Approach to the Patient with Diplopia

Valerie Purvin

Normal binocular vision is accomplished by focusing slightly different views of the same object on the fovea of each eye. When the visual axes are misaligned, the images fall on noncorresponding areas of the two retinas, usually experienced as diplopia. Occasional patients interpret this as blurring rather than doubling of the image; the tip-off here is that the “blur” is relieved by closing either eye. Absence of diplopia in the face of misalignment and with intact vision usually implies a very long-standing (often congenital) disorder.

I. MONOCULAR VERSUS BINOCULAR DIPLOPIA

Before embarking on an investigation of diplopia, it is important to determine whether the diplopia is monocular or binocular. For practical purposes, monocular diplopia is always due to some aberration of the ocular media, most often an uncorrected refractive error, corneal aberration, or lenticular change (cataract). Rare cases of monocular diplopia due to neurologic disease, termed “cerebral polyopia,” can be identified by the presence of diplopia in both eyes, associated homonymous visual field loss, and other symptoms of disordered visual integration. Finally, unlike monocular diplopia due to ocular disease, cerebral diplopia is not relieved with pinhole. Patients with monocular diplopia should be referred to an ophthalmologist for further evaluation and treatment.

II. HISTORY

A. Description.

After establishing that the diplopia is indeed binocular, it is helpful to inquire about some specific spatial and temporal aspects of the patient’s diplopia. Is it horizontal or vertical Is it affected by direction of gaze or by head posture Is the diplopia worse at distance or at near Horizontal diplopia at distance (usually prominent when driving) is usually due to sixth nerve palsy (NP) or other source of esotropia. Is the diplopia constant or intermittent A brief respite from diplopia immediately upon awakening is strongly suggestive of myasthenia gravis; in contrast, more prominence of diplopia with fatigue is nonspecific, frequently described by patients with an underlying phoria that periodically escapes fusion. Has the diplopia changed since onset

B. Associated symptoms.

Patients should also be questioned regarding accompanying symptoms. Is there eye or head pain The presence of pain with eye movement indicates an orbital condition, usually inflammatory. Pain in the distribution of the first division of the trigeminal nerve (V1) suggests a cavernous sinus/superior orbital fissure localiza tion. Headache in association with sixth nerve weakness is suggestive of increased intra cranial pressure (ICP). Other symptoms of increased ICP would include transient visual obscurations and pulsatile tinnitus. Elderly patients with diplopia should be questioned regarding symptoms of giant cell arteritis including scalp tenderness, jaw claudication, and proximal stiffness. In individuals with painless, pupil-sparing diplopia, myasthenia gravis is always a consideration; specific questions would include ptosis, dysphagia, dys arthria, fatigue with chewing, and limb weakness. Finally, patients should be questioned about other symptoms of potential brainstem localization such as dizziness, vertigo, ataxia, numbness, and weakness.

III. TERMINOLOGY

A. We distinguish between a tendency of the eyes to become misaligned, which comes out when binocular viewing is not allowed, termed a phoria, and an actual misalignment that is present even with both eyes viewing, termed a tropia. Phorias are sometimes referred to as latent deviations, tropias as manifest deviations. Many normal individuals have some degree of underlying phoria that, under normal viewing conditions, is easily held in check by fusional vergence mechanisms. The fusional system is fairly delicate, however, and can be disrupted by a number of factors including advancing age, intercur rent illness, minor trauma, and medications that have CNS depressant properties such as sedative/hypnotics, pain medications, and anticonvulsants.

B. The prefix attached to either term indicates the direction of the manifest or latent deviation: esotropia is an inward deviation of the eye, exotropia outward, hypertropia upward, and hypotropia downward. Common abbreviations used for charting include ET, XT, and HT, with the latter by convention referring to a hyper deviation. Orthophoria indicates that the eyes are aligned.

C. Ocular misalignment may be the same in all fields of gaze, termed comitant, or may vary depending on position of gaze, termed incomitant. This distinction is important because misalignment that is due to weakness of an extra-ocular muscle is always worse in the direc tion of action of that muscle whereas nonparetic misalignment (e.g., congenital esotropia) is the same in all fields of gaze. The term strabismus is nonspecific, simply referring to any form of ocular misalignment or deviation. Eye movements are usually tested with both eyes viewing, and such eye movements are called versions; in some cases it is helpful to observe eye movements in each eye separately (i.e., with just one eye viewing) and these move ments are called ductions. Infraduction indicates a downward movement, supraduction an upward movement. In vergence movements, the eyes are moving in different directions: convergence (inward movement) and divergence (outward movement).

IV. EXAMINATION

A. Head position and fixation.

The first step is to simply watch the patient looking at a target. Patients with a sixth NP often adopt a head turn toward the side of the lesion. A head tilt away from the side of the weak muscle is characteristic of a fourth NP. Such patients often find that their misalignment is better on up gaze (out of the field of the paretic muscle) and so they adopt, in addition, a chin-down posture. Patients with restrictive orbitopathy (e.g., thyroid eye disease) frequently have limitation of up gaze and so will often adopt a head back (chin-up) position. It is normal to have an occasional movement off the fixation target, but the presence of frequent saccadic intrusions indi cates loss of normal inhibition of brainstem pause cells and is characteristic of progres sive supranuclear palsy and cerebellar dysfunction. Simple inattentiveness to the target causing larger excursions may indicate frontal lobe disease or more global cerebral dysfunction.

B. Range of movement.

The patient is instructed to follow a target through the full range of normal eye movements. The range of motion can be quantified using degrees of excursion from primary position, normal being 45° to either side or down and 40° on up gaze or by measuring the amount of scleral show on side gaze. Limitation can also be graded with a series of hatchmarks from 1 to 4 with 1 indicating mild underaction and 4 signifying no movement past mid-position in that direction.

C. Testing saccades.

Limitation of eye movement due to cranial NP or supranuclear disorder is associated with saccadic slowing. In contrast, marked limitation with normal saccadic velocity is characteristic of orbital restrictive disease and of myasthenia. In the latter, large amplitude saccades may exhibit intrasaccadic fatigue, causing slowing at the end of the excursion. Small amplitude saccades, in contrast, will be quite rapid, sometimes appearing as small “quiver” movements. Slowing of medial rectus saccades is the most sensi tive sign of internuclear ophthalmoplegia (INO) and is extremely helpful for distinguishing this condition from other causes of adduction deficit.

D. Subjective diplopia testing.

Assessment of alignment based on simple observation of the eyes is extremely insensitive, particularly for vertical misalignments. Some infor mation concerning alignment can be obtained from subjective diplopia testing, which includes red glass and Maddox rod techniques, but objective methods are usually more informative. Whatever techniques are used, it is important to note that the displacement of the image is always opposite to the displacement of the eye. For example, in esotropia the eyes are turned in (crossed), but the images will appear uncrossed to the patient. When the eye is down, the image is up. Subjective methods are used to identify which image is coming from which eye in order to determine the direction and pattern of misalignment. For example, viewing a small white light with a red glass held over the right eye, a patient with a right sixth NP will report seeing the red light to the right of the white light and will report that the separation of the two images is greater when looking to the right and less when looking to the left. A Maddox rod transforms a point source of white light to a red line and is useful for demonstrating both phorias and tropias, whereas the red glass technique is only applicable to manifest deviations (tropias). This inexpensive and simple device is a valuable addition to the neurologist’s set of tools.

E. Objective diplopia testing.

Cover tests are the most accurate and preferred method for diagnosing strabismus. In the cover-uncover test, one eye is covered and then uncovered while the patient fixates a target first in primary position and then in differ ent positions of gaze. If a tropia is present, when the fixating eye is covered, the other eye will move to reacquire the target. The direction of this movement of redress will always be opposite to the direction of the deviation. For example, if the eye is exotropic, it will move in to take up fixation. In the alternate (or cross-cover) test, the occluder is quickly transferred from one eye to the other and back, thus preventing binocular view ing. As in the cover-uncover test, any movement of redress is noted. The cover-uncover test reveals tropias, whereas the alternate cover test will demonstrate both tropias and phorias. Although the results of such testing can be quantified with prisms, this aspect of the test is not necessary for obtaining meaningful information.

F. Head-tilt test.

Diagnosis of vertical diplopia is notoriously difficult if based on ductional deficits alone. Bielschowsky’s head-tilt test was designed to determine the paretic muscle responsible for vertical misalignment and is an enormously valuable technique (Fig. 12.1). The measurements that one plugs into the three-step test can be derived from alternate cover testing, red glass, Maddox rod, patient description, or any other technique that provides the necessary information.

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