Strabismus, commonly known as squint is a group of conditions in which one or both eyes are not aligned or pointed in the same direction. The condition is important since if one eye is not aimed at the same target as the other eye, then two images (double vision) will result. Although strabismus usually shows up in infancy or early childhood, adults can develop strabismus as well.

Each eye has six muscles controlling eye movement. These muscles constantly receive nerve signals from the brain to control eye movement so that the eyes make coordinated movements and both eyes are pointed at the same target. If the information received from these nerve impulses is incorrect, the eye may turn in, out, up or down. This can affect one eye or both eyes. The affected eye may deviate all the time or sometimes can become worse during times of fatigue or illness.

Patient with head tilt to the left shoulder. This patient has fusion in this habitual position. P.Ramkissoon, 2019

Both eyes must function properly for good depth perception (stereopsis). If one eye is not being used, then depth perception is limited. If one eye is not pointed at the same target, double vision may result. In order to deal with seeing two images, the brain may suppress one of the images so that you will only see one image, resulting in the suppressed eye becoming lazy (amblyopic). The brain soon learns to ignore the second image. Long-standing strabismus patients generally do not experience double vision but have single vision due to adaptation phenomena such as amblyopia and suppression. Recently acquired strabismus patients experience double vision because the adaptive phenomena have not had enough time to develop. Occasionally, some strabismic patients adopt an abnormal head posture in order to move the eyes to a position where the strabismus deviation is minimised and allow for fusion in order to see objects singly in three-dimensional space.

What are the different types of Strabismus?

Child with left esotropia (convergent squint) which developed during infancy. P.Ramkissoon, 2019.

Strabismus occurs in about 2% to 5% of the population. Some types of childhood strabismus are caused by abnormal development of the binocular system in the brain. Young children have huge amounts of focusing power. As a result, when a child has a large number of uncorrected hyperopia or farsightedness,
an attempt is made to make things clear by focusing extremely hard.

With this tremendous focusing effort, the binocular and focusing systems begin to get mixed signals. Usually, one eye will turn in and is called esotropia. On the other hand, when one eye has a large refractive error and the fellow eye does not, the brain may favour the better eye, leaving the eye with the larger error to wander outwards. This is referred to as exotropia or sometimes called wall-eye.

Adult with left esotropia (convergent squint) that has occurred recently. She developed a sixth nerve palsy from uncontrolled diabetes, high blood pressure and elevated bad cholesterol. P.Ramkissoon, 2019

In adults, strabismus occurs because of cranial nerve palsies that involve eye muscles and tend to occur in patterns. The main cause of third nerve palsies is vasculopathic diseases like diabetes and hypertension. Other causes of third nerve palsy are tumours and aneurysms. In third nerve palsies, the affected eye looks “down and out”. If the pupil is dilated, this can be caused by an aneurysm or tumour. Fourth nerve palsies have a “nasal up-shoot” where the deviated eye moves up and towards the nose and can be caused by vasculopathy, congenital conditions, tumour or trauma.

Congenital 4th nerve palsy tends to have a head tilt away from the side of the problem. Injury to the 4th nerve is very common because it is a long thin nerve. Sixth nerve palsy is also caused by vasculopathy diseases such as diabetes and hypertension. Besides, it can also be caused by high intracranial pressure (ICP). Sixth nerve palsies typically give a “crossed eye” appearance.

How is Strabismus diagnosed and treated?

A thorough case history guides the Eye Care Practitioner in making the diagnosis. Uncovering information about the age of onset, frequency, associated factors, medical and ocular history, and family medical and ocular history provides a wealth of information for making a proper diagnosis. Since vision and development are inextricably linked, developmental history is essential for the complete understanding of the patient’s ocular status. Incidence of strabismus is associated with: drugs, difficult pregnancy, birth trauma, metabolic diseases, and children with poor health, for example, cystic fibrosis, an increased prevalence associated with assisted delivery (forceps or cesarean section), low birth weight (including premature infants), neuro-developmental disorders.

To confirm the presence of strabismus, the ocular motility test is executed. The patient looks at a fixation stick tip (a tiny ball on a stick) which is moved in the different directions of gaze. Normally, both eyes should move in concert simultaneously. In strabismus, any eye that fails to move in a naturally smooth fashion is the eye with the cranial nerve palsy. The practitioner is able to diagnose the defective nerve and related strabismus type according to a diagnostic field chart. After the case history and ocular motility test, the light reflex test (especially in children) and the cover test are two of the earlier tests performed on the strabismic patient. Light reflex tests include the Brückner test, Hirschberg, and Krimsky tests. These tests are quick and very handy when consulting uncooperative children and adults. The Brückner test indicates the presence and laterality of strabismus while the Hirschberg and Krimsky quantify the angle of strabismus. The cover test (CT) is invaluable because it reveals information about the direction, magnitude, and frequency of the deviation.

Newly-acquired strabismus in an adult. P.Ramkissoon, 2019

The Eye Care Practitioner follows a particular sequence to treat strabismus.

Spectacles – The first step in strabismus management is the correction of refractive error with spectacles before effective amblyopia treatment engagement. An accurate spectacle Rx is critical to ensuring a clear retinal image in each eye. The use of cycloplegic drops (to paralyse the focusing mechanism) is necessary to obtain this information, particularly in pre-verbal children or those unable to successfully participate in the process of subjective refraction. Correcting the vision problem is the first step in helping the patient see clearly, and in some cases, eliminates most or all of the deviation.

Prism – Prism is a type of lens that bends light in a specific direction. Eye Care Practitioners can prescribe prism to move objects over to a position that allows the wearer to be able to fuse images so double vision does not occur. This is crucial for some patients who experience double vision as a result of strabismus.

Amblyopia treatment – Early strabismus detection is key in preventing long-term effects of amblyopia. The primary development of visual pathways occurs from birth to six to eight years of age. During this time, the visual system is most vulnerable to amblyogenic factors—the mechanisms that disrupt normal visual development. In humans, the critical period for amblyopia is 1-3 years, therefore in it advisable to see children as early as possible. After the patient receives the optimal spectacle prescription, amblyopia care is the next phase of the treatment regimen. Amblyopia treatment is initially passive such as direct patching of the good eye for up to 2 hours depending on amblyopia severity, while watching television, playing, skipping, hopping with gross targets and then engaging in activities requiring eye-hand coordination (playing and catching a ball, puzzles, colouring, making loom bands). Any significant improvement in visual acuity is always welcome since it provides a back-up of having two good eyes rather than one. Improvement should be seen within 6-8 weeks.

Adult patient wearing prism spectacles to eliminate double vision. P.Ramkissoon, 2019.

Vision Therapy – Vision Therapy (VT) encompasses a lot of different techniques to either correct strabismus or allow the person suffering from strabismus to learn when an eye deviates so that they can learn to use their eye muscles to minimise it. Sometimes certain machines and computer software programs are used to give the person behavioral feedback to control the eye muscle deviation. Vision therapy can improve visual skills such as depth perception, binocular coordination, binocular fusion, eye teaming skills, convergence, visual acuity, focusing skills, eye tracking, fixation skills, visual form discrimination, visual memory, and visual-motor integration (balance, body coordination, hand-eye coordination).

Surgery – Eye surgeons who specialise in eye muscle surgery use a variety of techniques that involve shortening or moving eye muscles for strabismus that warrant surgery. Adjustable sutures are used to fine-tune the eye muscle correction directly after surgery. Botox – Botulinum toxin is injected by some ophthalmologists directly into one of the muscles controlling eye movement. This temporarily blocks the nerve impulse and causes the muscle to be paralysed. This muscle relaxes and the other eye muscles take up the slack to straighten the eye.


The distinct appearance of strabismus may potentially interfere with social and psychological development of some patients. The aim of strabismus management is to achieve good visual acuity in each eye, restore normal ocular alignment and maximise the sensory cooperation between the two eyes.


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