Adult strabismus (AS), also known as squint eyes, is a person's inability to focus both eyes in one direction. Each eye focuses on a different region, hence compromising the vision. In the majority of the patients, strabismus is present from childhood.

How Can Adults Acquire Strabismus?

Squint in adults can be broadly classified into paralytic and non-paralytic.

1) Non-paralytic is when the squint remains the same in all positions of gaze.

2) Paralytic is when the nerve supply to specific eye muscles is affected. In this scenario, the squint will be in specific gazes where the muscle/muscles are involved.

Adult onset squint is the most common type, especially in diabetics and patients with brain disorders. Latent squint manifests when fusion is broken. The squint occurs in the eye where the visual axis is obstructed. In adults, if the vision is obstructed in one eye due to a cataract, corneal opacity, etc., the latent squint becomes a manifest squint. This is the most common cause of 'non-paralytic' squint in adults.

Who Is at Increased Risk of Getting Adult Strabismus?

People who are at increased risk of acquiring adult-onset strabismus include:

People with chronic systemic autoimmune diseases like diabetes, thyroid, and myasthenia gravis (autoimmune neuromuscular disorder).
History of past head injuries or accidents.
People who have a previous history of eye surgery.
Patients who suffer from Guillain-Barre syndrome.
Shellfish poisoning.

What Are the Different Forms of Squint?

Latent squint is a type of squint that is manifested when the eyelids are closed.
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An apparent squint appears due to faulty parallelism of the optical axis.
Manifest squint occurs when the eyes are open.

What Are the Clinical Presentations to Look For in Adult Strabismus Patients?

The incidence of adult-onset strabismus is steadily increasing. The most common symptom of a patient having adult strabismus is double vision.

Other signs and symptoms observed in adult strabismus are:

A sensation of the weakness of eye muscles surrounding the eyes.
Blurred vision.
Trouble in reading.
Constant head-turning to improve vision.
Loss of depth perception.
The movement of the eye is inward (esotropia), outward (exotropia), upward (hypertropia), or downward (endotropia) direction.

How Is a Diagnosis of Adult Strabismus Achieved?

The diagnosis of strabismus comprises complete history taking, physical examination of the vision, and other investigations that help aid the diagnostic process.

History taking includes acquiring complete information about the time of onset of the disease and its symptoms. Past medical history, history of trauma, infections, or poisoning are also obtained.

Physical evaluation of the eyes includes:

Visual acuity is the test performed mainly to assess the ability of the eyes to distinguish different shapes and objects. It is performed using various charts, such as Snellen chart, illiterate e charts, etc.
Stereo acuity is measured to determine the person's perception of depth. It is done by using the Titmus-stereo fly test and the lang cards test.
Compensatory head posture is most commonly seen in these patients. With adult paretic strabismus, there is a habitual positioning of the head to compensate for the double vision.
Ocular motility is tested for all the ocular muscles involved in the process of eye movement. The patient is checked for smooth eye movements, including the ability of convergence and divergence of the eye.
The field of single binocular vision is defined as the area where the fusion of both objects occurs. It is measured by using the Hess screen test. It is investigated in patients with extraocular muscle palsy or restriction history.
Refractive errors of the eye lead to blurred vision due to the light's inability to pass to the retina. Refractive error of the eye is measured by administering 1% Cyclopentolate hydrochloride followed by a retinoscopic examination.
A forced duction test is used to assess the mobility of fibrosed muscles of the eyeball. The patient is asked to move eyeballs after anesthetizing and pulling the conjunctiva.
Parks-Bielschowsky's three-step test is performed to identify the paretic muscle in diploic vision.
Fundoscopic examinations are performed to rule out other intraocular pathologies. Other pathologies include macular scarring, retinoblastoma, and optic disc hypoplasia.

Other investigations like neuroimaging are performed to investigate cerebral stroke, diabetes, and myasthenia gravis.

What Conditions Are Ruled Out Before Forming a Diagnosis of Adult Strabismus?

Adult-onset strabismus often shares similarities with the following conditions:

Cyclic esotropia is the occurrence of esotropia (inward alignment of the eyes) in an alternate day pattern.
Convergence insufficiency is a condition in which both eyes cannot converge into a single fused vision.
Poor unilateral vision is loss of vision in one eye due to occlusion of the retinal artery.

What Are the Various Treatment Options Present to Manage Adult-Onset Strabismus?

The mainstay of treatment for adult-onset strabismus is to correct the muscular impairment and treatment of the underlying cause.

Mere observation and treatment of the underlying cause are enough for patients suffering from other disease conditions.
Typically, surgery of the eyes remains the treatment of choice for adult-onset strabismus. Surgery is mainly performed as an outpatient procedure, and patients can return to their daily routine after the procedure. Occasionally more than one surgery is required. The surgeries performed for strabismus correction are of three types:
- Weakening procedures.
- Strengthening procedures.
- Vector-adjustment procedures.
Surgery helps in repairing the extraocular muscular defects to improve vision.
Nonsurgical management of adult-onset strabismus mainly includes:
- Use of prismatic glasses to correct diploic vision.
- Injection of Botulinum toxin in the muscular region surrounding the eye helps in proper eye alignment by paralyzing the muscles. The main disadvantage of the technique is the recurrence of the condition after some time.
- Ocular muscle exercises are advised to the patients by ophthalmologists. Pencil push-up exercises are recommended to patients that improve muscle activity.
- Miotics are prescribed to improve the peripheral accommodation of the eyes.

What Are the Complications That Are Associated With Adult-Onset Strabismus?

Untreated strabismus often leads to the following complications:

Amblyopia is the most frequently occurring complication. It is characterized by one eye's defective neural pathway resulting in wandering vision.
Diplopia that is present in the initial stages of strabismus worsens.
Nystagmus is involuntary movements of the eyes causing blurred vision and loss of depth perception.
Abnormal head and neck posture in adult strabismus to improve sight results in a characteristic head tilt.

Is It Possible to Completely Get Rid Of Adult-Onset Strabismus?

The prognosis of adult-onset strabismus primarily depends on the underlying cause. Prompt treatment of the underlying cause and strabismus leads to an overall good prognosis. Patients may still suffer from vision problems.

Conclusion

Adult strabismus is an ocular manifestation of many systemic conditions. The continued presence of double vision is a prerequisite for prompt ophthalmologist referral and management.