Median sensory conduction study (orthodromic) is performed by placing active electrode over the median nerve at wrist, (midline), reference electrode about 3 cm proximal to active electrode and ground electrode b/w stimulator and active electrode. Median nerve is stimulated at the index finger. (See figure)
Usually, a distal latency in excess of 3.5 ms is taken as abnormal. Similarly, SNAP (sensory nerve action potential) amplitude of less than 5 µv or conduction velocity of less than 50 m/s is taken as abnormal. However, these values may vary b/w various populations, machines etc, thus it is advisable to generate a normative data for each centre.
It is difficult to get SNAP response by single stimulation. Thus many responses are averaged to get final response. SNAP parameters are best judged by comparing values from contralateral sides.
Peroneal motor conduction study is performed by placing active electrode at extensor digitorum brevis (EDB) muscle, reference electrode at metatarsophalangeal joint of little toe and ground electrode b/w stimulator and active electrode.
Peroneal nerve is stimulated at the ankle, below fibular head and in popliteal fossa. Stimulation of nerve in popliteal fossa is difficult because nerve is lying deep in the fossa. Peroneal nerve is frequently damaged near fibular head.
Usually, a distal latency in excess of 5.0 ms is taken as abnormal. Similarly, CMAP amplitude of less than 2 mv or conduction velocity of less than 40 m/s is taken as abnormal. However, these values may vary b/w various populations, machines etc, thus it is advisable to generate a normative data for each centre.
Tibial motor conduction study is performed by placing active electrode at abductor hallucis brevis (AH) muscle, reference electrode at metatarsophalangeal joint of great toe and ground electrode b/w stimulator and active electrode. Tibial nerve is stimulated behind medial malleolus and in popliteal fossa. Stimulation of nerve in popliteal fossa is difficult because nerve is lying deep in the fossa.
Usually, a distal latency in excess of 5.0 ms is taken as abnormal. Similarly, CMAP amplitude of less than 5 mv or conduction velocity of less than 40 m/s is taken as abnormal. However, these values may vary b/w various populations, machines etc, thus it is advisable to generate a normative data for each centre.
Ulnar motor conduction study is performed by placing active electrode at abductor digiti minimi (ADM) muscle, reference electrode about 3 cm distal to active electrode and ground electrode b/w stimulator and active electrode. Ulnar nerve is stimulated at wrist, below elbow and above elbow. This is because elbow is a common site of nerve damage, thus identifying any evidence of focal demyelination in this segment of nerve is important. Usual evidence of focal demyelination is in the form of conduction block, which is defined as -fall in CMAP amplitude on proximal stimulation in excess of 50%, as compared to distal stimulation. Other evidence is in the form of presence of focal slowing. Identifying these features at elbow, and therefore stimulating the nerve both above and below elbow is important in this study.
Usually, a distal latency in excess of 3.0 ms is taken as abnormal. Similarly, CMAP amplitude of less than 5 mv or conduction velocity of less than 50 m/s is taken as abnormal. However, these values may vary b/w various populations, machines etc, thus it is advisable to generate a normative data for each centre.
Median motor conduction study is performed by placing active electrode at abductor pollicis brevis (APB) muscle, reference electrode about 3 cm distal to active electrode and ground electrode b/w stimulator and active electrode. Median nerve is stimulated at wrist and elbow.
Usually, a distal latency in excess of 3.8-4.0 ms is taken as abnormal. Similarly, CMAP amplitude of less than 5 mv or conduction velocity of less than 50 m/s is taken as abnormal. However, these values may vary b/w various populations, machines etc, thus it is advisable to generate a normative data for each centre.
The tibial nerve is the larger of the two major divisions of the sciatic nerve. It is derived from L5, S1 and S2 roots. It leaves the popliteal fossa between the heads of the gastrocnemius and supplies all muscles in the posterior compartment of the legs, i.e. gastrocnemius, soleus, plantaris, popliteus, flexor digitorum longus, flexor hallucis longus and tibialis posterior.
At the ankle the tibial nerve runs posterior to the medial malleolus under the flexor retinaculum (tarsal tunnel) to enter the foot. While coming out of (or within) tarsal tunnel, the nerve divides into four branches.
Two of these, the medial and lateral calcaneal nerves are purely sensory and supply sensation to the heel.
The other two branches, the medial and lateral plantar nerves innervate the intrinsic muscles of the foot and provide sensation to the medial and lateral sole respectively. Notably, medial plantar nerve supplies abductor hallucis brevis and lateral plantar nerve supplies abductor digiti quinti pedis.
Reference:
Richard S Snell, Clinical Anatomy: Lippincott Williams & Wilkins, 7th edition
Cimino WR. Tarsal tunnel syndrome: review of the literature. Foot Ankle 1990, 11:47.
Kimura J. Electrodiagnosis in disease of nerve and muscle: Principles and Practice, New York: Oxford V. Press, 3rd edition
Routine nerve conduction studies are performed to screen major nerves in both upper and lower limbs to get an idea about their normal function. The study is modified depending upon the clinical diagnosis or as requested by referring doctors (which are usually physicians, orthopedic surgeons, neurosurgeons or neurologists).
A routine test includes motor conduction studies performed on median and ulnar nerves in upper limb and posterior tibial and peroneal nerves in lower limbs. Sensory conduction studies are performed on median and ulnar nerves in upper limbs and sural nerves in lower limbs.
Therefore, if patient presents with wrist drop, then radial motor and sensory conduction study may be needed in addition to the above mentioned studies. Similarly, if patient presents with foot drop, superficial sensory studies may be required.
Frequency / duration Variable, from 1 Hz to 4-6 Hz
Distribution Occipital
Persistence They may last up to few seconds, and are seen mainly in stages 1, 2 & 3 of NREM sleep
Synchrony After 2 years of age, they are bisynchronous and symmetrical
Reference:
Erwin, CW, Somerville, ER and Radtke, RA. A review of electroencephalographic features of normal sleep. J. Clin. Neurophysiol. 1:253-274
Fisch BJ. Spehlmann’s EEG primer, Amsterdam: Elsevier, 3rd edition
Niedermeyer E, Lopes da Silva F. Electroencephalography: basic principles, clinical applications and related fields, Baltimore, Maryland: Williams and Wilkins, 4th edition
Shape Group of rhythmic waves characterized by gradually increasing and decreasing amplitude.
Amplitude Usually less than 50 µv, may decrease with age
Frequency / Duration 12-14 Hz / Duration more than 0.5 seconds
Distribution They are characteristically frontocentral in location
Persistence They may last up to few seconds, and are seen in mainly stages 2 and 3 of NREM sleep
Synchrony After 2 years of age, they are bisynchronous and symmetrical
Miscellaneous
Sleep spindles are usually well developed by 3-6 months of age, appearing in prolonged runs lasting 8s or longer separated by interval of less than 10 s. After that time, the duration of spindle bursts decreases.
Spindles are commonly asynchronous over the two hemispheres until the age of 8 months in normal infants; continuously asynchronous spindles after 2 years of age are abnormal.
Spindle bursts are fairly asymmetrical in normal infants, but a marked and persistent reduction on one side may suggest ipsilateral cerebral dysfunction.
Reference:
Hughes JR. Sleep spindles revisited. J. Clin. Neurophysiol. 2: 37-44.
Jankel, WR and Niedermeyer, E. Sleep spindles. J. Clin. Neurophysiol. 2: 1-36.
Fisch BJ. Spehlmann’s EEG primer, Amsterdam: Elsevier, 3rd edition
Niedermeyer E, Lopes da Silva F. Electroencephalography: basic principles, clinical applications and related fields, Baltimore, Maryland: Williams and Wilkins, 4th edition
Sciatic nerve originates form the L4 thru’ S2 roots. It leaves pelvis by passing thru’ the greater sciatic foramen and enters thigh. In the upper popliteal fossa, it divides into common peroneal and tibial nerves. Within the sciatic nerve, the fibers that eventually form the common peroneal and tibial division run separately from each other.
In the upper thigh, tibial division provides innervation to all hamstring muscles except short head of biceps femoris which is supplied by the peroneal division. Thus, short head of biceps femoris is the only peroneal innervated muscle above knee joint.
Soon after separating from tibial division, the common peroneal gives off the lateral cutaneous nerve of the calf, which innervates the skin over the upper third of the lateral aspect of the leg (not highlighted in figure). Then the peroneal nerve winds around the fibular neck and divides into its terminal braches, the superficial and deep peroneal nerves.
Superficial peroneal nerve The superficial peroneal nerve is predominantly sensory; it innervates the skin of the lower two thirds of the lateral aspect of the leg and the dorsum of the foot and sends motor branches to the peroneus longus and brevis.
Deep peroneal nerve The deep peroneal nerve is predominantly motor; it innervates tibialis anterior, extensor hallucis, extensor digitorum longus & brevis (all ankle and toe extensors) and peroneus tertius. It sensory branches supply the skin of the web space b/w the first and second toe.
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
Katriji MB, Wilbourn AJ. Common peroneal neuropathy: a clinical and electrophysiologic study of 116 lesions. Neurology 1988;38:1723.
SYNONYM = K WAVE Shape Consist of an initial sharp component, followed by a slow component that fuses with a superimposed fast component. It may or may not be followed by sleep spindles. It is easily differentiated from vertex waves by longer duration and greater complexity and variation.
Amplitude More than 200 µv in monopolar
Duration More than 500 ms
Distribution Frontal and vertex region
Persistence They are seen at irregular intervals in stages 2, 3 and 4 of NREM sleep.
Synchrony Bisynchronous
Miscellaneous
V waves and K complexes appear in well developed from for the first time at the age of 5-6 months.
They can be elicited during sleep by sensory stimulation (particularly auditory). The positive component usually occurs 0.75 seconds after the stimulus.
Reference:
Fisch BJ. Spehlmann’s EEG primer, Amsterdam: Elsevier, 3rd edition
Niedermeyer E, Lopes da Silva F. Electroencephalography: basic principles, clinical applications and related fields, Baltimore, Maryland: Williams and Wilkins, 4th edition
Stern JM, Engel J. Atlas of EEG patterns, Philadelphia: Lippicott Willams & Wilkins
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
Motor NCSs are performed by stimulating a motor or mixed peripheral nerve while recording the CMAP from a muscle innervated by that nerve. Standard anatomic locations of recording electrodes (and stimulation) provide reproducible potentials.
For motor nerve conduction studies, the preferred method is belly-tendon recording. In this arrangement, active electrode (known as G1) is placed on the belly of the muscle and reference electrode (known as G2) on the tendon. The ground electrode is usually placed between the stimulating and recording electrodes.
The nerve is stimulated at two or more points along its course. Typically, it is stimulated distally near the recording electrode and more proximally to evaluate its proximal segment. This is important, since for measurement of conduction velocity in motor conduction studies, single site stimulation may not be enough (see later).
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
Nerve stimulation is achieved with surface electrodes placed over a nerve where it is relatively superficial, such as ulnar nerve at elbow. The stimulator has a cathode and an anode. Normally both electrodes are placed over the nerve trunk, with the Cathode being Closer to the recording site (SEE INTRODUCTION).
Supramaximal stimulation of a nerve is required. This is to make sure that all axons have been depolarized. To achieve supramaximal stimulation, a gradually increasing stimulus current is applied, resulting in progressive increase in the size of the compound muscle action potential as more and more axons in the nerve are activated. When stage is reached, where no further increase in amplitude is seen, current is increased by additional 20-30% to ensure that no further change in amplitude occurs.
Reference: 1. Aminoff, MJ. Electrodiagnosis in Clinical Neurology: Nerve conduction studies, New York: Churchill Livingston, 4th edition 2. Kimura J. Electrodiagnosis in disease of nerve and muscle: Principles and Practice, New York: Oxford V. Press, 3rd edition
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
The frequency of EEG waves is divided into 4 frequency bands 1. Delta frequency band- under 4 Hz 2. Theta frequency band- 4 to 7 Hz 3. Alpha frequency band- 8 to 13 Hz 4. Beta frequency band- over 13 Hz
The activities seen in the EEGs of awake adults consist of frequencies in the alpha and beta ranges, with the alpha rhythm constituting the predominant background activity.
It is important to understand the difference b/w alpha activity and alpha rhythm. Alpha activity refers to any activity in the range b/w 8 and 13 Hz. While alpha rhythm is a specific rhythm consisting of alpha activity having following properties:
Rhythm at 8-13 Hz occurring during wakefulness over the posterior region of the head, generally with higher voltage over the occipital areas.
The amplitude of alpha waves often waxes and wanes, but is mostly below 50 µv in adults.
Alpha rhythm is best seen with eyes closed and under condition of physical and mental relaxation.
Alpha rhythm is blocked or attenuated by attention especially visual or mental effort (alpha reactivity).
Reference:
A glossary of terms commonly used by clinical electroencephalographers. Elcetroencephalogr. Clin. Neurogphysiol. 37: 538-548
Electroencephalography: General Principles and Adult Electroencephalograms in: Jasper R. Daube. Clinical Neurophysiology, Philadelphia: F. A. Davis Company
Normal and pathological waveforms arising in the brain are rarely less than 0.5 Hz or more than 100 Hz. Filters are used to attenuate or exclude waveforms of relatively high or low frequency from the EEG so that waveforms in the most important range can be recorded clearly, without significant attenuation or distortion.
Two types of filters are commonly used - low frequency filters and high frequency filters.
LOW FREQUENCY FILTERS (also called high-pass filters) These filters control the response of the instrument to lower frequencies while the response to higher frequencies remains unaffected. The low filter frequency setting specifies the cutoff frequency at which sine waves are reduced in amplitude by a set percentage. This percentage varies with different EEG machines.
In Grass models, the low frequency filter settings are denominated as 0.1, 0.3, 1, 3 and 10.. These numbers denote the frequencies in Hz at which the amplitude has dropped by 20%. Thus a sine wave of 0.3 Hz is nearly abolished by a filter setting of 3 Hz, severely reduced by a setting of 1 Hz and reduced by 20% by a setting of 0.3 Hz.
HIGH FREQUENCY FILTERS (also called low-pass filters) The high frequency response of the EEG instrument is controlled by high frequency filters.
In Grass instruments these filters switches are denominated 12, 17, 35 and 70 Hz. These are the values of the frequencies for which the response has declined by 20%.
CLINICAL IMPLICATIONS During the recording, the low frequency filter should be routinely set at 1 Hz and the high frequency filter at 70 Hz.
A low-frequency filter setting higher than 1 Hz should not be used routinely to attenuate slow-wave artifacts in the record. Vital information may be lost when pathologic activity in the delta range is present.
Setting the high frequency filter at a lower frequency than usual may give high frequency artifact (e.g. muscle artifact) the misleading appearance of cerebral potentials such as epileptiform spikes or fast background activity. It can also distort or attenuate spikes and other pathologic discharges into unrecognizable forms.
Reference:
Electricity and Electronics in Clinical Neurophysiology, in: Jasper R. Daube. Clinical Neurophysiology, Philadelphia: F. A. Davis Company
Fisch BJ. Spehlmann’s EEG primer, Amsterdam: Elsevier, 3rd edition
User Manual – Version 3.5 : Grass-Telefactor TWIN: Recording and analysis software 2005
Guideline One: Minimum Technical Requirements for Performing Clinical Electroencephalography. J. Clin. Neurophysiol. 11 (1) 2-5, Raven Press Ltd. New York
The selection of electrodes for inputs 1 and 2 for any one amplifier channel (see CHANNELS AND AMPLIFIERS) is referred to as the derivations. The combination of multiple derivations is referred to as a montage. Different montages provide different views of the electrical activity in brain.
Usually, two kinds of montages are used: common electrode reference (or referential) montage and bipolar montage.
COMMON ELECTRODE REFERENCE (REFERENTIAL) MONTAGE The common electrode reference montage consists of a series of derivations in which the same electrode is used in input 2 of each amplifier. In other words, electrodes in their various placements over the scalp are all referred to one single electrode, called the reference electrode, and montage is known as the referential montage.
The reference electrodes may be cephalic or non-cephalic. Usually the reference electrodes are placed over left and right ear lobules. All ipsilateral scalp electrodes are referred to these reference electrodes.
Referential montage is useful for assessing inter-hemispheric amplitude asymmetries.
BIPOLAR MONTAGE Here, the potential difference is measured b/w pairs of neighboring electrodes going serially in an anterior-posterior or transverse plane.Thus, they are longitudinal bipolar or transverse bipolar montages.
Bipolar montages are best suited for identification of highly localized cerebral activity.
References:
Fisch BJ. Spehlmann’s EEG primer, Amsterdam: Elsevier, 3rd edition
Niedermeyer E, Lopes da Silva F. Electroencephalography: basic principles, clinical applications and related fields, Williams and Wilkins, 4th edition
EEG is defined as “the difference in voltage recorded b/w two electrodes and plotted over time, when at least one of them is placed over the scalp (see definition of EEG).” One pair of electrodes usually make up a channel. Each horizontal tracing corresponds to an electrode pair placed on a particular area of the patient's scalp.
In usual EEG recordings, 16 channels are recorded in parallel - appearing as 16 traces (as shown in figure). However number can go up to 40 depending on the indication.
Each channel of EEG recording is produced by the output of one differential amplifier. This type of amplifier (also known as balanced amplifier), is constructed to amplify only the difference in the potential b/w its two inputs, identical voltages appearing at the two inputs are not amplified and create a flat line output.
This is one way of reducing contamination of the physiological signal by electrical noise, (for example, 60 Hz noise from line voltage devices) because this noise tends to be the same at all electrode positions and cancels out when a difference in potential is formed. On the other hand, physiologic signals are usually different at different electrode positions.
References:
Electricity and Electronics in Clinical Neurophysiology : Clinical Neurophysiology by Jasper R. Daube (F. A. DAVIS COMPANY)
Technological basis of EEG Recording : Electroencephalography - Basic Principles, Clinical Applications and Related Fields by Ernst Niedermeyer (WILLIAMS AND WILKINS)
A committee of the International Federation of Societies for Electroencephalography and Clinical Neurophysiology recommended a specific system of electrode placement - now known as the international 10-20 system. Specific measurements from bony landmarks (inion, nasion and preauricular point) are used to generate a system of lines, which run across the head and intersect at intervals of 10 or 20% of their total length (hence called 10-20 system).
The standard set of electrodes for adults consists of 21 recording electrodes and one ground electrode. Measurements are made as follows:
The distance b/w nasion and inion is measured along the midline (Let us assume it is 40 cm). Along this line, the frontopolar point, Fpz (see electrode placement for description), is marked at 10% above the nasion (i.e. 4 cm above the nasion). Frontal (Fz), central (Cz), parietal (Pz) and occipital (Oz) points are marked at intervals of 20% of the entire distance (i.e. 8 cm, 16 cm, 24 cm and 32 cm respectively, above Fpz) leaving 10% for the interval b/w Oz and inion. The midline points Fpz and Oz routinely do not receive any electrode.
The distance b/w two pre-auricular points across Cz is measured. Along this line, the transverse position for the central points C3 and C4 and the temporal points T3 and T4 are marked 20 and 40% respectively from the midline.
The circumference of the head is measured form the occipital point (Oz) thru’ temporal points T3 and T4 and the frontopolar point (Fpz). The longitudinal measurement for Fp1 is located on that circumference, 5% of the total length of the circumference to the left of Fpz. The longitudinal measurements for F7, T3, T5, O1, O2, T6, T4, F8 and Fp2 are at the distance of 10% of the circumference
F3 electrode is placed at the point of intersection of two lines drawn by joining Fp1 to C3 and F7 to Fz. Similarly F4 is placed on the right side. P3 electrode is placed at the point of intersection of two lines drawn by joining O1 to C3 and T5 to Pz. Similarly P4 electrode is placed on the right side.
Electrodes placed on or near both ears in positions are called A1 and A2 or on the mandibular angles in positions called M1 and M2.
Jasper, HH. Report of committee on methods of clinical exam in EEG. Electroencephalogr. Clin. Neurophysiol. 10:370-375
Jasper HH. The ten-twenty electrode system of the International Federation. In: Internal Federation of Societies for Electroencephalography and Clinical Neurophysiology: Recommendations for the practice of clinical electroencephalography Amsterdam: Elsevier, 1983:3-10
A committee of the International Federation of Societies for Electroencephalography and ClinicalNeurophysiologyrecommended a specific system of electrode placement for use in all laboratories under standard conditions. According to their recommendations, the recording electrodes are named with a letter and a subscript.
The letter is an abbreviation of the underlying region: Fp (frontopolar), F (frontal), C (central), P (parietal), O (occipital) and A (auricular). The subscript is letter z, indicating zero or midlinesagittal placement, or a number, indicating lateral placement. Odd numbers refer to electrodes on the left; even numbers refer to electrodes on the right side of the head. Therefore, F3 would be left frontal, Fz would be midline frontal and F4 would be right frontal position.
The inferior frontal electrodes F7 and F8 are often called ‘anterior temporal’ electrodes because they are placed closed to (and record activity from) the anterior temporal area.
References:
Jasper, H. Report of committee on methods of clinical exam in EEG. Electroencephalogr. Clin. Neurophysiol. 10:370-375
Guideline One: Minimum Technical Requirements for Performing Clinical Electroencephalography. J. Clin. Neurophysiol. 11 (1) 2-5, Raven Press Ltd. New York
Recording electrodes transfer electrical potentials at the recording site to the input of the recording machine. Electrode materials should be those, which do not interact chemically with the electrolytes of the scalp. Electrodes coated with tin, silver chloride, gold etc. are satisfactory. Experimental evidence suggests that silver-silver chloride or gold disc electrodes held on by collodion provide best results.
Electrode types most commonly used in clinical EEG are metal discs or cups attached to the scalp and other recording sites. They usually have diameters of 4-10 mm. Before electrodes are applied, the application site is prepared by wiping with alcohol or abrasive electrolyte gels . Electrodes are applied over the scalp, using a paste (such as Ten20) that can both hold the electrode in place and provide good electrical contact.
Clip electrodes are sometimes used for recording from the earlobes. The clips should contain cups or discs made from the same materials as the scalp electrodes .
References:
Recording techniques: montages, electrodes, amplifiers and filters. In: Halliday et al. A Textbook of Clinical Neurophysiology. Wiley, Chichester
Guideline One: Minimum Technical Requirements for Performing Clinical Electroencephalography. J. Clin. Neurophysiol. 11 (1) 2-5, Raven Press Ltd. New York
Hans Berger (1873-1941) was the discoverer of the human EEG1. The electroencephalograph (or EEG) records spontaneous electrical activity generated in the cortex. It is more precisely defined as “the difference in voltage recorded b/w two electrodes and plotted over time, when at least one of them is placed over the scalp”.2 For example, in referential montage (to be discussed later), one electrode is commonly placed over ear, while others are placed over the scalp.
Spontaneous electroencephalographic activity is a reflection of currents flowing in the extracellular space. Thus, main generators of spontaneous electrical activity (or EEG) are the excitatory and inhibitory postsynaptic potentials of pyramidal cells generated in cortex3, with contributions from intrinsic cell currents. This spontaneous activity of cortical neurons is much influenced and synchronized by subcortical structures, particularly thalamus and brainstem reticular formation.
The cortical neurons characteristically produce rhythmicity in the form of alpha waves and sleep spindles Current understanding of the mechanisms responsible for the production of “rhythmical EEG activity” is based largely on animal experimentations. Such experiments have suggested “thalamus” to be the pacemaker for EEG rhythmicity4. Thus, EEG rhythmicity appears to be dependent on interactions b/w the cortex and thalamus.5
References:
Brazier, M.A.B. A history of the electrical activity of the brain. The first half-century. London: Pitman
Fisch and Spehlmann’s EEG primer (ELSEVIER)
Clinical Neurophysiology by Jasper R. Daube (F. A. DAVIS COMPANY)
Morison, R. S. and Bassett, D. L. Electrical activity of the thalamus and basal ganglia in decorticate cats. J. Neurophysiol. 8: 309-314
Andersen, P., and Andersson, S.A. Physiological basis of alpha rhythm. New York: Appleton-Century-Croft
