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A 24 year old woman reports that her vision has recently become blurred and that she is having trouble with her balance. When she covers either eye, vision improves. Your examination reveals that visual function is apparently normal, and eye movements are full. But when you cover her right eye, her left eye moves down slightly. When you cover her left eye, there is no movement of the right eye. The amplitude of the downward movement in the left is present and of equal degree in all positions of eccentric gaze. You also notice a right-beating nystagmus in right gaze, left-beating nystagmus in left gaze, and upbeating nystagmus in upgaze.
Where is the lesion?
Incorrect
Incorrect
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Correct!
You should find this combination of neuro-ophthalmic abnormalities challenging. Why? First challenge: the complaint of blurred vision suggests something wrong with the
visual pathway. But patients will often interpret mild misalignment of the eyes as diplopia. The clue that the blurred vision is actually diplopia comes from the fact that
covering either eye improves vision!
Second challenge: localizing the pattern of nystagmus. It is important evidence of brainstem (rather than semicircular canal or otolith) dysfunction.
Third challenge: interpreting the pattern of ocular misalignment. The patient has a comitant vertical misalignment (“hypertropia”). In this context, it likely represents skew deviation.
Perhaps you were confused by the fact that the right eye did not move when the left eye was covered. Remember that the test performed here was the “cover test,” in which one eye is covered at a time. This patient is fixating with the right eye (maybe because she sees better with that eye). When the right eye is covered, the nonfixating left eye, which is higher than the right eye, moves downward to pick up fixation. If the examiner pauses before covering the left eye, that eye will move back up to its nonfixating higher position and the right eye will resume its fixating position. Covering the left eye, therefore, elicits no movement of the right eye! This phenomenon would not occur with the “alternate cover test,” in which the left eye will move down and the right eye will move up. Another way to detect the hypertropia is to use the Maddox rod, which would show a consistent vertical separation of the two images.
What is skew deviation? The lesion is affecting one side of the pathway carrying the vertical vestibulo-ocular reflex. That pathway travels from the medulla to the rostral midbrain and diencephalon. A unilateral or asymmetric lesion here creates an abnormal “ocular tilt reaction,” which includes torsional displacement of the eyes and vertical misalignment The patient may notice torsional displacement of the environment, and will report the vertical misalignment of the eyes (called “skew deviation”) as diplopia or blurred vision. In this case, the cause proved to be multiple sclerosis. The brainstem did not show any MRI signal abnormalities, but there were characteristic T2/FLAIR signal abnormalities in the cerebral white matter.
Second challenge: localizing the pattern of nystagmus. It is important evidence of brainstem (rather than semicircular canal or otolith) dysfunction.
Third challenge: interpreting the pattern of ocular misalignment. The patient has a comitant vertical misalignment (“hypertropia”). In this context, it likely represents skew deviation.
Perhaps you were confused by the fact that the right eye did not move when the left eye was covered. Remember that the test performed here was the “cover test,” in which one eye is covered at a time. This patient is fixating with the right eye (maybe because she sees better with that eye). When the right eye is covered, the nonfixating left eye, which is higher than the right eye, moves downward to pick up fixation. If the examiner pauses before covering the left eye, that eye will move back up to its nonfixating higher position and the right eye will resume its fixating position. Covering the left eye, therefore, elicits no movement of the right eye! This phenomenon would not occur with the “alternate cover test,” in which the left eye will move down and the right eye will move up. Another way to detect the hypertropia is to use the Maddox rod, which would show a consistent vertical separation of the two images.
What is skew deviation? The lesion is affecting one side of the pathway carrying the vertical vestibulo-ocular reflex. That pathway travels from the medulla to the rostral midbrain and diencephalon. A unilateral or asymmetric lesion here creates an abnormal “ocular tilt reaction,” which includes torsional displacement of the eyes and vertical misalignment The patient may notice torsional displacement of the environment, and will report the vertical misalignment of the eyes (called “skew deviation”) as diplopia or blurred vision. In this case, the cause proved to be multiple sclerosis. The brainstem did not show any MRI signal abnormalities, but there were characteristic T2/FLAIR signal abnormalities in the cerebral white matter.