Heterophoria or latent squint is defined as a condition in which eyes in the primary position or in their movement are maintained on the fixation point under stress only, with the aid of corrective fusion reflexes. When the influence of fusion is removed, the visual axis of one eye deviates.
Orthophoria is characterised by perfect alignment of two eyes in all positions of gaze and at all fixation distances so that the visual axes are parallel for distance and have proper convergence for near. Orthophoria as such is a rarity. A small amount of heterophoria is usually present.
George T Stevens (1886) introduced the term heterophoria and defined it as an abnormal adjustment of the eye muscles, or a tending of the visual lines in some other direction than parallelism, which applies to strabismus as well. In heterophoria binocular vision is habitually maintained, but by the expenditure of a greater amount of force than is demanded in the perfect equilibrium of the ocular muscles. Thus, deviation is kept latent by the fusion mechanism. In strabismus, diplopia is present and to overcome this, there is long suppression of one image. Therefore, the dividing line between heterophoria and strabismus rests on the ability or failure to maintain binocular vision.
The term heterophoria is derived from a Greek word, heteros meaning other, different from; and phoria meaning bringing, comparing.
All the anomalies of ocular alignment are divided into two classes
In heterophoria there is relative deviation of the visual axis held in check by a fusion mechanism and when this mechanism is disrupted the phoria breaks into tropia (squint) and thus the deviation becomes manifest.
Agarwal Sunita, Agarwal Athiya, Apple David J, Buratto Lucio, Aliό Jorge L, Pandey Suresh K, Agarwal Amar. Textbook of Ophthalmology Vol 1. Jaypee Brothers Medical Publishers (P) Ltd 2002. P 452- 456.
Stevens GT. A system of terms relating to the conditions of the ocular muscles known as ‘insufficiencies’. N Y Med J 1886; 44: 624.
Depending upon the symptoms, heterophoria may be divided into compensated and de-compensated types.
I. Compensated heterophoria: It is not associated with any symptoms. Compensation depends upon the reserve neuromuscular power to overcome the muscular imbalance.
II. De-compensated heterophoria: Symptoms arise when the fusion amplitudes are inadequate to control deviation. Even debilitating illness may precipitate symptoms in a previously asymptomatic patient. It is associated with symptoms such as
- Asthenopia (eyestrain).
- Photophobia (increased sensitivity to light).
- Difficulty in changing focus from near to distance and vice versa.
- Blurring of vision.
- Crowding of words while reading.
- Difficulty with stereopsis.
- Intermittent diplopia.
- Intermittent squint without diplopia.
- Problem in judging distances and positions, especially of objects in motion.
Causes of heterophoria are grouped as
- Orbital asymmetry: May be due to size, orientation and shape of orbits.
- Interpupillary distance (IPD) abnormalities: Wide IPD is associated with exophoria and small IPD with esophoria.
- Size and shape of globes.
- Abnormal strength or structure of extra-ocular muscles.
- Volume of retro-bulbar tissue, orbital fascias and ligaments.
- Anomalous central distribution of the tonic innervations of eyes.
- Variation to the optical axis of the eye.
- Age: Esophoria is common in younger age groups as compared to exophoria. Exophoria is more often seen in elderly group.
- Convergence: Excessive use of convergence may cause esophoria as is seen in bilateral congenital myopes. Decreased use of convergence leads to exophoria as is present in presbyopia (age-related diminution of vision).
- Accommodation: Increased accommodation leads to esophoria as is seen in hypermetropia (far-sightedness) and also in individuals doing excessive near work. Decreased accommodation is associated with exophoria as is seen in simple myopia.
- Dissociation factors: Prolonged constant use of one eye may result in exophoria. This is seen in people using uniocular microscope and in watchmakers using uniocular magnifying glass.
Risk factors for de-compensated heterophoria include
Sensorial adaptations such as abnormal retinal correspondence and suppression of image may develop to avoid foveal diplopia. Intermittent heterotropia is heterophoria breaking into heterotropia. In this, patient sometimes has heterophoria and sometimes heterotropia.
There are different methods for diagnosis of heterophorias. There is no reason to believe that heterophorias are of smaller magnitude as compared to heterotropia. Heterophoria may be as large as 25˚ and heterotropia may be as small as 5˚ (microtropia). Nature of deviation depends upon the degree of fusion amplitude.
Examination of patient comprises of detailed history and determination of refractive error (retinoscopy) under cycloplegia. Ocular movements should also be tested in all positions of gaze. Important tests in the evaluation of heterophoria include
Clinical types of heterophoria: These are
- Divergence excess type.
- Basic type.
- Convergence weakness type.
- Convergence excess type.
- Basic type.
- Divergence weakness type.
- Incyclophoria: Incyclophoria is characterised by inward rotation of the upper pole of the cornea.
- Excyclophoria: Excyclophoria is characterised by outward rotation of the upper pole of the cornea.
Heterophoria should be differentiated from heterotropias.
Management should be carried out under medical supervision.
Management is indicated for decompensated heterophoria. Lesser degrees of heterophoria without any symptoms require no treatment except for correction of refractive error, if present.
- Full correction of the refractive error is given to the patient when refraction shows significant amount of hypermetropia (+1.25 dioptres or more).
- Patients with high AC/A ratio and a symptomatic esophoria without hypermetropia may be treated with miotics or bifocal glasses.
- Base-out prisms for visual comfort in patients with non-accommodative esophoria. To prevent total inactivity of fusion divergence mechanism, one-half to one-third only of the angle of deviation is corrected. This does not correct underlying cause of latent deviation. It is used mainly in elderly patients with symptomatic esophoria who do not respond to orthoptic treatment. It may also be used in younger patients prior to surgery.
- Patients without asthenopia do not require any treatment.
- Significant refractive errors, especially anisometropia, aniseikonia, aphakia, intraocular lens (IOL) with wrong power (pseudophakia), and astigmatism, should be treated to produce sharp retinal images which increase stimulus to fuse. Hypermetropia of less than 2 Dioptres may be left uncorrected in children, though in older patients this needs to be corrected to avoid asthenopia. Presbyopia patients are given the weakest bifocal lenses which provide comfortable near vision. Half of exodeviation may be treated with base-in prisms for near vision. Myopic errors are fully corrected.
- Minus lenses may decrease exodeviation in patients with high AC/A ratio.
- In younger children with convergence insufficiency exodeviation, minus lenses as lower segment bifocals may be tried, as a temporary measure. In divergence excess exodeviation, minus lenses as upper segment bifocals may be tried.
- Prisms may be used for postoperative overcorrection of exodeviation. It is sometimes used preoperatively to enforce bifoveal stimulation.
- Prisms in glasses may also be tried in selective cases of hyperphoria. The prism is prescribed with the apex toward the direction of hyperphoria to correct one-half or at the most two-third of heterophoria only.
This is the mainstay of treatment. Patients with moderate degrees of exophoria or esophoria who have fair degree of binocular function, orthoptic execises are the treatment of choice. Patients with convergence insufficiency are given convergence exercises on synoptophore. Likewise, patient with divergence insufficiency are given divergence exercises.
III. Miotic drugs:
Miotic drugs may be useful in near esophoria due to high AC/A ratio. These facilitate peripheral accommodation so that less than normal innervation is required and consequently less than normal accommodative convergence occurs.
This is indicated in patients with intermittent exotropia who show signs of worsening in the form of
The aim of surgical procedure is to strengthen the weak muscle or to weaken the strong muscle. Elderly patients should be treated conservatively due to decreased fusion capacity. Younger patients with small surgical overcorrection may be easily treated by fusion convergence since the fusion capacity is good.
Surgical procedures which may be performed are