Thread: Ocular Motility Tests[Cover Tests & Subjective ClinicalMethods]

MCQS In OCULAR MOTILITY - COVER TESTS Click Here


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-The Cover Test
-The Uncover Test
-The Cover-Uncover Test
-The Alternate Cover Test
-Bonus: Self-Assesment [MCQs]

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The Cover Test


Introduction

This module uses animations extensively to demonstrate important concepts. The material is best viewed online in a browser window. Viewing the material on a printed page may not give sufficient information for a good understanding of the material.

The cover test is a simple procedure, using only the occluder, that is used to detect the presence of an eye muscle imbalance known as a tropia.

Ortho

A patient who does not demonstrate a deviation of eye alignment is termed "ortho" or "orthophoric".



Tropia

A tropia is an eye turn or deviation that the patient has no, or very little, control over. The patient is unable to keep the eye straight with the power of fusion. Even though there is such a thing as an intermittent tropia, a tropia is a manifest deviation, meaning it is evident upon inspection and is not hidden.

Phoria

A phoria is a muscle imblance that is hidden by fusion. The eyes remain straight as long as fusion is present. The phoric deviation only becomes evident when fusion is disrupted.

Identifying deviations by direction and by deviating eye

While the fellow eye fixes on a visual target, the tropic eye deviates either toward the nose (esotropia), temporally (exotropia), superiorly (hypertropia), inferiorly (hypotropia), or a combination of a horizontal and a vertical deviation.




The deviation is further identified by the eye that is deviating.

Possibilities are:

right esotropia (RET)

right exotropia (RXT)

right hypertropia (RHT)

right hypotropia

left esotropia (LET)

left exotropia (LXT)

left hypertropia (LHT)

left hypotropia

alternating esotropia (ALT ET)

alternating exotropia (ALT XT)

A hypotropia is a bit confusing because they may be labeled as a hypertropia of the other eye. For example, a left hypotropia may be labeled as a right hypertropia, even though the left eye is deviating and the right eye is fixing. To avoid confusion only use the RHT to indicate a right eye that is deviating upward. If the left eye is deviating downward you can simply write out "left hypotropia".


An alternating tropia occurs when a patient always has a deviating eye, but can and does fixate with either eye. For example, the patient may be fixing with the right eye and the left eye may be deviating outward (XT). A moment later the patient may switch fixation to the left eye and the right eye will move to an outward deviation.


A combination left exotropia and left hypertropia might be labeled:

LXT with LHT.

Procedure

When performing the cover test, our job as technicians is to answer the question: Is there a tropia present? Yes or no. If the answer is yes, we should identify (name) the tropia. The Uncover Test (Sections 2 and 3) and the Alternate Cover Test (Section 4) will be used to detect a phoria.

Have the patient view a distant target (the test is also performed at near).

While you observe the right eye, use the occluder to cover the left eye. One of two things will happen:

1) If the right eye does not move and remains fixed on the visual target, then the right eye is not tropic.

2) If the right eye is observed to move in order to take up fixation, then the eye is tropic and the tropia is identified according to the direction the eye moved from, and which eye it is that moved.

Look at the two demonstrations below. In the top demo a right esotropia (RET) is demonstrated.

The patient is viewing a distant target. It is obvious in this example that the left eye is fixing on the target and the right eye is turning in. Small angle tropias are not this obvious. As the demo starts, an occluder moves over the left eye, forcing the right eye to search for the target. The right eye moves to take up fixation. The right eye is moving outward from an inward position, so the deviation is termed a right esotropia.

ESOTROPIA DEMO



The demo below illustrates a right exotropia (RXT) revealed by the cover test.

EXOTROPIA DEMO



We have checked the right eye with the cover test. Now we complete the cover test examination by testing the left eye.

Remove the occluder from the left eye and allow the patient's gaze to stabilize on the target.

Now cover the right eye with the occluder while you observe what happens to the left eye. As discussed above, the left eye will either remain straight, or it will move from a deviated position to take up fixation.


Exceptional situations

It is possible that an obviously tropic eye will not move to take up fixation. This typically occurs when the vision is poor in the deviating eye. The eye cannot see the fixation target, so it doesn't pick up fixation. In these cases the tropia is usually so obvious that a cover test is not needed.

A confusing situation for beginners is the patient with an alternating tropia. This patient may fixate first with the right eye and a moment later shift fixation to the left eye, and may do so back-and-forth. As fixation is shifting, the fellow eye moves to a deviated position. You may not even need an occluder for this patient. The trick is to recognize that the patient is shifting fixation from one eye to the other and to observe which way the deviating eye is moving.

The cover test detects tropias—constant visual axis deviations.

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The examiner observes the uncovered eye for movement as its fellow is covered with a paddle, thumb, or remote occluder ( Fig. 70-6 ).[6]

A nasal movement implies exotropia ( Fig. 70-7, A ), temporal movement esotropia ( Fig. 70-7, B ), upward movement hypotropia, and downward movement hypertropia ( Fig. 70-7, C ) of the uncovered eye.[7]

Each eye is covered in turn.
An accommodation-controlling fixation target of about 20/40 (6/12) size is presented to the patient, who ideally describes the target.

Small toys are suitable for young children, but bright white lights are avoided as the patient cannot accommodate on the contours of a light.

Tropias established by the cover test may be measured using the simultaneous prism and cover test; a prism of appropriate strength held in the appropriate direction is introduced before one eye as its fellow is covered ( Fig. 70-8 ).

Prism strength is increased until eye movement ceases; this prism strength corresponds to the size of the strabismus. The test is then repeated with the prism before the other eye.






Figure 70-6 Cover test.

A, The patient's left eye is examined for movement as the right eye is covered. B, The patient's right eye is examined for movement as the left eye is covered.

Figure 70-7 Cover test for tropias.

A, For exotropia, covering the right eye drives inward movement of the left eye to take up fixation; uncovering the right eye shows recovery of fixation by the right eye and leftward movement of both eyes; covering the left eye discloses no shift of the preferred right eye. B, For esotropia, covering the right eye drives outward movement of the left eye to take up fixation; uncovering the right eye shows recovery of fixation by the right eye and rightward movement of both eyes; covering the left eye discloses no shift of the preferred right eye. C, For hypertropia, covering the right eye drives downward movement of the left eye to take up fixation; uncovering the right eye shows recovery of fixation by the right eye and upward movement of both eyes; covering the left eye shows no shift of the preferred right eye. D, For exophoria, the left eye deviates outward behind a cover and returns to primary position when the cover is removed. An immediate inward movement denotes a phoria, a delayed inward movement denotes an intermittent exotropia. (Redrawn with permission from Diamond G, Eggers H. Strabismus and pediatric ophthalmology. London: Mosby; 1993.)

Figure 70-8 Simultaneous prism and cover test with right eye fixing.

The prism is moved before the fixing right eye simultaneously with the cover held before the left eye.

The uncover test requires observation of the covered eye as the cover is removed. If that eye deviated under cover, it may regain fixation or may remain deviated. The former implies the presence of a phoria, a latent deviation held in check by sensory fusion, or an intermittent tropia; the latter implies a tropia with fixation preference for the fellow eye.

Phorias may be detected more directly using the alternate cover test, in which each eye is occluded alternately to dissociate the visual axes maximally.

Care must be taken to permit time for each eye to reside behind the cover (the cover must not be “fanned” before the eyes).

Appropriately held prisms enable quantitation of the phoria ( Fig. 70-9 ). Some patients have poorly defined end points and a range over which eye movements shift from one direction to the opposite as prism strength is increased; the strabismus measurement may be estimated as the midpoint between clearly defined movements in each direction. For most clinical purposes, measurements within 2D are sufficiently accurate.

Cover test measurements are influenced by the presence of eccentric fixation; its presence must be investigated in patients who have severe amblyopia.



Figure 70-9 Alternate cover test.

Prisms of increasing strength (here base out for an esotropic patient) are held before the eye and an alternate cover test is performed. As the cover is moved from eye to eye, the amount and direction of eye movement are noted and prism strength is adjusted until no eye movement occurs. The prism strength at this point is defined as the deviation measurement.

The eye behind the prism is the “fixing” eye. If the cause of strabismus is paralytic or restrictive, patients may have greater cover test measurements when the paretic or restricted eye fixes in a given gaze position (secondary deviation) than when the sound eye fixes (primary deviation).

This phenomenon arises from Hering's law, which demands equal innervation to yoke muscles; thus, the yoke of a paralyzed or restricted muscle receives excess innervation when the pathologic eye is fixing.

Strabismus should be detected and measured in primary position at distance and near fixation and in gaze up, down, right, and left 30° from primary position.

The nine “diagnostic gaze positions” include the above plus up and right, up and left, down and right, and down and left; these are useful to measure cyclovertical muscle palsies.

For patients who have oblique muscle dysfunction, measurements are taken with the head tilted 30° right and left at distance fixation.

Subjective clinical methods include diplopia tests and haploscopic tests, which require cooperation, intelligence, and the ability of the patient to communicate the sensory percept to the examiner.

The red glass test requires the patient to alert the examiner when the red light viewed behind a red filter before the right eye and a white light viewed with the left eye are superimposed or displaced one from the other. Fusion is disrupted by the red glass and thus horizontal and vertical phorias are uncovered and measured; torsional deviations are not detected by this method. The gaze position of maximal image separation is a clue to the identity of paretic or restricted muscles. This is a useful bedside test, but accommodation is not controlled.

The Maddox rod consists of closely aligned, powerful glass or plastic cylinders. When illuminated, these cylinders project a line upon the patient's retina perpendicular to the groove orientation. The line is aligned horizontally to detect and measure horizontal phorias (accommodation cannot be controlled with this test).

Torsion may be detected and quantitated; the Maddox rod is placed in a trial frame scaled in degrees ( Fig. 70-10 ). It is common to place two Maddox rods of different colors in each trial frame cell and permit the patient to rotate each to his or her perception of the horizontal.

The torsional position of each eye may be read directly in degrees from the angular scale used for cylinder axes. Figure 70-11 exhibits a red Maddox rod before the right eye of a patient who has 10° of excyclotorsion, as read directly from the trial frame.



Figure 70-10 Maddox rod.
A series of aligned strong cylinders, here placed in a paddle handle.



Figure 70-11 Red Maddox rod.
A red Maddox rod in a trial frame may be used to evaluate subjective ocular torsion. The grooves must be aligned with the mark on the rim, as they tend to rotate within.



Haploscopic devices, such as the major amblyoscope, present each eye with a target significantly different from that presented to its fellow eye. These devices measure horizontal, vertical, and torsional deviations by alternately illuminating the fixation targets presented to each eye; the tubes are positioned appropriately until no eye movements occur.

Diplopia tests also present different images to the two eyes. These images may be identified by red-green (anaglyph) glasses or by mirrors.
The Lancaster red-green test uses a screen marked in 2° increments viewed from a distance of 6.6 ft (2 m). The examiner projects a red or green line on the screen; the patient views through the anaglyph glasses and projects the appropriate colored line upon the examiner's projected line.
The results are recorded and the glasses reversed or the projecting wands switched. Torsion may be evaluated.

The Hess screen contains illuminated red lights at fixed distances on a black screen; the patient places a green pointer light on the red lights while wearing anaglyph glasses. The glasses are reversed to test the other eye. Torsion cannot be detected.

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