Chapter 12: Strabismus
Heterophoria is deviation of the eyes that is held in check by binocular vision. Almost all individuals have some degree of heterophoria, and small amounts are considered normal. Larger amounts may cause symptoms depending on the level of effort required by the individual to control latent muscle imbalance.
The symptoms of heterophoria may be clear-cut (intermittent diplopia) or vague ("eyestrain" or asthenopia). Diplopia may come on only with fatigue or with poor lighting conditions, as in night driving. Usage requirements for the eyes and personality type are additional factors. Thus, there is no degree of heterophoria that is clearly abnormal, though larger amounts are more likely to be symptomatic. Except for hyperopia, high AC/A ratios, and mild cases of muscle paresis not resulting in frank heterotropia, the fundamental causes of heterophorias are unknown.
Asthenopia is sometimes caused by uncorrected refractive errors as well as by muscle imbalance. One possible mechanism is aniseikonia, in which an image seen by one eye is a different size and shape from that seen by the other eye. Spectacles with unequal lens powers in the two eyes can cause asthenopia by creating prismatic displacement of the image in one eye for gaze away from the optic axis that is too large to control (induced prism). Another mechanism that may produce symptoms is a change in spatial perception due to the curvature of the lenses or astigmatic corrections. (See Chapter 20.)
The symptoms encountered in asthenopia take a wide variety of forms. There may be a feeling of heaviness, tiredness, or discomfort of the eyes, varying from a dull ache to deep pain located in or behind the eyes. Headaches of all types occur. Easy fatigability, blurring of vision, and diplopia, especially after prolonged use of the eyes, also occur. Symptoms are more common for near visual work than for distance. Frequently, an aversion to reading develops. Symptoms can be brought on by fatigue or illness or following the ingestion of medications or alcohol.
The diagnosis of heterophoria is based on prism and cover measurements. Relative fusional vergence amplitudes are measured. While the patient views an accommodative target at distance or near, prisms of increasing strength are placed in front of one eye. The fusional vergence amplitude is the amount of prism the patient is able to overcome and still maintain single vision. Measurements are done with base-out, base-in, base-up, and base-down prisms. The important feature is the size of the amplitudes in comparison to the angle of heterophoria. While one cannot give exact norms for normal relative fusion vergence, guidelines for typical normal findings are as follows: at distance, convergence is 14Δ, divergence is 6Δ, and vertical is 2.5Δ; at near, convergence is 35Δ, divergence is 15Δ, and vertical is 2.5Δ.
Heterophoria requires treatment only if symp-tomatic. Untreated heterophoria or asthenopia does not cause any permanent damage to the eyes. Treatment methods are all aimed at reducing the effort required to achieve fusion or at changing muscle mechanics so that the muscle imbalance itself is reduced.
A. Medical Treatment:
1. Accurate refractive correction-
Occasionally, poor visual acuity is found in the presence of symp-tomatic heterophoria. Spectacles providing clear vision are sometimes all that is needed to alleviate symptoms. The clearer image allows the patient's fusional capacity to function to its fullest.
2. Manipulation of accommodation-
In general, esophorias are treated with antiaccommodative therapy and exophorias by stimulating accommodation. Plus lenses often work well for esophoria, especially if hyperopia is present, by reducing accommodative convergence. A high AC/A ratio may be effectively treated with plus lenses, sometimes combined with bifocals or miotics.
The use of prisms requires the wearing of glasses; for some patients, this is unacceptable. A trial of plastic Fresnel press-on prisms should be made before ground-in prisms are ordered. For optical reasons, larger amounts of prismatic correction produce visual distortions limiting the use of prisms in higher strengths. Furthermore, very thick lenses can result. The usual practice is to prescribe about one-third to one-half of the measured deviation, which often allows fusion to occur. Prisms can be useful for esophoria, exophoria, and vertical phorias as well.
4. Botulinum toxin type A (Botox) injection-
This treatment is well suited to producing small to moderate shifts in ocular alignment and has been used as a substitute for surgical weakening of one muscle. The main disadvantage is that the resulting effect may be variable or wear off completely months later.
B. Surgical Treatment:
Surgery should be done only after medical methods have failed. Muscles are chosen for correction according to the measured deviation at distance and near in various directions of gaze. Sometimes only one muscle needs adjustment. Adjustable sutures can be very helpful (Figure 12-8).
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