Frequency / duration Less than 2 Hz / Duration less than 200 ms
Distribution Maximum at vertex (C3, C4) but may have wider distribution
Persistence They are seen at irregular intervals in stages 1 and 2 of NREM sleep
Synchrony Bisynchronous, may be unilateral
Miscellaneous
V waves and K complexes appear in well developed from for the first time at the age of 5-6 months.
They are most likely secondarily to auditory evoked potentials that converge from their cortical projection areas to a region underlying the vertex electrodes.
Reference: 1. Fisch BJ. Spehlmann’s EEG primer, Amsterdam: Elsevier, 3rd edition 2. Niedermeyer E, Lopes da Silva F. Electroencephalography: basic principles, clinical applications and related fields, Baltimore, Maryland: Williams and Wilkins, 4th edition 3. Jasper R. Daube. Clinical Neurophysiology, Philadelphia: F. A. Davis Company 4. Kooi, K. A. et al. Polarity and field configuration of the vertex components of the human auditory evoked response: a reinterpretation. Electroencephalogr. Clin. Neurophysiol. 31:166-169
The radial nerve is the largest branch of the brachial plexus. The radial nerve is derived primarily from the C5, C6, C7, C8 and T1. The radial nerve enters the arm from axilla along the medial side of the humerus to reach the spiral groove. From the axilla to the spiral groove, the radial nerve gives off motor branches to triceps and anconeus. It also receives the posterior cutaneous nerve of the arm, the posterior cutaneous nerve of the forearm and the lower lateral cutaneous nerve of the arm in this region. At the spiral groove, the radial nerve is in contact with the humerus as the nerve travels laterally, and then it pierces the lateral intermuscular septum. Here, it is bordered medially by the brachialis (Br) muscle, and laterally (from proximal to distal) by the brachioradialis (BR), the extensor carpi radialis longus (ECRL), and the extensor carpi radialis brevis (ECRB). All these muscles receive motor supply from radial nerve. The radial nerve then crosses the elbow joint anterior to the lateral epicondyle of the humerus. At the elbow the radial nerve divides into a motor nerve, the posterior interosseus nerve; and a sensory nerve, the superficial radial nerve.
Posterior Interosseus Nerve The posterior interosseus nerve enters and innervates the supinator (Sup) muscle. The nerve then gives motor branches to - extensor digitorum communis (EDC), extensor digiti minimi (EDM), extensor carpi ulnaris (ECU), abductor pollicis longus (APL), extensor pollicis longus (EPL), extensor pollicis brevis (EPB), and extensor indicis proprius (EIP).
Superficial Radial Nerve (SRN) At the elbow, the superficial radial nerve stays superficial to the supinator and proceeds anterolaterally, deep to the BR muscle. Approximately at the junction of the proximal two thirds and the distal one third of the forearm (approx 8 cm from tip of radial styloid), the SRN becomes superficial and crosses over to the posterior aspect of the distal radial forearm, passing superficial to the tendons of the anatomical snuffbox (APL, EPL, and EPB) and traversing the wrist over the extensor retinaculum. It supplies cutaneous sensation to the dorsal surface of the lateral hand, as shown in the figure.
Reference:
Richard S Snell, Clinical Anatomy: Lippincott Williams & Wilkins, 7th edition
Preston DC. Distal Median Neuropathies. In: Entrapment and other focal neuropathies; Neurologic Clinics: WB Saunders company, August 1999
The ulnar nerve derives from C8 and T1 nerve roots. It runs on the medial aspect of upper arm, and gives off no branches in the upper arm. It passes posterior to the medial epicondyle of the humerus to enter the cubital tunnel. Near elbow, ulnar nerve gives motor branches to flexor carpi and medial portion of flexor digitorum profundus.
In forearm, it gives rise to a palmar cutaneous branch which arises near the middle of the forearm and supplies the skin on the medial part of the palm, and the dorsal cutaneous branch which arises in the distal half of the forearm and supplies cutaneous sensation on the dorsal, ulnar surface of the hand and digits 4 and 5.
At the wrist, the nerve enters Guyon's canal and divides into a superficial sensory and deep motor branch. The superficial branch supplies sensation to the palmar surface of the ring and the little fingers. The deep motor branch supplies abductor digit minimi flexor digit minimi muscles, opponens digiti minimi, third and fourth lumbricals, the palmar and dorsal interossei, the flexor pollicis brevis and adductor pollicis brevis and first dorsal interosseous. In essence, ulnar nerve supplies all small muscles of hands except abductor pollicis brevis, flexor pollicis brevis, opponens pollicis and 1st and 2nd lumbricals (which are supplied by median nerve).
Reference:
Richard S Snell, Clinical Anatomy: Lippincott Williams & Wilkins, 7th edition
Preston DC. Distal Median Neuropathies. In: Entrapment and other focal neuropathies; Neurologic Clinics: WB Saunders company, August 1999
Sensory axons are evaluated by stimulating a nerve while recording the transmitted potential (known as sensory nerve action potential or SNAP) from the same nerve at a different site. (See orthodromic and antidromic studies) SNAPs are of much lower amplitude (measured in millivolts) than compound muscle action potentials, and they often require averaging of multiple responses. Three main parameters recorded with sensory nerve conduction studies are – latencies, amplitudes and conduction velocity.
LATENCY
Latencies reflect time taken (in milliseconds) for an impulse to travel from the point of stimulation to the recording electrode. Two types of sensory distal latencies are used – peak latency and onset latency.
SNAP AMPLITUDE This is a semiquantitative measure of the number of sensory axons that conduct between the stimulation and recording sites. It is expressed in microvolts.
CONDUCTION VELOCITY This requires stimulation at a single site only because unlike CMAP, SNAP is true nerve action potential. (See motor conduction studies – parameters).
Reference:
Aminoff, MJ. Electrodiagnosis in Clinical Neurology: Nerve conduction studies, New York: Churchill Livingston, 4th edition
Kimura J. Electrodiagnosis in disease of nerve and muscle: Principles and Practice, New York: Oxford V. Press, 3rd edition
When a motor or mixed nerve is stimulated and recording is made by placing electrodes over a muscle supplied by that nerve, the recorded potential is known as compound muscle action potential or CMAP. There are three main parameters of CMAP, which are routinely evaluated during motor nerve conduction studies. They are latency, amplitude and conduction velocity.
LATENCY This is the time in milliseconds between nerve stimulation and initial deflection from baseline. It reflects the time required for action potential to travel along the fastest-conducting axons to activate the muscle fibers.
The latency includes not only the time taken for impulse to travel along the nerve till it reaches nerve terminal, but also the time taken for neuromuscular junction transmission and muscle activation.
Whenever possible, the nerve is stimulated at two points: a distal point near the recording site (distal latency) and a more proximal point (proximal latency).
Prolonged latencies are usually taken as evidence of demyelination.
AMPLITUDE This is usually measured as height in millivolts of CMAP, from the baseline to the negative peak.
CMAP amplitude is a semiquantitative measure of the number of axons conducting between the stimulating and the recording points.
Decreased CMAP amplitudes usually suggest either axon loss or conduction block from demyelination located b/w the stimulation site and recorded muscle. But it can be due to reasons other than motor nerve dysfunction (e.g. neuromuscular junction, muscle fiber etc).
CONDUCTION VELOCITY
Measurement of differences in distance and latency b/w proximal and distal stimulation sites allows calculation of conduction velocity in the segment of nerve b/w the site of stimulation and is expressed in meters per second.
Normal conduction velocities are from 40-50 m/sec in the legs and from 50-70 m/sec in the arms.
Motor conduction velocity can not be calculated by performing a single stimulation. This is because, the latency of compound muscle action potential reflects transmission across nerve, junction and muscle, measurement of true conduction velocity across the nerve will necessarily require stimulation at two points.
Decrease in conduction velocities is usually taken as sign of demyelination.
Reference:
Kimura J. Electrodiagnosis in disease of nerve and muscle: Principles and Practice, New York: Oxford V. Press, 3rd edition
Preston DC, Shapiro BE. Electromyography and Neuromuscular Disorders, Boston: Butterworth-Heinemann
The arrangement of active (G1) electrode, reference electrode (G2), and ground electrodes follows the same principle as in motor conduction, except that, in place of muscle, G1 and G2 electrodes are placed over the sensory nerves.
Sensory nerves are evaluated by stimulating a nerve while recording from the same nerve at a different site.
Antidromic sensory NCSs are performed by stimulating nerve proximally and recording distally along the sensory nerves, whereas orthodromic studies are obtained by stimulating it distally and recording it proximally along the nerve (i.e. recording sensory potentials as they travel towards CNS). See figure.
Ring electrodes are convenient to record the sensory potentials from digital nerves over the proximal and distal interphalangeal joints.
Reference:
Sethi RK, Thompson LL. The Electromyographer’s Handbook, Boston/Toronto: Little, Brown and Company, 2nd edition
Aminoff, MJ. Electrodiagnosis in Clinical Neurology: Nerve conduction studies, New York: Churchill Livingston, 4th edition
