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
- Latent deviation.
- Manifest deviation.
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.
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
- Symptoms of muscular fatigue: These result due to continuous use of reserve neuromuscular power. Common symptoms are
– Asthenopia (eyestrain).
– Photophobia (increased sensitivity to light).
– Difficulty in changing focus from near to distance and vice versa.
- Symptoms due to failure in maintaining binocular single vision (BSV): These are
– Blurring of vision.
– Crowding of words while reading.
– Difficulty with stereopsis.
– Intermittent diplopia.
– Intermittent squint without diplopia.
- Symptoms due to defective postural sensations: These causes
– Problem in judging distances and positions, especially of objects in motion.
Causes of heterophoria are grouped as
- Static causes or anatomical factors: Anatomical factors causing heterophoria include
– 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.
- Kinetic causes (physiological factors): These may be
– 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.
- Neurogenic causes: Lower motor neuron disease leads to incomitant heterophoria (deviations which vary with the direction of gaze) and upper motor neuron disease leads to comitant heterophoria (deviations are same in all directions of gaze for a particular fixation distance).
Risk factors for de-compensated heterophoria include
- General debility and lowered vitality.
- Mental stress.
- Inadequacy of fusion reserve.
- Advancing age.
- Precision jobs.
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
- Cover- Uncover test: It should be performed for both distance and near. One eye of the patient is covered while fixating with other eye at a distant target. The eye is then uncovered and any movement of the eye to take up fixation is noted. The test is repeated with other eye. The direction of deviation, the degree of deviation and speed of recovery is noted. If the right eye is deviated while under cover, a re-fixation movement (recovery to BSV) is observed on being uncovered. Adduction of the right eye indicates exophoria and abduction esophoria. Upward or downward movement indicates a vertical phoria. After the cover is removed, the speed and smoothness of recovery indicates the strength of motor fusion.
- Alternate cover test: The alternate cover test induces dissociation to reveal total deviation when fusion is disrupted. It is performed after cover-uncover test. The occluder is shifted quickly back and forth from one eye to other, several times. After the cover is removed, the speed and smoothness of recovery is noted as the eyes return to their pre- dissociated state. A patient with well compensated heterophoria will have straight eyes before and after the test has been performed, whereas a patient with poor control may decompensate to manifest deviation.
- Maddox rod test: The Maddox rod consists of a series of fused cylindrical red glass rods that convert the appearance of a white spot of light in to a red streak. The optical property of rod causes the streak of light to be at an angle of 90˚ with the long axis of the rods. The Maddox rod is placed in front of one eye and patient fixates with both eyes on a spot of light. The patient is asked whether the streak produced by Maddox rod passes through the fixation light seen by the other eye. When rod is placed horizontally, a vertical streak is produced and it gives an idea about exophoria or esophoria. But a vertically placed rod produce horizontal streak, and it delineates hyper- or hypophoria. Prisms may be inserted in front of one eye to make the streak pass through the point of fixation, which estimates the degree of error. Maddox tangent scale placed at one meter or five meters may be used to measure degree of heterophoria directly from the scale.
- Maddox wing test: This is used to measure degree of heterophoria at near fixation. The instrument is constructed in such a way that the right eye sees only a white vertical arrow and a red horizontal arrow, whereas the left eye sees only horizontal and vertical rows of numbers. Patient rests the front piece of Maddox wing on nose and looks through the slit in eyepieces. Right eye of patient sees white scale and the left eye sees arrow. The number on scale through which white arrow passes, gives the measurement of horizontal heterophoria. Similarly, reading on vertical scale measures vertical phorias. Cyclophoria may be found by adjusting shaft of red arrow parallel to the horizontal scale.
- Synoptophore: This may also be used to measure degree of heterophoria.
- Near point of convergence: The near point of convergence is the nearest point on which the eyes can maintain binocular fixation. It may be measured with royal air force (RAF) rule, which rests on cheeks of patient. A target is slowly moved along the rule towards the eyes of patient until one eye loses fixation and drifts laterally. The subjective near point of convergence is the point at which the patient reports diplopia. Normally, the near point of convergence should be nearer than 10 centimeter without undue effort.
- Near point of accommodation: The near point of accommodation is the nearest point on which the eyes can maintain clear focus. It may also be measured with the RAF rule. The patient fixates a line of print, which is then slowly moved towards the patient until it becomes blurred. The distance at which blurring is reported first is read off the rule and it denotes the near point of accommodation. The near point of accommodation recedes with age. When near point of accommodation recedes sufficiently far away to render reading difficult without optical correction, presbyopia is present.
- Measurement of fusion reserves: Fusion amplitudes measure the efficacy of vergence movements. These may be tested with prisms bars or the synoptophore. An increasingly strong prism is placed in front of one eye, which then abduct or adduct (depending on whether the prism is base-in or base-out), in order to maintain bifoveal fixation. When a prism of greater than fusion amplitude is reached, diplopia occurs or one eye drifts in the opposite direction, indicating the limit of vergence ability.
- Assessment of binocular vision.
Clinical types of heterophoria:
- Exophoria: It is characterised by a tendency of the eyes to diverge, which is checked by fusion amplitudes. It is a passive process unlike esophoria (which is an active process). An exophoria of less than 9 prism dioptres is usually not significant. There are three types of exophoria
– Divergence excess type.
– Basic type.
– Convergence weakness type.
- Esophoria: It is characterised by a tendency of the eyes to deviate inwards, which is held in check by fusion impulses. It is either due to hypermetropic refractive error or high accommodative convergence/ accommodation (AC/A) ratio or increased amount of near work. There are three types of esophorias.
– Convergence excess type.
– Basic type.
– Divergence weakness type.
- Hyperphoria: Hyperphoria is characterised by a tendency of the eyes to deviate upwards in a vertical direction, which is held in check by fusion amplitudes.
- Hypophoria: Hypophoria is characterised by a tendency of the eyes to deviate downwards in a vertical direction, which is held in check by fusion amplitudes.
- Cyclophoria: Cyclophoria is characterised by a tendency of the eyes to rotate around their sagittal axis (antero-posterior), which is held in check by fusion amplitudes. This can be
– 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.
- Anisophoria: Anisophoria is that type of heterophoria in which the degree of muscular imbalance varies with the direction of conjugate gaze.
Heterophoria should be differentiated from heterotropias.
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
- Exotropia lasts for more than 50% of the waking hours.
- There are symptoms of asthenopia or diplopia.
- Patient develops abnormal retinal correspondence or suppression.
- There is decrease in stereo-acuity.
- There is development of secondary convergence insufficiency.
- There is increase in deviation.
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
- Bi-medial recession: Bi-medial recession is done in cases of convergence excess.
- Bilateral resection: Bilateral resection is done in cases of divergence insufficiency.
Bowling Brad, Kanski’s Clinical Ophthalmology- A Systematic Approach. Eighth Edition. Elsevier 2016. P 748- 749.