Clinical electrodiagnostic examination is composed of two main tests: nerve conduction studies and needle electromyography (EMG). Additional electrodiagnostic procedures include F waves, H reflexes, and repetitive nerve stimulation.
Nerve conduction studies assess peripheral motor and sensory functions by recording the evoked response to stimulation of peripheral nerves. There are two main types of NCS: motor and sensory.
Motor nerve conduction studies are performed by stimulating a peripheral nerve. This evokes compound muscle action potential (CMAP), which is recorded from the muscle innervated by that nerve.
Sensory nerve conduction studies are usually performed by stimulating mixed nerve and recording from cutaneous nerve or vice versa (see later).
Nerve conduction studies assess only large, heavily myelinated nerve fibers.
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
Median nerve arises from C5-T1 roots. In upper arm, it does not give any branches. Once in the forearm, the nerve passes between the two heads of the pronator teres (PT) muscle, and innervates them. The nerve then innervates flexor digitorum superficialis (FDS) and flexor carpi radialis (FCR).
The median nerve’s largest branch, the anterior interosseous nerve originates approximately 5 cm distal to the radial epicondyle, travels between the flexor pollicis longus (FPL) and flexor digitorum profundus (FDP) muscles, and finally reaches pronator quadratus, supplying all the three muscles.
Just before reaching the carpal tunnel, the palmar cutaneous branch (a sensory nerve) leaves the trunk of the median nerve and enters the palm above the flexor retinaculum, outside the carpal tunnel. Palmar cutaneous branch supplies sensation over the thenar eminence. The majority of the median nerve enters the hand via the carpal tunnel. In the palm, the median nerve terminates into motor and sensory divisions. The motor division supplies first and the second lumbricals, opponens pollicis, abductor pollicis brevis, and superficial head of the flexor pollicis brevis.
The sensory fibers of the median nerve that pass through the carpal tunnel supply sensation to the index and the middle fingers in addition to the medial thumb and lateral half of the ring finger.
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
Patient: ABC DOB: 4/8/1994 Age: 14 yrs 05 mos Sex: F Complaint: Known case of epilepsy, with increased seizure frequency Medication: Carbamazepin 200 mg three times a day
Procedure:
This is a 16-Channel EEG monitoring study, performed using conventional (10-20 system) scalp electrode placements. The EEG was reviewed in longitudinal, transverse bi-polar and referential montages. The patient was recorded while awake. High frequency filter was kept at 70 Hz and low frequency filter was kept at 1 Hz.
Artifacts are unwanted signals that are generated by sources other than those of interest. Thus, EEG artifacts are recorded signals that are non cerebral in origin. It is important to identify these artifacts and not confuse them with pathological brain activity. Commonly encountered artifacts are blink artifacts, muscle artifacts, electrode artifacts and ECG artifacts.
BLINK ARTIFACT Metabolic activity of the retina generates a steady potential difference b/w the cornea and the retina, with cornea (or front of the eye) being positive in relation to the retina. These movements cause potentials changes that are picked up mainly by frontal electrodes. The electrodes that record the largest potential changes with vertical eye movements (e.g. eye blink) are Fp1 and Fp2 because they are placed directly above the eye.
MUSCLE ARTIFACTS
The scalp muscles responsible for muscle artifact are frontals, temporalis and occipitalis muscles which lie directly under the recording electrodes. Muscle artifacts from scalp and face muscles occur mainly in the frontal and temporal regions but may be recorded by electrodes nearly anywhere on the head. Contraction of a skeletal muscle causes very short duration potentials that usually occur in clusters or periodic runs. Muscle artifacts are usually easily identified by their shape and repetition.
ELECTRODE ARTIFACTS These occur usually due to poor contact of electrodes with scalp, which may occur due to poorly applied electrodes or less commonly poorly made electrodes.
ECG ARTIFACTS Electrical potentials arising from heart are very high in amplitude; hence they readily spread to the scalp. Usually the electrical field of the cardiac activity is equipotential on the scalp, so that bipolar montages do not show significant ECG artifact. In referential recording, especially if ear is used for reference electrode placement, the ECG pickup may be appreciable. Cardiac artifacts are mostly due to QRS complexes, where R wave is most prominent. They are recognized by their characteristic form and regularity.
Reference:
Jasper R. Daube. Clinical Neurophysiology, Philadelphia: F. A. Davis Company
Fisch BJ. Spehlmann’s EEG primer, Amsterdam: Elsevier, 3rd edition
Dunn, A. T. identification of artifact in EEG recording. Am. J. EEG Technol., 7:61-71
Motor nerve conduction studies are performed by stimulating a peripheral nerve. This evokes compound muscle action potential (CMAP), which is recorded from the muscle innervated by that nerve.
Sensory nerve conduction studies are usually performed by stimulating mixed nerve and recording from cutaneous nerve or vice versa (see later).
