Oscillating or wiggling eyes are a big diagnostic challenge. You should consider two groupings, nystagmus and saccadic intrusions. Let's take nystagmus first. Nystagmus is involuntary rhythmic oscillations initiated by a slow conjugate drift of the eyes. If the slow drift is followed by an oppositely directed fast conjugate movement, call it jerk nystagmus.
If the slow drift is followed by an oppositely directed slow conjugate movement, call it pendular nystagmus. Sometimes you will struggle to tell if the nystagmus is jerk or pendular. That confusion is especially true of infantile nystagmus syndrome, which can be idiopathic or caused by an underlying vision disorder, including an intracranial tumor.
Nystagmus may occur in the horizontal plane, in the vertical plane, or in the torsional plane. In one form of pendular nystagmus, the eyes actually oscillate in a circular or oval plane. This form of nystagmus, called acquired pendular nystagmus, is most common in multiple sclerosis.
Sometimes the eyes oscillate in opposite directions. The most common form of this disconjugate nystagmus is seesaw nystagmus. One eye moves up and rotates inward, while the other eye moves down and rotates outward.
Seesaw nystagmus is associated with disorders of the diencephalon, including tumors. Nystagmus can occur in any position of gaze. You will find most jerk nystagmus limited to the extremes of side gaze, up gaze, or down gaze. On the other hand, most forms of pendular nystagmus occur in straight-ahead gaze.
There are many causes of nystagmus. Often there is a structural lesion somewhere in the brainstem, like this medullary cyst. The pattern of the nystagmus may direct you to the lesion site.
For example, downbeat nystagmus usually arises in lesions in the hindbrain, like Chiari malformation. One of the biggest diagnostic challenges is to distinguish nystagmus from another type of ocular oscillation called saccadic intrusions. In saccadic intrusions, the initial conjugate eye movement is not a slow drift, as in nystagmus, but a fast movement.
That movement is followed by an oppositely directed fast recovery conjugate movement. This common type of saccadic intrusion is called square wave jerks. The eyes move together to one side, hold fast briefly, and then move back to center.
Square wave jerks are common in progressive... supernuclear palsy, but they can be seen in any brainstem disorder, especially after severe head trauma. Here is a saccadic intrusion called ocular flutter. The eyes oscillate back and forth quickly in the horizontal plane.
An even more dramatic version is called opsoclonus. The eyes oscillate rapidly in any plane. Ocular flutter and opsoclonus are signs of a diffuse toxic, metabolic, or inflammatory disturbance that disinhibits brainstem burst cells that drive saccades. Ocular flutter and opsoclonus are often accompanied by tremor of the limbs and trunk, ataxia, and sudden body jerks called myoclonus.
Ocular flutter can also be volitional. Patients can start it by converging their eyes. It is limited to brief bursts. Convergent spasm is a common accompaniment. There should be no other neurologic manifestations.
Sometimes it is very hard to tell if the oscillations are nystagmus or saccadic intrusions. Look at these eye movements of superior oblique myokymia. The eye is making torsional oscillations because the superior oblique myokymia is making torsional muscle is being inappropriately activated, usually for unknown reasons.
Superior oblique myokymia, which is always confined to one eye, is not a nystagmus but a saccadic intrusion. reports intermittent twitching and blurring of vision in the affected eye and sometimes diplopia. Anticonvulsant medications sometimes quiet these disturbing eye movements.
When the slow drift of nystagmus has a large enough amplitude, the patient will report that the viewed objects are moving, a symptom called oscillopsia. Oscillopsia does not occur when nystagmus has developed within the first few years. years of life. But if the ocular oscillations are frequent, they will reduce foveation time and cause blurred vision.
Treatments aimed at reducing ocular oscillations have been disappointing. The best approach is to go after the underlying problem, so concentrate your efforts on interpreting what you see. What's the pattern of the nystagmus? Or could you be looking at a psychotic intrusion?
These distinctions are critical to correct diagnosis.