Introduction
Heterotropia, also called strabismus, is a disorder mainly affecting children. It is also known by other terms such as ‘squint,’ ‘wall eyes,’ or ‘crossed-eyes.’ This is because both eyes cannot fixate simultaneously and on the same object. It is a quite common condition affecting one out of twenty children. It usually affects children at the age of three years.
How Is Heterotropia Caused?
The cause is not known. It is an inherited familial disorder. Normally, each eye's muscles help focus on nearby and far objects. This information is then carried to the brain via nerves through a coordinated mode. Any issue with the muscles or nerves can result in this disease.
What Are the Types of Heterotropia?
Depending upon the clinical presentation, four types of heterotropia are known, namely,
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Exotropia - The affected eye deviates outwards. This is also called ‘wall eyes.’
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Esotropia - The affected eye deviates inwards. This condition is sometimes referred to as ‘crossed eyes.’
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Hypotropia - The affected eye deviates downwards.
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Hypertropia - The affected eye deviates upwards.
What Are the Signs and Symptoms of Heterotropia?
The following are the signs and symptoms:
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Both eyes do not look in the same direction.
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The affected eye is closed in bright sunlight.
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Patient tilts their head to look at an object.
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Double vision.
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Hitting objects while walking.
What Are the Conditions Associated With Heterotropia?
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Neurological Conditions: Heterotropia is caused due to neurological problems such as cerebral palsy, stroke, and brain tumors, mainly in elderly individuals. Any damage to the nerves associated with eye movements and blood vessels can also result in heterotropia. Hydrocephalus (a condition resulting in fluid accumulation in the brain) affects children.
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Paralysis of Eye Muscles: Lateral rectus and superior oblique muscles are associated with eye movement. Paralysis of the said muscles has been linked to heterotropia.
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Faulty Refraction: Problem with accurately focusing the light.
Syndromes: Various syndromes are associated with this condition:
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Mobius Syndrome: There is the absence of incomplete development of the sixth and seventh cranial nerves (important for eye function).
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Down’s Syndrome: Children with Down’s syndrome are at a higher risk for developing heterotropia and far-sightedness. About 60 percent of children with Down’s syndrome are affected by heterotropia.
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Browne Syndrome: This syndrome concerns the superior oblique tendon (a tissue that attaches muscle to bone). An injury or inflammation may lead to damage to the tendon.
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Duane Retraction Syndrome: It is a disorder of the eyelids. Surgery is indicated for the correction of the defect.
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Cranial Nerve Palsy: Third cranial nerve palsy (oculomotor nerve) can lead to drooping of the eyelid and additional heterotropia.
How Is Heterotropia Diagnosed?
Usually, by six months, an infant starts focusing on near and far objects. Another condition called pseudostrabismus is caused due to an extra fold of skin on the inner corner of the eyes (epicanthus). Therefore, it should be ruled out. However, if heterotropia appears suddenly in an adult, it points towards something more dangerous such as an underlying brain or eye tumor (retinoblastoma).
A complete eye examination and other tests are imperative to arrive at a diagnosis of strabismus.
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History and Physical Examination: A history of premature birth, low birth weight, and allergy to eye dilating drops may point towards heterotropia. Parents often claim that the child has crossed eyes since birth. A series of family photographs can be helpful in these cases. The doctor should also include an abnormal head position, double vision, involuntary eye movements, and headaches.
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Visual Acuity: Visual acuity examination varies for different ages. The ‘CSM’ (central, steady, and maintained) method is used for infants. A torch is used to check the corneal reflex. Snellen charts, E charts, and Sheridan Gardiner tests are examples of a few visual acuity tests for children.
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Refraction Eye Test: This is a routine eye exam, also known as a vision test, that uses instruments such as corrective glasses or lenses.
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Light Reflection Test: A light is shined into the patient’s eyes. In the case of aligned straight eyes, the light will reflect in the center of the patient’s eyes. If a patient has heterotropia, the light will shine on the eye's iris or away from the eye’s center.
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Cover Test: In this test, the eye looking at the target is covered. As a result, the malaligned eye will automatically align itself and focus on the target.
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Retinal Examination: After administering dilating eye drips to the concerned eye, internal structures such as the retina are examined for structural issues.
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Other Investigations: Investigations such as brain imaging should be done to rule out neurological diseases in adults suffering from heterotropia. Thyroid disorders, namely Grave’s disease, have been associated with heterotropia. Hence, thyroid function tests should be routinely done.
How Is Heterotropia Managed?
The management is aimed at the disease severity. Therefore, an eye specialist takes a thorough history and a complete physical examination of the patient before starting the treatment. The following are treatment modalities for heterotropia:
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Refractive Error Correction: Patients can wear corrective glasses to correct mild heterotropia. In some children, bifocal lenses are needed to straighten the eye gaze.
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Eye Exercises: Three simple exercises are advised, namely, barrel cards, brock string exercise, and pencil pushups.
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Eye Patches: These are very effective in treating children suffering from heterotropia. A patch may correct malalignment after vision correction.
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Medications: Eye drops containing Atropine can help rectify refractive errors. Atropine temporarily blurs the vision from the affected eye and conditions the brain to realign and correct the vision. As a result, children can safely use it. Another example of corrective eye drops is Ecothiopate iodide, used in 0.125 % once daily as a short-term treatment.
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Surgery: Surgery should be the last resort in treating heterotropia. The ideal age for surgery in infants is less than two years. Surgery is of two types; weakening or strengthening, depending on the muscle condition. In children, general anesthesia is used. Minimal discomfort is expected, and patients can resume day-to-day activities after a few days of surgery. Follow-up is very important.
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Botulinum Toxin Type-A: Botulinum toxin has been tried along with surgery showing promising results. It causes temporary paralysis of eye muscles leading to condition improvement.
What Are the Complications of Heterotropia?
If untreated, patients can suffer from the following complications:
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Amblyopia (Lazy Eye): Reduced vision can develop from heterotropia. As a result, vision loss in one or both eyes can occur. Heterotropia and Amblyopia can occur at the same time. Patching, eye drops, exercise, and surgery can correct Amblyopia at an earlier stage.
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Blurred Vision: This can affect the patient’s performance at school or work.
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Blindness: Permanent loss of vision can also occur.
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Diplopia: Diplopia or double vision makes the patient see two objects instead of one.
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Headache: Headache is caused due to difficulty in focusing and eye strain.
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Low Confidence: Heterotropia has a negative psychological and social impact on a child’s development. Hence, early correction improves the child’s overall welfare.
Conclusion
Heterotropia is a condition during which both the eyes cannot focus on the same object simultaneously. It mainly affects children. Early detection and intervention in a heterotropic patient significantly improve vision and protect against vision loss. It can be initially managed by eye exercises and medications and has a good prognosis.
