The goal of this article is to explain how eye movements contribute to vision and the performance of activities of daily living, how PSP can affect the motor system of the eyes and to suggest treatments directed toward alleviating some of the symptoms associated with this condition.
In 1996, an international workshop of PSP researchers convened by the Society for PSP and The National Institute of Neurological Disorders and Stroke (of the National Institutes of Health) reached consensus on clinical criteria for the diagnosis of PSP. In the words of the report, "Limitation of the range of voluntary downward vertical gaze, which is overcome by reflexive stimulation, is the established ocular motor criterion for the diagnosis of PSP." This means that there is difficulty moving the eyes downward on the physician's request, but that the eyes can move downward when the patient is asked to stare at a stationary target and the physician gently rotates the head backward. Voluntary eye movement depends heavily on the "supranuclear" eye movement circuits in the brain, while the reflex movement induced by passive head rotation uses the lower, "nuclear" eye movement centers. The consensus report allowed a diagnosis of PSP to be made without a limitation of downward gaze, but only when there are other, minor eye movement abnormalities and certain non-ocular signs such as loss of balance. Limited upward eye movement also commonly occurs in PSP, but this is a frequent finding in healthy seniors and in people with other progressive neurological conditions such as Parkinson's disease.
Generally, difficulties with eye movements in PSP are manifested by changes in the ability to see well that cannot be corrected by glasses. Probably the most common symptom is double vision, when the two eyes are not correctly aligned and pointed at the visual target. Other symptoms are more subtle: difficulty reading from one line to the next, or guiding one's utensil accurately to the mouth. According to one review, what physicians term "vertical supranuclear gaze palsy" was present in 79% of patients at the time PSP was diagnosed, while only 13% of these patients reported visual symptoms such as double vision, blurred vision, burning eyes and light sensitivity. Within the first year, 2/3 of people with PSP develop visual symptoms. Often such patients visit an ophthalmologist who checks visual fields and acuity, and finds nothing wrong, yet the symptoms are troubling: difficulty seeing moving objects, adjusting to bifocals, reading or watching television in bed. Other complaints might not be initially attributed to problems with vision or eye movements: difficulty going down stairs, spilling food from utensils, not eating food from the near half of the plate, or trouble tying shoelaces. The limited ability to move the eyes downward is often compounded by increased tone in the muscles of the neck, making downward head movement also difficult. Patients may have difficulty making eye contact, and appear to stare off into the distance and not take notice of people or objects in their environment.
In a full neuro-ophthalmologic evaluation, several different types of eye movements are examined; how patients perform on these bedside tests can often provide clues to the diagnosis of PSP. The first type of eye movement is actually not a movement at all, but is fixation - holding one's eyes stationary on a target. Normally, the eyes can be held quite still, and both eyes are pointed in the same direction. In PSP, there are an excessive number of "square wave jerks," which are small (.5 to 3 degree), rapid, side-to-side movements which typically occur once every second or two, but can be more or less frequent than that.
Sometimes, eye movement problems in PSP start with a "phoria," a tendency for the two eyes not to line up correctly when one eye is covered, but where the brain can keep the eyes aligned when both eyes have access to the visual target. When a phoria "breaks down," as often happens in PSP, there is misalignment of the eyes all the time, resulting in double vision. Together with a decreased ability to look downward, this can make reading difficult.
A second kind of eye movement is used to change the position of the eyes, as with looking from one point to another. These are called saccades, and are usually so quick that they cannot be appreciated by an observer. They are usually completed in a tenth of a second or less and are usually quite accurate, landing the eyes very close to the desired target. In PSP, these movements (especially vertical ones) are slower and smaller than usual, so that several individual movements are needed to get the eyes to their new position. Such saccades are termed "hypometric." The saccades in PSP may be slow enough for an observer to actually observe the eye movement in progress. Eventually, horizontal saccades may become affected as well.
Pursuit eye movements are used to track objects that are moving. Usually, this can be performed smoothly, matching the speed of the target and the eyes. In PSP, this type of tracking is described as "cogwheel," with many small saccadic movements required to bring the eye on target because it has fallen behind. While this is not a specific finding to PSP, it may be an early finding which can alert the physician to check for other abnormalities.
Opticokinetic eye movements are similar to pursuit, in that the eye is following a visual stimulus, but in this case the eyes are following an entire visual scene, such as the view of roadside fenceposts from the window of a moving car. This type of eye movement is normally called nystagmus; a slow following movement matched in speed to the visual surround, followed by a rapid saccadelike movement in the opposite direction to re-center the eyes. Early in PSP, patients often lose the "quick phase" of nystagmus, even in the horizontal direction. When a physician moves a striped flag in front of the patient and asks the patient to silently count the stripes as they go past, the eyes tend to get stuck at the extremes of their range of motion, and fail to jump back so they can begin to track again.
This absence of the quick phases of nystagmus can be observed following vestibular stimulation as well. The vestibular system, which comprises the parts of the inner ear and brain responsible for maintaining one's orientation in space, generates commands to move the eyes to counter the movement of the head, so that vision remains stable. With prolonged rotation of the head, such as when being gently spun in a rotating chair designed for this purpose, the eyes move in the appropriate direction, but again "get stuck" at one end of their range and no nystagmus is recorded. On the other hand, in patients who cannot voluntarily move their eyes downward, rotating the head upward can often result in a compensatory downward eye movement driven by the vestibular system.
Finally, convergence eye movements, which normally cross the eyes when looking at something close up, are often lost in PSP. This results in double vision when looking at near objects.
Eyelid movements are also affected in this disease. The number of blinks per minute is significantly reduced in patients with PSP, which can lead to dry eyes and irritation. Blepharospasm, the involuntary forced closure of the eyelids, is common in PSP, as is difficulty with eyelid opening, sometimes called "apraxia of eyelid opening." This difficulty can sometimes be overcome by relaxing or by having a specific visual target.
The eyelids are generally retracted in PSP patients, contributing to the characteristic facial expression observed. Other findings include slow eyelid movements during vertical saccades and the inability to suppress blinking when a bright light is repeatedly directed toward the eyes.
There are several different types of treatment available for the problems discussed above, although drug therapy has not been very successful. There have been some reports suggesting modest benefit from methysergide (Sansert), tricyclic antidepressants (e.g. amitriptyline, desipramine), physostigmine (a Mestinon-like drug) or dopamine agonists such as bromocriptine (Parlodel). However, none of these produces enough benefit in enough patients to justify their risks of side effects. Optical therapy consists of using prisms, slabs of glass or plastic that are triangular in cross section, to redirect the gaze. They can be ground into glasses and mounted in ordinary eyeglass frames, or glued onto one's existing eyeglass lenses. Many optometrists and ophthalmologists can fit prisms. To increase the success of these interventions, the prisms should be fitted for the specific task required. Different corrections may be required for reading versus watching television.
Double vision is sometimes treated with strabismus surgery, in which the eye muscles are re-attached to the eyeball at a different location. This procedure is often only temporarily successful, however, because as the disease progresses, the severity of double vision changes and the surgery only corrects the eyes' alignment at one point in time. It is easier to have prisms changed than to undergo repeat surgery. Sometimes the surgery might bring the eyes into position where prisms might be more effective. Patching one eye, or putting tape over one lens, will also eliminate double vision due to eye misalignment. Mirrored prisms (available from Swift Instruments, Inc. in Boston, MA) can be fitted to glasses frames to redirect patients' vision to the inferior field, allowing them to see food or reading material directly in front of themselves. A similar arrangement can be used to view television while lying in bed, and is available commercially (BedSpec TV Viewer recumbent spectacle). A reading stand or swivel-tilt-adjustable wheelchair table is often useful for keeping reading materials in a comfortable location.
Photo-gray treated or tinted lenses and side shields when going outside helps to reduce the photophobia (intolerance to bright light) and the risk of foreign objects entering the eyes because of the decreased blink rate. Eyedrops can be used to reduce irritation and keep the eye lubricated.
Lid problems can be treated as well. Patients with difficulty opening the eyelids can have lid crutches attached to the inside of the glasses frame. These are stiff wires which can gently hold the lids open. Blepharospasm and apraxia of eyelid opening can often successfully be treated with injections of botulinum toxin (Botox) into the muscles around the eye, which weakens them for months at a time, preventing their involuntary closing. Further information about blepharospasm can be obtained through: