Definition
Strabismus or squint also called crossed eyes, is a condition in which the eyes don’t look towards an object together. One of the eyes may look in or out, or turn up or down. The eye turning can occur all of the time or only sometimes, such as during stressful situations or illness. Strabismus is a common condition occurs among children.
Types of strabismus
There are different types of strabismus. They can be described by the cause or by the way the eye turns. The following terms describe strabismus by the positions of the eye:
- Hypertropia is when the eye turns upwards
- Hypotropia is when the eye turns downwards
- Esotropia is when the eye turns inwards
- Exotropia is when the eye turns outwards
Pathophysiology of Strabismus
- The extraocular muscles control the position of the eyes. Thus, a problem with the muscles or the nerves controlling them can cause paralytic strabismus.
- The extraocular muscles are controlled by cranial nerves III, IV, and VI. An impairment of cranial nerve III causes the associated eye to deviate down and out and may or may not affect the size of the pupil.
- Impairment of cranial nerve IV, which can be congenital, causes the eye to drift up and perhaps slightly inward. Sixth nerve palsy causes the eyes to deviate inward and has many causes due to the relatively long path of the nerve.
Epidemiology of Strabismus
- Strabismus is one of the most prevalent ocular problems among children, affecting 5 in every 100 US citizens, or some 12 million people in a population of 245 million.
- Infantile esotropia accounts for 28-54% of all esotropias. A population-based study from 1965 to 1994 reports the birth prevalence of infantile esotropia to be 25 per 10,000 or 1 in 403 live births.
- It is thought to affect about 1% of full-term, healthy newborns and a much higher percentage of newborns with perinatal complications due to prematurity or hypoxic/ischemic encephalopathy.
Causes of Squint
Whilst the causes of strabismus are not completely understood it is known that abnormalities in the muscles and nerves surrounding the eyes are both contributing factors. Most strabismus is caused by
- Refractive error
- Muscle imbalance
- Rare causes include retinoblastoma or other serious ocular defects and neurologic disease.
- Strabismus may be infantile or acquired. The term infantile rather than congenital is preferred because the presence of true strabismus at birth is uncommon, and the term infantile permits inclusion of varieties that develop within the first 6 months of life. The term acquired includes varieties that develop after 6 months
Risk factors of Strabismus
- Family history (1st- or 2nd-degree relative),
- Genetic disorders such as Down syndrome and Crouzon syndrome
- Prenatal drug exposure (including alcohol),
- Prematurity or low birth weight,
- Congenital eye defects,
- Cerebral palsy.
How can squint be manifested?
Most kids with strabismus don’t complain of eye problems or notice changes in their vision. Usually, it’s a family member, teacher, or health care provider who notices that the eyes are not straight.
- Double vision (seeing two objects when there’s only one in view)
- Have trouble seeing things in general
- Eyes don’t move together as they should.
- They could be crossed inward or outward.
- A child who has it may rub one or both eyes often.
- They could also squint, tilt her head, or close one eye to see things better.
- Loss of depth perception.
Complications associated with the squint
- Because the eyes focus on two different areas, the brain receives two different images.
- To make up for this, your baby’s brain ignores the image from the crossed eye and processes only the image from the stronger eye.
- This can affect her depth perception.
- It can also lead to amblyopia or lazy eye. This happens when the crossed eye doesn’t develop good eyesight or even loses vision.
How to diagnose and Test
Physical and neurologic examinations at well-child checkups
- Strabismus can be detected during well-child checkups through the history and eye examination. Evaluation should include questions about family history of amblyopia or strabismus and, if family or caregivers have noticed deviation of gaze, questions about when the deviation began, when or how often it is present, and whether there is a preference for using one eye for fixation.
- Physical examination should include an assessment of visual acuity, pupil reactivity, and the extent of extraocular movements. Slit-lamp examination is done to detect signs of cataract, and funduscopic examination is done to detect signs of retinoblastoma.
- Neurologic examination, particularly of the cranial nerves, is important.
Tests
Corneal light reflex test
The corneal light reflex test is a good screening test, but it is not very sensitive for detecting small deviations. The child looks at a light and the light reflection (reflex) from the pupil is observed; normally, the reflex appears symmetric (ie, in the same location on each pupil). The light reflex for an exotropic eye is nasal to the pupillary center, whereas the reflex for an esotropic eye is temporal to the pupillary center. Vision screening machines operated by trained personnel are being introduced to identify children at risk.
Alignment and focusing testing.
An optometrist will assess how well your eyes focus, move and work together. In order to obtain a clear, single image of what you are viewing, your eyes must effectively change focus, move and work in unison. This testing will look for problems that keep your eyes from focusing effectively or make it difficult to use both eyes together.
Refraction.
An optometrist can conduct a refraction to determine the appropriate lens power you need to compensate for any refractive error (nearsightedness, farsightedness or astigmatism). Using an instrument called a phoropter, the optometrist places a series of lenses in front of your eyes and measures how they focus light using a handheld lighted instrument called a retinoscope.
Cover test
The child is asked to fixate on an object. One eye is then covered while the other is observed for movement. No movement should be detected if the eyes are properly aligned, but manifest strabismus is present if the uncovered eye shifts to establish fixation once the other eye, which had fixed on the object, is covered. The test is then repeated on the other eye.
Alternate cover
In a variation of the cover test, called the alternate uncover test, the child is asked to fixate on an object while the examiner alternately covers one eye and then the other, back and forth. An eye with a latent strabismus shifts position when it is uncovered. In exotropia, the eye that was covered turns in to fixate; in esotropia, it turns out to fixate.
Prisms
Deviations can be quantified by using prisms positioned such that the deviating eye need not move to fixate. The power of the prism is used to quantify the deviation and provide a measurement of the magnitude of misalignment of the visual axes. The unit of measurement used by ophthalmologists is the prism diopter. One prism diopter is a deviation of the visual axes of 1cm at 1 m.
Treatment and care
People with strabismus have several treatment options to improve eye alignment and coordination. They include:
Eyeglasses or contact lenses. This may be the only treatment needed for some patients.
Prism lenses. These special lenses have a prescription for prism power in them. The prisms alter the light entering the eye and reduce how much turning the eye must do to view objects. Sometimes the prisms can eliminate the eye turning.
Vision therapy. Your optometrist might prescribe a structured program of visual activities to improve eye coordination and eye focusing. Vision therapy trains the eyes and brain to work together more effectively. These eye exercises can help problems with eye movement, eye focusing and eye teaming and reinforce the eye-brain connection. Treatment can occur in your optometrist’s office as well as at home.
Eye muscle surgery. Surgery can change the length or position of the muscles around the eye so they appear straight. Often, people who have eye muscle surgery will also need vision therapy to improve eye coordination and to keep the eyes from becoming misaligned again.
Botulinum toxin injection, or botox. This is injected into a muscle on the surface of the eye. The doctor may recommend this treatment if no underlying cause can be identified and if signs and symptoms appear suddenly. The Botox temporarily weakens the injected muscle, and this can help the eyes to align properly.
Eye drops may help and Eyepatch can be worn over the good eye, a patch can get the other eye, the one with the squint, to work better.
Prevention of Strabismus
- Regular eye exams are the best way to detect strabismus.
- Strabismus cannot be prevented. However, complications of strabismus can be prevented by detecting the problem early and treated properly.
- Early diagnosis and treatment better a child’s chances of having straight eyes and developing good vision and depth perception.
- Premature babies, and especially those with retinopathy, should have formal eye exams.