Heterophoria: Description, Causes and Risk Factors:Heterophoria may be defined as that condition in which binocular vision, being temporarily suspended, the visual lines of the two eyes do not intersect at the point of fixation. It is characterized by a change in the innervation of the ocular muscles when the binocular fusion of images is prevented. Under normal conditions, binocular vision for a given point is, maintained by the co-ordinate action of the entire group of these muscles, and in the ideal eye, at least within certain limits, the innervation of these muscles is not a necessary function of the binocular act. That is to say, the binocular fusion of images being suspended, the innervation remains unaltered. It becomes a function of this act only in states of heterophoria.Each eye is surrounded by six muscles which allow us to move the eyes and point both eyes at the object we are looking at. Controlling these muscles is a delicate balancing act and occasionally the balance is upset and the eyes have a tendency to become misaligned. This can be caused by the incorrect placement of a muscle, a weakness of one or more of the muscles or a problem with the nerves supplying the muscles.In heterophoria there is a relative deviation of the visual axes held in check by the fusion mechanism. The relative position of the visual axes is determined by the equilibrium or disequilibrium of forces that keep the eyes properly aligned and of forces that disrupt this alignment. Clearly, the fusion mechanism and its anomalies are involved in some manner in producing comitant heterotropias. To understand the etiology of neuromuscular anomalies of the eyes, therefore, one should also gain an insight into other factors that determine the relative position of the visual axes.Other Risks Factors May Include:The form and position of the eyeballs (orbits).
The place of insertion of the ocular muscles.
The essential and relative power of the ocular muscles (amplitude of convergence).
The ratio of the positive and negative portions of the relative accommodation, together with the ratio of the convergence and accommodation for the point in question.
Symptoms:Heterophoria does not manifest itself as long asimage fusion is unimpaired. Where fusion is impaired as a result of alcoholconsumption, stress, fatigue, concussion, or emotional distress, the muscularimbalance can cause intermittent or occasionally permanent strabismus. Thisis then typically associated with symptoms such as headache, blurred vision,diplopia, and easily fatigued eyes.Diagnosis:Heterophoria is diagnosed by the uncover test.This test simulates the special conditions under which heterophoria becomesmanifest (decreased image fusion such as can occur due to extreme fatigue orconsumption of alcohol) and eliminates the impetus to fuse images. In contrastto the cover test, the uncover test focuses on the response of the previouslycovered eye immediately after being uncovered. Once uncovered, the eyemakes a visible adjustment to permit fusion and recover binocular vision.Treatment:The treatment of heterophoria should begin with the determination of the following points:1. The amount and character of the heterophoria:
For reading distance, 1/3 meter.
2. The mobility of the eyeballs in various directions.3. The refraction.4. The muscular power:
Amplitude of convergence.
5. The relative accommodation for infinity and for reading distance.In most cases heterophoria does not cause any symptoms so treatment is not required. If you are suffering headaches, eye strain or diplopia (double vision) as a result of a heterophoria, your optometrist will advise you on the most appropriate form of treatment.In some cases the symptoms can be relieved by carrying out exercises to strengthen the eye muscles. In other cases, spectacles may be required.Persons can overcome a heterophoria, provided there is no interference with fusion, by maintaining a tonus distribution in the extraocular muscles so that their visual axes are parallel for distance and are properly directed in near vision. Under certain circumstances, this task may be too dif?cult and may cause subjective symptoms consisting of discomfort of varying degree and location, so-called asthenopic symptoms, or diplopiaNOTE: The above information is educational purpose. The information provided herein should not be used during any medical emergency or for the diagnosis or treatment of any medical condition.DISCLAIMER: This information should not substitute for seeking responsible, professional medical care.
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