NEUROLOGY INVESTIGATIONS

Sep 2, 2008

NERVE CONDUCTION STUDIES - INTRODUCTION

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 ANATOMY

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

NORMAL EEG REPORT

Patient Information:

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.

EEG Summary:

Background consists of 8-9 Hz, 40 -50 µv, posterior dominant activity, showing normal reactivity. Occasional C4 electrode artifacts were noted during recording.

No epileptiform discharges seen.
No lateralizing or localizing signs seen.
HV and PS did not reveal any additional abnormality.

Impression:

Normal EEG record.

ARTIFACTS

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

Aug 28, 2008

ALPHA RHYTHM

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

Aug 25, 2008

FILTERS

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.