The Phyisiology of Double Vision New Occlusion Techniques
Thomas Politzer, O.D., FCOVD, FAAO

Diplopia is a serious and intolerable sequelae to the problems of strabismus, ophthalmoplegia, gaze palsy, and decompensated binocular skills that occur in patients with head injury, stroke and other neurologically compromising conditions.
According to Morgan (1948) diplopia is, "the ocular condition characterized by the use of one eye for fixation while the other eye is directed to some other point in the field of vison." According to Gunter von Noorden (1990), "Heterotropias are manifest deviations not kept in check by fusion."

Etiology and incidence of diplopia

Causes of Strabismic Deviation

  1. Donders: Abnormal accommodative /convergence ratio and hyperopia
  2. Duke-Elder: Failure in the development of the secondary fixation reflex; disruptions of the central, peripheral and postural oculomotor mechanisms; and impaired development of the optical, sensory or motor systems
  3. Scobee: Innervational, accommodative, mechanical, functional, sensory and motor anomalies
Factors Responsible for Devaiations (von Noorden 1990)
Disruptions of Motor and Sensory Fusion
Mechanical / muscular anomalies
Structural anomalies
Accommodative and refractive factors
Neurologic factors
Paretic elements
Anomalies of the brainstem
Anomalies of convergence and divergence
Vestibular abnormalities
Anomalies of the visual pathways
Brain damage
Facial and orbital deformities
Causes of paralysis of CN III, NIV, and N VI palsies (%) (Rush and Younge 1981)
Cause III IV VI
Undetermined 23 36 27
Head Trauma 16 32 17
Neoplasm 12 4 15
Vascular 21 19 18
Aneurysm 14 2 4
Other 15 8 18
Acquired third nerve palsy (Parks 1990)
Brainstem lesion
Inflammatory conditions (meningitis, encephalitis)
Vascular (Aneurysm)
Tumor
Demyelinating disease
Trauma
CN III paralysis
The eye will be in a position of abduction, slight depression, and intorsion. Ptosis from paralysis of the levator palpebrae and possible slight proptosis. Motility limited abduction, small degrees of depression while abducted, incycloduction, and adduction that does not go beyond the primary position.
CN IV paralysis
Vertical deviation. No motility of depression when adducted. Head tilt.
CN VI paralysis
Eso deviation with gaze to the affected side. Impaired abduction beyond primary gaze.
INO (Internuclear ophthalmoplegia)
Esotropia and loss of lateral gaze to the contraleteral side. Binocular internuclear ophthalmoplegia causes esotropia and bilateral loss of lateral gaze.
Strabismus can result from an injury or anomaly of the visual cortex and pathways; midbrain nuclei; cranial nerves (III, IV, and VI); brainstem; origin, insertion and/or innervation of the extraocular muscles; vestibular system; refraction; orbital deformities and the effects of orbital space occupying lesions. Acquired nerve palsies which cause strabismus and diplopia can result from head trauma, neoplasm, vascular disease, aneurysm, brainstem lesions, inflammatory conditions (e.g. meningitis and encephalitis) and demyelinating disease.
Historical treatment of diplopia
Methods
Efficacy
Ramifications of patching
Prisms, lenses, vision therapy, surgery and pharmacologic measures have been used to help patients achieve fusion (alignment of the eyes) and alleviate diplopia. Some patients will adapt by suppressing the vision of one eye to eliminate their diplopia. If treatment is not successful and the patient does not suppress vision in one eye, intractable diplopia ensues.

Although several treatment modalities (refractive, orthoptic, surgical, and pharmacologic) are available for strabismus, the treatment of third nerve palsy and paralytic strabismus remains a challenge. von Noorden, writes, "The surgical management of a complete N III paralysis is a formidable challenge to the ophthalmologist, and the therapeutic possibilities are limited. At best, the surgeon will succeed only in moving the paretic eye into the primary position without restoring adduction, elevation, or depression to a significant degree." Parks, regarding treatment for third nerve palsy says, "Treatment involves relief of the patients diplopia. Occlusion is the best solution for the patient's diplopia."

In the population of neurologically compromised patients, patching has frequently been used to eliminate diplopia. Although patching is effective in eliminating diplopia, it creates problems by rendering the patient monocular. The chief problems of monocular vision are loss of stereopsis and reduction of peripheral visual field.

Compared to binocular vision, monocular vision results in roughly a 25% decrease in the field of vision, absence of stereopsis, decreased visual acuity (due to a lack of binocular summation) and impaired spatial orientation. Monocular individuals are disadvantaged in visual motor skills, exteroception of form and color, and appreciation of the dynamic relationship of the body to the environment, which facilitates control of manipulation, reaching and balance.

Problems from monocular vision will manifest as difficulties in eye hand coordination, clumsiness, bumping into objects and / or people, ascending or descending stairs or curbs, crossing the street, driving, various sports and other activities of daily living which require stereopsis and peripheral vision.

Preferred methods of treatment
Prisms
Fusion training

Selective partial occlusion (spot patch)

A new method of treating diplopia that does not have the limitations of traditional patching has been successfully evaluated. The "spot patch" is a procedure that eliminates diplopia without compromising peripheral vision. It is a small, usually round or oval, patch made of 3-M TransporeTM tape, 3-M blurring film (or another such translucent tape). It is placed on the inside of the lenses of glasses and directly in the line of sight contributing to the diplopia. The diameter is generally about one centimeter, but will vary on the individual angular subtense required for the particular strabismus, ophthalmoplegia, or gaze palsy. Final size and placement is determined by evaluating different sizes and shapes to arrive at the smallest one, which effectively eliminates the diplopia.

The "spot patch" works because it effectively eliminates central vision in the partially occluded eye. Diplopia is perceived as a central visual phenomena when the visual axes do not align. The size of the diplopic zone is not known for certain, but is believed by this author to correspond to Panum's fusional area, which is approximately 25 by 25 minutes of arc. Diplopia does not seem to be perceived outside of this zone.

Central vision is necessary for examining small areas of detail, visual acuity and stereopsis. Peripheral vision is necessary for evaluating space in general around the body, motion detection, orientation and mobility. With the "spot patch" central vision is sufficiently blurred so as to eliminate the diplopic image, but not so much as to completely eliminate vision. Since peripheral vision is not eliminated with the "spot patch" the patient does not lose peripheral fusion, visual field, or many of the visual components of orientation, balance and mobility.