients. They tend not to be quite as sensitive to subtle pathology, but are important in assessing severity and are critically important in the localisation of certain entrapment neuropathies, such as ulnar compression at the elbow and peroneal compression at the fibular head.

In the case of motor studies, the response is recorded from a muscle and is referred to as a CMAP (compound muscle action potential). This means that for motor studies the latency from stimulation to response always includes the time for neuromuscular and muscular transmission, along with time for conduction along the motor nerve itself.

“H” reflexes and “f” waves.

These are both long latency responses that are useful in studying proximal nerve conduction.

The “f’ wave is obtained by stimulating the distal motor nerve in the usual manner and then recording a response that comes much later than the usual distal CMAP. The action potential travels up the motor axon (antidromic), to the motor neurone, then back down the motor axon (orthodromic). The “f’ wave is a purely motor response and can be studied for any of the major motor nerves.

The “H” reflex or wave is the electrical equivalent of the tendon jerk. Following stimulation of a sensory nerve the action potential travels to the spinal cord and produces trans-synaptic activation of motor neurones at the same segment and a motor response is recorded. “H” reflexes can be recorded at several sites, but most often the tibial/soleus response is recorded, a test of the S1 segment that is similar to testing the ankle jerk.

Nerve conduction values.

The main measurements are the response amplitude and onset latency. The conduction velocity is then derived from the latency measurement. Most laboratories (ours included) don’t quote normal values beside these measurements, and this is chiefly because values vary with a wide range of factors, such as temperature, age, sex and race. Temperature is particularly important and cold hands can lead to quite marked slowing of distal nerve conduction in normal individuals. Normal conduction velocities are usually 50 m/s or greater for the upper limbs and 40 m/s or greater for the lower limbs. Normal SNAP amplitudes are usually 6 μV or greater for the sural and 12 μV or greater for the upper limb nerves.

Which nerves to study?

Many of the peripheral nerves can be studied, but some of the smaller and more proximal nerves present technical challenges and results from these nerves are often unreliable. Thus in routine clinical testing we mainly study the larger peripheral nerves, particularly the median, ulnar and radial nerves in the arm and the tibial, peroneal and sural nerves in the leg. Some other nerves will be studied if there is a pressing indication and technical factors permit. Many patients who are referred for NCS will not require EMG, especially for relatively straightforward diagnoses such as CTS.

EMG.

Electomyography (EMG) is the recording of muscle electrical activity using a fine needle electrode. It can provide important information about the health of the nerve supplying the muscle and can also give information about the health of the muscle itself. The EMG of a normal muscle has the following features: the muscle is silent at rest; with gentle contraction of the muscle, motor units (MUs) appear that are relatively small, narrow and of simple morphology; with maximal contraction the screen is filled by numerous MUs firing together to form a normal and complete interference pattern. Any deviation from this pattern can give important diagnostic information. The commonest abnormality is denervation. In denervation, there will typically be spontaneous activity (such as fibrillations and positive sharp waves or fasciculations), the MUs will be large and complex (often polyphasic) and the interference pattern will be reduced and often made up of only a few large MUs firing together at a high frequency. Often it is the pattern of denervation in different muscles that provides the most critical diagnostic information, e.g. can the pattern of denervation be explained by a lesion of a single peripheral nerve or nerve root?

How to interpret the findings?

In common with many hospital neurophysiology departments, we have consciously chosen not to quote normal values with our test results, as discussed above (nerve conduction values). This is because the test is always individualised and must be interpreted in the specific clinical context. Isolated “abnormal” results can be essentially normal, or at least of no clinical significance, in certain contexts, e.g. small sural or ulnar sensory potentials in some elderly subjects. A report along the lines of “these findings provide some support for a peripheral neuropathy of axonal type, but such findings are within the broad normal range at this age”.

Often the most important information is obtained by comparing values between different nerves in the individual patient being studied. This particularly applies when we are looking for evidence of a focal nerve lesion, such as Carpal Tunnel Syndrome. In the specific situation of Carpal Tunnel Syndrome we are looking for an isolated abnormality of median conduction across the wrist. Even a “normal” conduction velocity for distal median conduction (say greater than 50 m/s) might be abnormal if it is significantly slower than corresponding ulnar or radial conduction.

In the clinical context of suspected generalised peripheral neuropathy we must resort to population normal values, as all of the individual patient’s nerves are liable to be affected. The sural nerve is a pure sensory nerve in the lower limb that is involved early in generalised neuropathy and is only very rarely subject to localised compression or other processes. An amplitude of below 6 µV is usually abnormal, but even a value of 10 µV might be considered abnormal in an otherwise healthy and thin young female.

Which patients might benefit from the test?

Any patient with a suspected disorder of the peripheral nervous system is likely to require NCS/EMG. In general terms, the test is usually indicated where the clinical indication includes some combination of numbness, parasthesiae, neuropathic pain, weakness and cramps. Several specific clinical scenarios are considered below.

Specific clinical indications.

Suspected Carpal Tunnel Syndrome (CTS).

Suspected CTS is easily the commonest indication for NCS referral. Classical symptoms are intermittent hand numbness and/or parasthesiae waking the patient from sleep, on waking and on maintaining a grip. Predominant upper limb pain is a less common presentation.

There is no absolute “gold standard” for the diagnosis of CTS, but NCS is the best test currently available. An accurate diagnosis of CTS can usually be achieved by combining clinical assessment with NCS. Sometimes imaging and carpal tunnel steroid injection are required to clarify the diagnosis. It is important to be aware that even the most carefully performed NCS is probably negative in up to 10% of cases of CTS. It is equally important to recall that NCS can reveal incidental asymptomatic CTS. Thus clinical correlation is essential. Carpal Tunnel decompression is associated with substantial short term, and occasionally long term, morbidity and thus should generally only be undertaken when the diagnosis is made with confidence.

A wide range of techniques for the diagnosis of CTS are available. An isolated and localised abnormality of median nerve conduction across the wrist is the salient finding. Gross abnormalities of amplitude and conduction velocity may be evident, but it is often necessary to carefully compare median with ulnar and/or radial values to obtain a diagnosis in mild cases. In uncertain cases we commonly compare median and radial sensory conduction to the thumb over an equal distance – a difference of 0.4 ms or greater is often diagnostic, but a difference of 0.2 ms is supportive in some cases. A similar test that is also commonly used is comparison of motor conduction from the wrist to interossei supplied by median and ulnar nerves.

Other focal nerve lesions/compression neuropathies.

Common examples include ulnar lesion at the elbow, wrist and foot drop and meralgia parasthetica. NCS is highly informative in many such patients. The characteristic findings are abnormalities of conduction (and possibly EMG) confined to a single peripheral nerve. The responses for the specific nerve may be small and slow. It is sometimes possible to demonstrate focal conduction abnormalities at the site of the lesion, such as across the elbow for the ulnar nerve or the fibular neck for the peroneal nerve. In such cases there may be a significant drop in conduction velocity or response amplitude (conduction block) across a short segment of nerve. As with CTS, a normal NCS does not exclude a mild focal nerve lesion.

Peripheral neuropathy.

The NCS is often able to provide a definitive diagnosis in the clinical context of suspected generalised peripheral neuropathy. There is a wide range of possible clinical presentations, but the commonest pattern is of sensory symptoms and/or signs predominant for the feet. The commonest abnormality is a low amplitude sural sensory response and in more severe cases there may also be low amplitude upper limb sensory and lower limb motor responses. “H” and “f” waves may also be delayed or absent. Most neuropathies are of “axonal” type, with dying back of the longest axons, and this pattern shows normal or near normal conduction velocity. Examples include most cases of diabetic neuropathy, alcohol and nutritional neuropathies. If the conduction velocities are slow then the possibility of primary demyelination needs to be considered, specific diagnoses including Guillain Barre Syndrome, Chronic Inflammatory Demyelinating Polyradiculoneuropathy and Charcot Marie Tooth neuropathy. As with the other conditions considered above, the NCS can be normal in some cases of definite neuropathy. The commonest situation is where the patient has a “small fibre” neuropathy with neuropathic pain and parasthesiae, with minimal numbness and no definite abnormalities on examination. NCS only examines the relatively large diameter sensory fibres and thus a process confined to small diameter fibres can be missed.

Weakness of lower motor neurone type.

Where there is objective weakness the diagnostic yield of NCS/EMG is usually high. The details of the study performed will depend on the pattern of weakness and may encompass the studies for focal nerve lesions and generalised peripheral neuropathy discussed above.

If there is a relatively focal pattern of weakness then a combination of targeted NCS and needle EMG will usually provide a specific diagnosis. In cases of radiculopathy/nerve root compression the sensory studies will be normal, even if sensory abnormalities are severe, because peripheral conduction is only affected if pathology is at, or distal to, the dorsal root ganglion. Motor response amplitudes will be low, in proportion to weakness and wasting, as motor fibres are in continuity from the anterior horn cell to the distal axon, without any intervening synapse. Needle EMG will show denervation confined to muscles supplied by the involved nerve roots. Paraspinal EMG is sometimes performed, as it is only abnormal with pathology at nerve root level.

In the case of generalised weakness, NCS and needle EMG will nearly always need to be performed. NCS will reveal any neuropathy. The closely related technique of repetitive nerve stimulation might need to be used to help disclose a neuromuscular junction disorder, such as Myasthenia Gravis. The needle EMG will generally be abnormal and will help to distinguish a myopathic from a neuropathic process. In myopathies and motor neurone disease the only abnormalities will be found on needle EMG, in most cases. In rare cases it is difficult to distinguish between myopathic and neuropathic abnormalities on EMG and sometimes only muscle biopsy is definitive.

Limb pain of suspected neuropathic cause.

NCS/EMG is more likely to be normal in this clinical setting than in those considered previously. Focal nerve lesions can present with predominant pain, but there are usually other symptoms and signs to assist the diagnosis. Radiculopathy/nerve root compression is often the primary consideration. Standard sensory and motor NCS are usually normal in radiculopathy where there is no objective weakness and thus the routine studies are often used to help exclude peripheral nerve lesions rather than to enable a positive diagnosis. Studies of proximal nerve conduction can be abnormal in radiculopathy, with the soleus/tibial H wave particularly useful in the context of a suspected S1 radiculopathy. Needle EMG has a higher diagnostic yield in this clinical context, but even this test is often negative where the radiculopathy is presenting with pain or sensory disturbance without clinically apparent weakness. Given that it is a relatively invasive and uncomfortable test, the neurologist will sometimes elect not to proceed to needle EMG if the presenting problem is isolated limb pain.

What should patients expect to experience when they have the test?

For nerve conduction studies, a relatively weak electrical impulse is delivered using surface electrodes across the skin. Usually multiple stimuli are given (and the responses averaged) for each nerve tested. Often multiple nerves need to be tested. The stimulation will often result in twitching or jumping of the muscles supplied by that nerve. Most patients find the study easily tolerable. The sensation is usually described as strange, unusual or uncomfortable. There is however a wide range of tolerance and occasional patients describe the stimulation as frankly painful and rare patients cannot tolerate even very mild stimulation. Nerve conduction studies are always individualised, but usually last between 30 and 60 minutes, depending on the complexity of the clinical problem and various technical factors. In our laboratory the testing is either performed directly by a neurologist or by a highly trained (university qualified) neurophysiology technician. If testing is performed by a technician it is always guided, supervised and reported by a neurologist. If the patient has both upper and lower limb problems we usually perform separate studies on different days, both to ensure that there is sufficient time to perform a detailed assessment without “cutting corners” and to minimise waiting time for other patients. Needle EMG is only performed by a neurologist and is required in a minority of cases. A fine needle is inserted quickly into the muscle or muscles of interest and the patient then needs to contract the muscle with the needle in place. Needle EMG is often uncomfortable and thus is generally only performed if specifically requested or if the neurologist considers it will provide critical clinical information. Needle EMG is not typically performed in a standard case of suspected Carpal Tunnel Syndrome or sensory neuropathy.

Written by Dr Shaun Watson, 2020.