Evaluation of the Ocular Motor System Flashcards
Strabismus is the term we use for misalignment of the visual axes. There are two basic types:
Paralytic (non-comitant) and non-paralytic (comitant)
Describe paralytic (non-comitant) strabismus
Paralytic or non-comitant is characterized by the presence of variable angles of deviation in different fields of gaze. Weakness of one or more extraocular muscles is the usual cause. These usually affect adults and cause diplopia (double vision) in the direction of gaze where the affected muscle or muscles is (are) supposed to be working the hardest. Since they occur generally in adults, amblyopia does not result from these deviations.
Describe non-paralytic strabismus
Non-paralytic or comitant strabismus is seen when the visual axes are misaligned early in childhood, usually before the age of 6, The angle of deviation is similar in all fields of gaze.
No diplopia occurs as the brain suppresses one of the images leading to amblyopia in that eye.
No specific muscle weakness is identified.
There are two types of ocular deviation, namely:
tropia and phoria.
Describe tropia. Phoria?
A patient with a tropia has the deviation of the visual axes present at all times;
A patient with a phoria has a deviation of the visual axes only when fusion is disrupted (such as covering one eye). With both eyes uncovered the fusion mechanism of the brain of the patient with a phoria sends signals to the extraocular muscles that corrects the misalignment.
An ESO deviation means the eye and the visual axes are crossed (turned inward). An EXO deviation means that the eyes are turned outward.
A HYPER deviation means that one eye is higher than the other (hypertropia). HYPO means that one eye is lower than the other. Clinically, we usually refer to the HYPER (higher) eye.
The same prefixes are used for describing phorias.
This slide shows examples of a large angle ESOtropia (upper photograph) and EXOtropia (lower photograph).
What tests can be used to verify the alignment of the visual axes?
the alternate cover test and the light reflex test
Describe how an alternate cover test is performed
The alternate cover test is performed by covering one eye and then quickly moving the cover to the other eye while the patient is fixated on a small target.
This motion is repeated several times, always pausing long enough to observe the action of the uncovered eye.
How are the results of an alternate cover test interpreted?
If no motion of the uncovered eye is noted, then the visual axes are in normal alignment.
If there is motion of the uncovered eye, the direction of the motion is noted. If the uncovered eye moves INWARD, then it was turned OUT to begin with. The inward motion brings the visual axis to fixate on the target. This would be described as an EXO deviation.
If the uncovered eye moves OUTWARD, then it was turned IN to start. This would be described as an ESO deviation.
If the uncovered eye moves DOWNWARD, then the eye was UP to begin with. This would be described as a HYPER deviation.
How is a light reflex test performed?
The light reflex test is performed by having the patient look at a distant target while the physician holds a light source in the patient’s midline at approximately arm’s length from the patient. The light is directed onto the patient’s corneas and the position of the light reflex there observed.
How should a light reflex test be interpreted?
If normal alignment of the eye is present, the light reflexes should be symmetrically positioned on the cornea. If one light reflex is displaced TEMPORALLY when compared to the other side, that eye is deviated INWARDLY (ESO deviation). If one light reflex is displaced NASALLY when compared to the other side, that eye is deviated OUTWARDLY (EXO deviation). If one light reflex is displaced DOWNWARD when compared to the other side, that eye is deviated UPWARDLY (HYPER deviation).
This test is used in patients that are not very cooperative or those with good vision in only one eye.
This diagram shows the position of the corneal light reflexes with normal alignment of the visual axes. Small deviations will NOT be visible when the patient is tested by this technique.
This diagram shows the position of the corneal light reflexes in a patient with ESOtropia. The left eye is fixating the target and the right eye is turned inward.
This diagram shows the position of the corneal light reflexes in a patient with EXOtropia. The left eye is fixating the target and the right eye is turned outward.
Movements of only ONE EYE are called ______
ductions.
ADduction means that the eye moves toward the nose (toward the midline).
ABduction mans that the eye moves towards the ear (away from the midline).
Elevation means the eye moves upward, while Depression means that the eye moves downward.
Ocular _______ are movements of both eyes in the same direction. Clinically we usually refer to these as right gaze, left gaze, etc.
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