Clinical Optometric Procedures: Ocular Motilities

Clinical Optometric Procedures: Ocular Motilities

CHAPTER 6 – OCULAR MOTILITIES

• This chapter includes a review of:
• Types of eye movements
• Binocular eye movements

INTRODUCTION

Ocular motilities refer to eye movements. The purpose of these eye movements is to assess the patient’s ability on conjugate eye movements. Vertical and lateral gaze movements direct the lines of sight along the Y and X axis respectively. Rotations of the eyes along these axes result in the eyes moving toward targets within any of the four quadrants. In this process the lines of sight of the eyes are directed up, down, right or left, away from the primary position of gaze (straight ahead position of gaze).

Reflexive and voluntary eye movements are controlled by a cortical network involving the frontal, parietal and occipital areas of the brain that send premotor signals to the nuclei of the 3rd, 4th and 6th cranial nerves. Voluntary movements originate in the frontal areas of the brain, while reflexive movements originate in the posterior portion of the brain.

TYPES OF EYE MOVEMENTS

Binocular Eye Movements

Conjugate eye movements

Versions are movements of both eyes in the same direction, i.e. the lines of sight are parallel to each other. They may be to the right (Fig. 6.1), left (Fig. 6.2), up or down. These movements are tested to determine if the neuromuscular systems controlling the movements are functioning and intact.

Figure 6.1 Schematic of eye movement on dextroversion (to the right) International Centre for Eyecare Education Figure 6.2 Schematic of eye movement on levoversion (to the left) Disconjugate eye movements

Vergences are movements of both eyes in opposite directions e.g. convergence (toward the nose) or divergence (away from the each other or the nose) where the lines of sight are not parallel to each other (Fig.6. 3).

• Figure 6.3 Schematic of eye movement in convergence and divergence

Torsional eye movements

These eye movements allow a rotation of the eyes clockwise and counterclockwise. They can be incyclotorsion refers to a movement when the eye rotates toward the nose and excyclotorsion is the movement of the eyes when the top of the eye rotates away from the nose.

Control of conjugate eye movements

The signal for eye movements originates in the cerebrum and is transmitted to the gaze centers in the midbrain and motor nuclei in the Pons. Signals from here are transmitted via the 3rd, 4th and 6th nerves to the EOMs.

Supranuclear pathways conduct impulses to the gaze centers while internuclear pathways coordinate the gaze centers with the motor nuclei. Abnormalities detected with conjugate eye movements informs the practitioner of lesions which involve one or more of the 3 cranial nerves controlling eye movements or lesions located at the motor nuclei in the midbrain, Pons and gaze centers in the upper midbrain or the cerebral centers where the eye movements are initiated.

Monocular Eye Movements

Ductions refer to monocular eye movements. The muscle or muscles that contract to produce the movement of the eye are the agonist(s). When 2 muscles move the eye in the same direction to produce a movement, then the muscles are synergists. For each agonist that contracts to move the eye, an antagonist muscle which produces a movement in the direction opposite the agonist, relaxes.

BINOCULAR EYE MOVEMENTS

Bifoveal fixation is the goal of the coordinated action of the EOMs. Movement of the eyes to the right is referred to as dextroversion (Fig. 6.1) while movement to the left is levoversion (Fig. 6.2). An assessment of motilities allows us to evaluate the oculomotor system. Abnormal eye movements may be caused by fatigue, poor attention span, aging, drugs or medications, and neurological problems.

Achievement of Binocular Eye Movements Conjugate eye movements are produced when the direction, speed and magnitude of rotation of the eyes are equal. This is enabled since the EOMs of both eyes are yoked together. There is identical excitatory or inhibitory innervation supplied to the corresponding yoked muscles. The equal, simultaneous innervation sent to the yoked EOMs to produce voluntary conjugate eye movements is governed by Hering’s Law of Equal Innervation. This law applied whether the eyes are fixating binocularly or monocularly.

Table 6.1 Yoked Pairs of Ocular Muscles

RIGHT EYE LEFT EYE

• Lateral rectus Medial rectus
• Medial rectus Lateral rectus
• Superior rectus Inferior oblique
• Inferior rectus Superior oblique
• Superior oblique Inferior rectus
• Inferior oblique Superior rectus

Types of Conjugate Eye Movements

There are 3 types of conjugate eye movements, viz. pursuits, saccades and vestibular eye movements. Pursuits and saccades are initiated in the cerebral hemispheres and are mediated by supranuclear pathways. Vestibular (reflex) eye movements are initiated in the ear canal and mediated by the cerebellum and brain stem.

Pursuits

Pursuits are slow, smooth tracking conjugate eye movements which are stimulated by target motion. They are elicited by asking a patient to follow a slow moving target.

Saccades

Saccades are rapid, voluntary or reflex fixational movements. It is elicited by having the patient fixate various targets.

Vestibular eye movements

Vestibular eye movements are a mix of smooth image stabilizing movements or rapid saccadic eye movements.

They occur in response to continuously moving stimuli in the visual field or to stimulation of the semicircular canals. International Centre for Eyecare Education

Pursuit Eye Movement Testing / Broad-H Test

A pursuit or broad H test refers to a following eye movement when the eye moves to follow the visual clue of an object moving in the field of vision.

The findings of the Broad H test consider the fields of action of the six extrinsic ocular muscles. The field of action of a muscle refers to the field in which a particular muscle has its greatest action, e.g. RLR has its field of action in the right hand field while RMR has its field of action in the left hand field. The consideration of the lateral and medial rectii muscles is simpler than that of the vertically acting muscles. The fields of action of these muscles are based on their muscle planes. For example, the SR and IR muscles lie in a plane that makes an angle of 23 degrees with the primary position (Fig. 6.4) and when the eye is orientated in this direction, the SR and IR muscles function as pure elevators and depressors respectively. In other words, in order to isolate a problem with the SR muscle, the practitioner would have to instruct the patient to move their eyes 23 degrees from fixation and then attempt to elevate the globe.

The SO and IO muscles of the eye lie in a plane that is 55 degrees from the primary position. Therefore, when the eye is turned inward by 55 degrees, the SO would function as a pure depressor while the IO would function as pure elevator. Limitations of eye movements when the eye looks first inward by 55 degrees and then either up or down can be isolated to a problem with the IO or SO respectively.

It is not practical to have the patient look exactly 23 degrees outward or 55 degrees inward from the primary position when testing motilities. It has been found that limitations of movement can be detected when the eye changes gaze approximately 30 to 40 degrees in either direction.

• Figure 6.4 Muscle planes of the rectii (left) and oblique (right) EOMs.

Instrumentation

Various targets may be used to evaluate the motilities of a Px, viz. penlight or transilluminator, tip of a knitting needle, Wolff ball, finger puppet or another interesting target in the case of children. However, if the practitioner wishes to observe the correspondence in the corneal reflexes on both eyes, then the penlight/transilluminator is the ideal target. International Centre for Eyecare Education Figure 6.5 Broad H testing on a patient with restricted lateral rectus function Procedure

• The examiner stands directly in front of the Px in a well lit room.
• The patient is seated comfortably with his/her head in the primary position and directly in front of the patient.
• The success of the test depends on proper instruction to the patient. The examiner should tell the patient “I

am now going to evaluate whether your eye muscles work well together. For me to do this, I need you to follow the movement of the target with your eyes while keeping your head still”. The examiner must also ensure that he/she does not move the target too fast as fixation may be lost and this would produce inaccurate fixation characteristics.

• The loss of the corneal reflex can help to indicate that the examiner has moved the target out of the binocular

field.

• The target is positioned in the primary position at a distance of 40cm away from the patient’s eyes.
• The target is moved into the 9 diagnostic positions of gaze by moving the target in a broad H pattern (Fig.

6.5).

• It may be necessary to hold the lids up when observing the patient’s eye position during downgaze to
• observe any misalignment of the corneal reflexes.
• In addition, a pattern X is also done, which checks for any oblique muscle problems.
• Observations
• One needs to take note of the following:
• Is the movement smooth / jerky / inaccurate?
• Is the movement full and unrestricted / restricted? (are there any over or under actions)
• Is the Px using head movements to follow the target?
• Is the Px feeling any eye pain or discomfort on eye movement (in conditions like retrobulbar neuritis)?
• Does the Px detect diplopia at any stage of the test?

Recording

If the patient follows the light smoothly, accurately, and with full extent of movement, the pursuits are recorded as being SAFE. SAFE means that eye movements were S = Smooth; A = accurate; F = full and E = extensive.

• Alternatively they may be recorded as being FROM = Full range of motion.
• Ocular motility may also be graded on a scale where:
• 4 = smooth and accurate
• 3 = one obvious jerky movement (fixation loss)
• 2 = two obvious jerky movements (fixation losses)

1 = more than two obvious jerky movements

• If the patient has a limited range of motion or restriction in a particular field of gaze, the practitioner must

record either an overaction or limited range of motion on the diagram of the diagnostic action fields. A plus (+) sign would indicate an overaction in a particular direction of gaze and a minus (-) sign would indicate an underaction in a particular direction of gaze.

Other findings

• End point nystagmus: When testing the horizontal meridian one may observe a slight end-point nystagmus.

This is a physiological nystagmus and is therefore not an abnormal finding.

• Midline jump: One may also observe a midline jump in the oblique meridians. This is usually found in

children at up to 6 years of age. If the Px uses head movements to follow the target then it could imply that the Px may have a muscle restriction.

Saccadic Eye Movements

Background

A saccadic eye movement refers to rapid eye movements to bring a point of regard onto the fovea. It is an example of a fast binocular eye movement. Saccades are employed in tasks like reading or shifting fixation from one object to another in daily life. Saccades are characterized by extremely high initial acceleration of up to 30000/second. They usually begin with a latency of about 200 milliseconds. Saccades are known to end just as abruptly as when they begin. Saccadic movements are produced and controlled by various central structures including the occipito-parietal cortex, the frontal lobes, the superior colliculus, the basal ganglia, the cerebellum and the brainstem. These movements may be voluntary or reflexive. Reflexive saccades are initiated by the appearance of a new target while voluntary saccades are initiated by the volition of the subject.

There are 3 aspects that are crucial in the assessment of saccadic eye movement. They include:

• Latency of the saccade which is the difference in time of the presentation of the stimulus and the movement
• intended to acquire the target. The mean latency of normal subjects is around 200ms.
• Velocity refers to the peak velocity obtained during the eye movement.
• Accuracy refers to the exactness of the movement for the target displacement. It can be either hypermetric

(overshoot) which is larger than appropriate for the target displacement or hypometric (undershoot) which is smaller than needed. Saccadic hypometria is usually normal and is only regarded as abnormal if it is extremely undershooting the target. Even a small amount of hypermetria is considered abnormal and indicates the presence of a cerebellar problem. International Centre for Eyecare Education Instrumentation

• 2 pursuit targets.
• Procedure
• 1. Targets are presented at 40cm in front of the Px in primary position of gaze.

2. Targets are held 25cm apart.

3. The Px is directed to shift fixation from one target to the other as the examiner calls them out. The examiner must ensure that the patient is maintaining a straight and stationery head position.

• 4. Targets must be presented along the horizontal, vertical and oblique meridians.
• Observations
• The examiner needs to note the following:
• Is the fixation accurate?
• If fixation is not accurate, is the Px under- or overshooting. If so, by how much.
• Is fixation on the object of regard well sustained?
• Recording
• Accurate, overshooting or undershooting.
• One must always record the directions/meridians in which the movement is produced.
• e.g. Patient is undershooting in all meridians.

Findings

• Normal individuals show a small degree of undershooting. This undershooting may be more pronounced

with aging and fatigue.

• Overshooting is less common and abnormal, and may be a sign of neurological disease. International Centre for Eyecare Education

Section A – PRELIMINARY EXAMINATION