|Year : 1973 | Volume
| Issue : 1 | Page : 1-4
Electromyography of extraocular muscles (EOM)
RC Patel1, M Shahani2
1 Dept of Ophthalmology, King Edward Memorial Hospital, Bombay -12, India
2 Laboratory of Electrophysiology, Gordhandas Sunderdas Medical College, Bombay-12, India
R C Patel
Dept of Ophthalmology, King Edward Memorial Hospital, Bombay -12
Source of Support: None, Conflict of Interest: None
|How to cite this article:|
Patel R C, Shahani M. Electromyography of extraocular muscles (EOM). Indian J Ophthalmol 1973;21:1-4
| Introduction|| |
The electrophysiological studies in eye muscles can be based widely on knowledge of the electrical behaviour of peripheral skeletal muscles.
DUBOIS-REYMOND in 1848, established the `action current' principle or wave of the electrical negativity of the nerve impulse.
PIPER in 1907, obtained first recorded electromyogram.
ADRIAN AND BRONK in 1929 introduced concentric needle electrodes which made it possible to pick up potentials developed by a single motor unit.
As early as 1930, COOPER AND ECCLES studied the mechanical and electrical response of medial rectus muscle to tetanic stimulation.
With respect to extraocular muscles pioneering work was done by SHERRINGTON on reciprocal innervation and HERRING on equal distribution of nerve impulse between contralateral synergists. The outstanding papers of BJORK AND KUGELBERG have presented the first detailed study of electromyography of extraocular muscles in man.
BREININ  and his colleagues have studied various aspects of electromyography of extraocular muscles like electromyography of normal extraocular muscles, EMG of extraocular muscles in conditions of limited mobility chiefly due to neurogenic paralysis, as a tool in neurogenic diagnosis.
TAMLER, MERG AND JAMPOLSKY  studied electromyography of co-activity of human extraocular muscles in following movements, in saccadic eye movements and following movements of the eye between tertiary positions.
The recording of potentials from the extraocular muscles in man is a technique which may provide a greater insight in understanding their function.
| Material and Method|| |
Normal subjects, 13 males and 2 females between the ages of 20-70 years were chosen from patients attending out-patient department of K.E.M. Hospital, Bombay. They had no clinical manifestations of the involvement of extraocular muscles. Electromyography was carried out with concentric bipolar needle electrodes using Medelec MS-4 EMG machine.
Conjunctiva was anaesthetised with anaethaine 0.5% at intervals of five minutes. An eye speculum was put. A concentric needle electrode 25 to 50 mm. long and a diameter of 0.45 mm. made of platinum lead and insulated from steel cannula was used.
Autoclaved electrode was ,inserted through the conjunctiva into the belly of the muscle almost parallel to the muscle axis. A burst of impulses and a high crescendo sound in loud speaker indicates the proper placement of the electrode.
Electrical activity from the muscle is picked up by electrode and led off by means of shielded wires to a box type pre-amplifier connected to the machine.
The visual display of the electrical activity was made by cathode ray oscilloscope. Recordings were made photographically by 35 mm. still camera. Tracings can be suitably displayed by adjusting the sweep speed.
The electrical activity of the muscle was made audible by a loud speaker.
Recordings of electrical activity were made in primary position of the eyes, in the direction of the action of the muscle and the direction opposite to the action of the muscle. Electrical activity was studied mainly in lateral and medial recti in normal subjects.
As soon as the electrode was introduced in the muscle, the electrical activity was noted on the oscilloscope screen as well as by sharpness of sound in the loud speaker. The degree of activity recorded was dependant upon the correct site of the electrode tip in the muscle.
With the eyes in primary position and the electrode inserted in the muscle, continuous electrical activity was noted in all the subjects. Out of 15 cases, 3 cases showed lateral rectus (LR) and 2 cases medial rectus (MR) interference pattern. The range of amplitude noted was 25-149μv in case of LR and 17-125 μv in case of MR during rest. [Figure - 2]
When the eyes were moved in the direction of the action of the muscle, there was distinct increase in the amplitude, and pattern of the electrical activity from single oscillation or mixed pattern became a complex pattern of interference in all cases. [Figure - 1] The pitch of the electromyographic sound as noted from the loud speaker rose to sharp crescendo into sharp loud sound like a fast moving train in all cases. The amplitudes noted ranged from 33-958μv and 58-917 µv in cases of LR and MR respectively during the action of the muscles. The frequency noted was very fast, several hundreds sec.
Electrical activity of the muscle during the position of the eyes in the direction opposite to cardinal action of the muscle were found to be markedly inhibited. In two cases in LR no activity was noted while in all other cases single discrete potentials were noted. The duration in LR and MR ranged from 1.6 to 2.5 m sec. and 2.5 to 3.3 m sec. respectively. The amplitude ranged from 12.5 to 125 μv in LR and 50 to 125 µv in MR. [Figure - 3]
Low amplitude values were noted in old patients (60 years and above) both in MR - and LR while higher amplitude values were noted in those below 40 years.
| Discussion|| |
A striking characteristic of extraocular muscle is continuous electrical activity during resting state in primary position, as against no electrical activity in other skeletal muscles at rest. [Figure - 2]
The electrode in an extraocular muscle may record from a single unit (motor unit) which is recognised by its rhythmicity and constant size and shape. When a single unit responds a pattern of single oscillations in the form of spikes is observed.
If several units are being recorded, they will appear as separate impulses unless two or more happen to fall synchronously. In this case they will summate or show the total of their individual amplitudes. Here the response is so great that the individual units cover one another so that they cannot be individually resolved, and one gets an interference pattern. This summated pattern will show notches or humps where motor units have their maxima or minima. This pattern in extraocular muscle is seen commonly when the muscle is contracting.
When more than one unit is responding but still the response is not so great so that individual units cannot be masked, the picture is called mixed pattern.
The change from simple pattern to interference pattern [Figure - 1] is due to
(1) Increase in frequency of discharge of motor units.
(2) Recruitment of additional motor units which may be of larger amplitude.
Electrically there is no position of rest; the so called position of rest of extraocular muscles, the straight ahead position of the eyes involves constant contraction of number of motor units in all the muscles.
Sherrington in his epic work enunciated the law of reciprocal innervatic-ti. This is confirmed by electromyography of the antagonist muscles. In all cases we noticed that there was marked diminution of electrical activity, when the eyes were made to look in the direction opposite to that of the action of that particular muscle. In two cases there was no electrical activity, while in the remaining cases discrete single potentials were noted. The amplitude ranged from 12 to 150 µv, while the duration ranged from 1.6 to 3.3 m sec. Frequency ranged from 19 to 61 /sec. [Figure - 3]
The motor unit of ocular muscle is much smaller than that of the skeletal muscle as evidenced by small discrete amplitudes in the electromyograph of the antagonist muscle. The small duration of these potentials may be due to smaller fibre diameter of the ocular muscle (15.9 to 22.7 μv) than that of the other striated muscles.
In summarising it may be said that electrical activity in the ocular muscle differs in some respects from that in skeletal muscles. The ocular motor unit has a lower amplitude and shorter duration. The frequency is much higher. The individual motor units can recruit faster than the skeletal motor units. There is constant electrical activity in the so called position of rest in extraocular muscles while skeletal muscles are electrically silent in position of rest. [Table 1]
| Summary and Conclusions|| |
Electromyography was done in 15 normal volunteers.
Electromyography of extraocular muscles in man is clinically feasible, relatively simple and without ill effects. It provides important tool for physiological study of innervational factors of extraocular muscles.
Electromyography of extraocular muscles show that motor unit is simpler, and smaller than skeletal muscles, shorter in duration and can fire at a greater frequency.
There is no position of innervational rest. Reciprocal innervation in vergences and versions is exhibited by gradual decrement of the antagonists and augmentation of agonists. The field of action of a muscle can be readily demonstrated.
| References|| |
Breinin, G. M. and Moidaver J.: Electromyography of human extraocular muscles. Arch. Oph. 54: 206, 1955.
Hughes. W. F.: EMG of Eye Muscles Year Book of Ophthalmology, 1963-64, p. 66.
Jampolsky, A., Tamler and Merg: Artefacts and normal variations in human EMG. Arch. Oph. (Chicago) 61: 402, 1959.
Licht, S.: Electrodiagnosis and Electromyography. Elizabeth Licht Publications, 1956.
Merg, E. et al.: Elements of human extraocular muscle electromyography. Arch. Oph. (Chicago) 61: 258, 1959.
Miller, J. E.: The EMG of Vergence movement. Arch. Oph. (Chicago) 62: 790, 1959.
Moses, R. A.: Adler's Physiology of the Eye. 5th Edition, The C. V. Mosby Company.
Tamler, E., Merg, E. and Jampolsky, A. et al.: EMG of human saccadic eye movements. Arch. Oph. (Chicago) 62: 657, 1959.
Tamler, E. and Jampolsky, A. et al.: EMG study of following movements of the eye between tertiary positions. Arch. Oph. (Chicago) 62: 804, 1959.
[Figure - 1], [Figure - 2], [Figure - 3]