To use this program as a guide in treating a specific heterophoria case, it is first necessary to ascertain from the clinical diagnosis whether there is a low, normal, or high distance tonic vergence (exo, normal, or esophoria at distance) and whether it is associated with a low, normal, or high AC/A ratio. Once this information is known, use the next portion of the program to enter these data plus any associated conditions (amblyopia, vertical phoria, etc.). The program will then give you a list of and suggested order of treatment options for your individual patient. Additionally, the program will allow you to ask for extra information about each individual treatment option to use when making decisions so that treatment is directed toward the most efficacious management. Thus, use of the model allows one to be guided into thinking about the order in which different treatment procedures should be considered for efficient and successful management of a specific "kind" of binocular anomaly case.
Functional correction for phorias involves different parameters than in strabismus where the principal goal is overcoming the turning of the eye and establishing good binocular vision. For phorias, the chief concern of the patient is overcoming symptoms and/or some visual performance deficiency (such as limited ability to maintain clear vision while reading or reduced stereoacuity) --- while the goal of the practitioner is usually to improve certain visual functions in some measurable way. Relieving symptoms and improving vision functions do not always go hand in hand. Possibly for this reason, the criterion that practitioners use to define functional correction of phoria cases usually involves some combination of decrease in symptoms and improvement in measured visual functions.
Based on epidemiological data of Morgan (1944) the prevalence of significant distance phorias in the population is in the neighborhood of 5% (about the same as strabismus); the prevalence of significant near phorias is considerably higher because of the additional large variance in AC/A ratios in the population. In this system for treating phorias, the direction of the distance phoria (eso or exo) does not determine the sequence of treatment options. It is the size of the AC/A ratio (low, normal, and high) that establishes the treatment sequence. The direction of the distance phoria determines certain particulars of treatment, such as whether base-out or base- in prism should be prescribed given that prism treatment is under consideration.
There is belief that better results are obtained with exophores than esophores, but this may result from the fact that some practitioners are more reluctant to prescribe base-out than base-in prisms or to prescribe the larger amount of prisms needed for esophorias than exophorias. It may also result from the fact that training an esophore's divergence usually takes longer than training an exophore's convergence. Reluctance by some practitioners to prescribe bifocals for esophoric children adds to the belief of better success with exophores than esophores. When all treatment methods are fully utilized, the prognosis is remarkably high for both esophores and exophores.
Accommodative problems are frequently associated with both esophorias and exophorias. Differences, however, occur. Exophores often have a problem when looking from a near task to a distant one --- the distance object is slow to clear or may continue to be seen blurred binocularly. One explanation for this effect is that accommodation is stimulated to promote singleness through accommodative vergence and it is slow to be released. Another explanation, not incompatible with the first, is that convergence accommodation brought about by the continually required positive fusional vergence is similarly not easily inhibited. Conversely, esophores tend to see near objects blurred often when looking from distance to near and sometimes when sustaining near fixation. The explanations are opposite to those for exophores; inhibition of accommodation at near to facilitate fusion and/or fusional divergence that reduces accommodation. Accommodative insufficiencies in these phoria patients may stem from the binocular oculomotor problem (or conversely); irregardless, the concomitant poor accommodative facility is clearly evident in these patients, often is exhibited in monocular vision, and can be quickly remediated with added lens power or more effectively with accommodative facility training. For unknown reasons, some patients without significant phorias exhibit similar accommodative difficulties; on a purely pragmatic basis clinicians are quite successful in improving accommodation and alleviating the symptoms with training.
Amblyopia that occurs in phoria patients is almost always associated with anisometropia (Flom and Bedell, 1985). Even though anisometropia is about equally common among hyperopes and myopes, amblyopia is more common and tends to be deeper among hyperopic than myopic anisometropes (Jampolsky et al., 1955). The degree of amblyopia appears to be directly correlated with the amount of hyperopic anisometropia, starting with as little as a diopter or so of anisometropia. Among myopes, however, significant amblyopia (20/40 or worse) does not appear to occur very often until the difference in myopia is quite large, say more than 6 diopters.
Clinicians have the impression that esophoria is more common than exophoria among hyperopes, and the opposite among myopes; but we are unable to find research to support this opinion. To the extent that hyperopia is more common among esophores, and myopia among exophores, amblyopia will be more common and deeper among esophores than exophores. Patients who have anisometropic amblyopia will have an average acuity of 20/60 (Flom and Bedell, 1985) and most of them will be hyperopic. Regardless of whether more of the anisometropic amblyopes have esophoria or exophoria, the treatment of the amblyopia and any associated suppression always needs to be considered immediately after prescribing prism to compensate for a vertical phoria. Occlusion needs to be either part-time and complete or full-time and partial to avoid the possibility of precipitating a strabismus. And, active treatment (orthoptics) can be mainly binocular to break down suppression and improve acuity simultaneously.
Having developed the rationale for considering the various treatment options for the three kinds of heterophoria cases, we summarize here the key points in the treatment of heterophoria.
LOW AC/A:
refractive correction
prisms
suppression treatment
sensory/motor training
Normal AC/A:
refractive correction
prisms and added lens power
suppression treatment
sensory/motor training
High AC/A:
refractive correction
added lens power and prisms
suppression treatment
sensory/motor training