Other Diseases of the Eye and Eyelid Muscles
3. Mixed rotary and jerk
4. Acquired pendular—Present in many directions (ie, vertical or hori-zontal) and may be different in the two eyes. It is often accompanied by signs of disease of nearby structures, for example, slurred speech, ataxia of the limbs and gait, diplopia, and tremor.
Vestibular (Peripheral or End Organ) Lesions
Nystagmus resulting from vestibular lesions occurs in one direction only, always away from the lesion. It is usually mixed, lateral, and rotary. It is never vertical or rotary only, is reduced by visual fixation, and is always ac-companied by marked vertigo and usually by tinnitus and hearing loss.
Diencephalon
This is a seesaw, pendular nystagmus with no fast component. There are conjugate rotational eye movements plus a bizarre vertical movement. The eye that rotates inward rises as the eye that rotates outward drops. The verti-cal seesaw movements may be elicited in downward gaze only. It can be congenital. The site of the lesion responsible for this type of nystagmus is not firmly established.
ASSOCIATED SIGNS AND SYMPTOMS
1. With nystagmus of brain stem or cerebellar origin,
a. Ataxia, dysarthria, diplopia, internuclear ophthalmoplegia (see Chapter 7), other cranial nerve lesions, long tract motor and sensory signs
b. Vertigo, absent or relatively mild and improved by lying still with the eyes closed
2. With nystagmus of vestibular origin,
a. Vertigo and vomiting are intense, onset of both is abrupt, and the pa-tient usually cannot be induced to move or get up; the papa-tient feels that the environment is rotating away from the side of the lesion.
b. Other signs of eighth cranial nerve dysfunction are present (see Chapter 9).
c. There are no signs of brain stem or cerebellar disease but the eyes are forced to the side of the lesion.
d. There is usually a history of tinnitus or reduced hearing.
e. If the patient can be induced to stand and walk, he staggers to the side of the lesion.
MISCELLANEOUS End-Point Nystagmus
Normal, seen at the extremes of lateral gaze, end-point nystagmus is of the small, irregular jerk type, best seen in the abducting eye. It is more marked and common in association with even minor alcohol intake.
Convergence Nystagmus
Convergence will usually dampen congenital nystagmus and peripheral vestibular nystagmus. Nystagmus seen only during convergence can be part of the dorsal midbrain syndrome.
Rebound Nystagmus
With this condition, the patient looks, for example, to the left and has left beating jerk nystagmus. It stops and when he moves his eyes back to the mary position, nystagmus reappears as right beating nystagmus in the pri-mary position. After a few seconds it stops again. This signifies intrinsic brain stem/cerebellar disease. It is important to watch the patient’s eyes as they come back to the primary position, as this may be the only position in which you see any nystagmus.
Periodic Alternating Nystagmus
This is an uncommon, startling type of nystagmus. The patient will have a spontaneous constant jerk nystagmus in one direction; it will stop after some seconds and, after a short interval of no nystagmus, will resume in the oppo-site direction. This alternating goes on constantly. It is significant in a num-ber of acquired and congenital posterior fossa disorders. It can occur in oth-erwise normal people.
Convergence-Retraction Nystagmus
On attempting to look upward, rapid repetitive convergent movements with retraction of the eyes occur. The lesion is in the midbrain (see the sec-tion on “Parinaud’s Syndrome” in Chapter 7).
Voluntary Nystagmus
Voluntary nystagmus is not a true nystagmus. It is a very rapid, usually side-to-side, oscillation or rotation with the eyes in the primary position and with voluntary overconvergence. Like the ability to voluntarily move one’s ears, a few of us can do it, but most of us cannot. The person with voluntary nystagmus can keep it up for only a few seconds at a time. This is not a man-ifestation of disease. When present in a medical student, it can win small
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gers from other medical students, as in “Did you know I can have nystagmus any time I want to?”
Optokinetic Nystagmus
Optokinetic nystagmus (OKN) is also referred to as “railroad” nystagmus.
If you have ridden on a train and watched the utility poles go by, you have had OKN in the direction (ie, fast phase) that the train is moving.
• Clinically, patients are tested for this by having them watch a revolving drum that has alternating black and white vertical stripes.
• At the bedside you may use a cloth 1.0-m measuring tape; however, if you want even better contrast, buy a roll of black electrician’s tape and stick 15-mm pieces onto your 1.0-m cloth tape, 15 mm apart. Draw it through your fingers from left to right, asking the patient to look at a number on the tape as it appears and follow it until it disappears and then look at another number and follow it.
Repeat the process from right to left. Watch the patient’s eyes. For a few seconds after the tape has stopped moving or the drum has stopped rotating, spontaneous nystagmus is seen.
Most people have OKN. The slow phase is in the following direction, and the rapid phase is the direction the tape is coming from. It can be evoked in the horizontal or vertical plane.
Who Does Not Have OKN?
1. The drowsy or uncooperative patient, who looks “through” the tape. He has no OKN to the right, left, up, or down.
2. The patient with a defect of fixation. Fixation requires a. A functioning macula and normal visual acuity
b. Contours or contrast in the object being seen; that is, you cannot fix your vision on a perfectly cloudless blue sky
c. The object seen must arouse the attention of the subject
3. The patient with a gaze palsy (see Chapter 7). OKN is absent in the di-rection of the gaze palsy.
4. The patient with a homonymous field defect—absolute or inattention—
may have absent OKN to the side of the defect. This is most consistent when the lesion is in the middle or posterior part of the optic radiation, that is, the parietal lobe. If OKN is equal to the right and left and a homonymous defect is present, the lesion is probably in the oc-cipital lobe.
5. The patient with congenital nystagmus (see the section earlier in this chapter). The OKN response in most of these patients is the opposite of what one would expect and is known as “inverted.” The OKN response can be used to differentiate acquired from congenital nystagmus; that is, if the patient has vestibular nystagmus to her right and a tape is pre-sented moving to her left, the OKN will be added to the vestibular nys-tagmus. If her nystagmus stops or reverses its direction after she looks at the tape, her nystagmus is congenital. This is a reliable way to differ-entiate congenital from acquired nystagmus and may save a patient from expensive, possibly dangerous, and needless, investigations.