Tests of nerve conduction are used to identify damage to a nerve trunk as a result of pressure, transection or neuropathy, and to may also be used to trace the extent of regeneration of axons following injury.
Motor and Sensory axons can be examined separately, and the tests can be performed on major nerves in upper and lower limbs.
One common disorder, carpal tunnel syndrome is due to pressure on the median nerve within the carpal tunnel (at the wrist), and can be investigated using nerve conduction tests and electromyography.
Other compression syndromes include ulnar nerve compression at the elbow, and peroneal nerve compression at the neck of the fibula. In both cases the nerve can be damaged by external pressure on a section of the nerve overlying bone.
Neuropathies are often the consequence of systemic conditions, such as diabetes or toxins, and usually affect many nerves in the body. Conduction of action potentials is slowed in many nerve trunks as a result of demyelination or axonal retraction.
A commonly performed test to examine the median nerve is shown in the diagram. Electrical stimuli are applied through the overlying skin to the median nerve the wrist, and again at the elbow. In each case the action potentials generated in the nerve result in electrical activity of the muscle, but the latency is longer in the case of stimulation at the elbow. The difference in timing and the distance between the two stimulation sites are used to calculate the conduction velocity of the largest axons innervating that muscle.
Nerve Conduction Velocity
Nerve Conduction Tests include the distal motor terminal latency of motor nerves and the conduction velocity of motor or sensory nerve fibres. Tests of nerve conduction velocity are relatively simple to perform and give an indication about the condition of the fastest sensory or motor fibres in the nerve.
In the case of the median nerve, distal stimulation (i.e. at the wrist) causes an action potential in the small muscles of the thumb. The distal terminal latency is the time taken for the action potential to be conducted from nerve underlying the cathodal stimulating electrode to the nerve muscle junction plus the time taken to initiate the action potential in the muscles.
Proximal stimulation of the median nerve at the elbow initiates a similar series of events but the proximal latency is longer because it includes the time taken for the action potential to be conducted along the nerve - the distance between the two stimulating electrodes (wrist and elbow).
So the conduction velocity of the median nerve in the forearm can be calculated by dividing the distance between the two stimulating sites by the difference in latencies in mm/mmsec (i.e.metres/second).
If the motor conduction velocity (CV) of the fastest fibres is greater than 40 metres/second then the result is considered normal. The result is considered abnormal if the CV is less than 40 m/sec. In practice, it is important to be ensure that the limb is warm (because low temperature reduces the CV of peripheral nerves and would produce a spurious result).
If the temperature of the limb is normal, and the CV is less than 40 m/sec, then it can be concluded that the nerve is abnormal, possibly due to neuropathies associated with disorders such as diabetes, or nerve compression.
Image source: www.hkma.org
The setup for recording the Sensory Nerve Action Potential (SNAP)
Conduction Velocity of the fastest Sensory Axons
The conduction velocity of the fastest sensory axons can be examined by applying electrical stimuli to the nerve in the index or middle fingers (using ring electrodes), and recording a compound action potential from the median nerve in the wrist, using surface electrodes.
The conduction velocity is the distance between the cathodal stimulating electrode and the onset of the action potential, divided by the latency - expressed in metres/second.
Because no synapses are involved, the distance between stimulating and recording electrodes (mm), is divided by the latency of the sensory action potential (msec). This is probably the most sensitive index of peripheral nerve function.
Nerve Compression Syndromes - Upper Limb
Carpal Tunnel Syndrome. In the Carpal Tunnel Syndrome, pressure on the median nerve in the carpal tunnel causes the distal terminal latency to be greater than normal.
However the proximal part of the median nerve in the forearm is unaffected by the compression, and the CV in the forearm is unaffected.
Decompression of the carpal tunnel can return the distal terminal latency towards it normal value (<4.5 msec).
Other Nerve Compression Syndromes.
The ulnar nerve can become compressed at the elbow (student's elbow - the 'funny bone') , and the radial nerve may be affected by fractures of the humerus in the region of the radial groove.
Nerve Compression Syndromes - Lower Limb
In the lower limb, the common peroneal nerve can be compressed as it winds round the neck of the fibula, as a result of direct pressure of fracture of the bone.
The motor nerve tests in the lower limb are focussed on the flexor digitorum brevis muscle, which is innervated by the common peroneal nerve. Loss of control of this muscle results in footdrop.
Sensory testing in the lower limb examines the potentials generated in the sural nerve.
Peripheral Nerves are affected are affected by many systemic disorders including common diseases such as long-term diabetes.
Segmental De-myelination is one of the pathological chenges in Neuropathies and can be detected because demyelination slows the passage of the action potential along the axon.
Changes in the conduction velocity, motor and sensory, in the upper and lower limbs is a characteristic of neuropathies with segmental demyelination.
Unlike the nerve compression syndromes, where a nerve trunk is affected distal to the point of compression, neuropathic changes affect many nerves throughout their lengths.
Neuropathies affecting unmyelinated axons
The non-invasive techniques mentioned previously monitor the state of myelinated axons. However unmyelinated axons are also affected by neuropathy, and needle electrodes may be inserted into nerves to examine these changes.
Autonomic Neuropathy affects the unmyelinated and finely myelinated axons of the autonomic nervous system, and typically affects the functions of the heart and circulation, gastro-intestinal tract and bladder.
Sensory changes, such as paraesthesia and pain are also characteristic of neuropathy, and are associated with changes in the unmyelinated afferent fibres in sensory nerves.