Simply put, A squint is a condition where the eyes do not look together in the same direction. Whilst one eye looks straight ahead, the other eye turns to point inwards, outwards, upwards or downwards.
UNDERSTANDING THE EYE MUSCLES
The movement of each eye is controlled by six muscles that pull the eye in specific directions. The lateral rectus muscle pulls the eye outwards. The medial rectus muscle pulls the eye inwards. The superior rectus muscle is mainly responsible for upwards movements, whilst the inferior rectus muscle mostly pulls the eye downwards. Finally, the superior and inferior oblique muscles help to stabilise the eye movements - particularly for looking downwards and inwards, or upward and outward movements.For example, to look to the left, the lateral rectus muscle of the left eye pulls the left eye outwards and the medial rectus of the right eye pulls the right eye inwards towards the nose.A squint develops when the eye muscles do not work in a balanced way and the eyes do not move together correctly.
CLASSIFICATION
*Apparent squint or pseudostrabismus
-Pseudoesotropia
-Pseudoexotropia
-Latent squint (heterophoria)
-Esophoria
-Exophoria
-Hyperphoria
Hyperphoria is a physical mis-alignment of the two eyes so that one eye is set higher in the head than the other.
-Hypophoria
Hypophoria is a physical mis-alignment of the two eyes so that one eye is set lower in the head than the other.
-Cyclophoria
*Manifest squint (heteropia)
.Concomitant squint
.Convergent squint or Esotropia
.Divergent squint or Exotropia
.Vertical squint
Hypertropia
Hypotropia
*Incomitant squint
-Paralytic strabismus
Forms of paralytic strabismus include
.Third (oculomotor) nerve palsy
.Fourth (trochlear) nerve palsy
.Congenital fourth nerve palsy
.Sixth (abducent) nerve palsy
.Total (external) ophthalmoplegia
.Progressive external ophthalmoplegia
*Other
-Kearns-Sayre syndrome
-A and V pattern heteropias
-Restrictive squint
-Mechanical strabismus
-Brown's sheath syndrome
-Duane syndrome
*Other and unspecified heterotropia
-Microtropia
-Monofixation syndrome
-Heterophoria
-Esophoria
-Exophoria
CAUSES
About 5 in 100 children aged five years old have a squint. It is quite common to notice a brief squint when tired or daydreaming. Babies sometimes cross their eyes - it is quite normal for this to happen occasionally, especially when they are tired. Some squints are much more obvious than others. You might notice your child has an eye that does not look straight ahead. Another sign of squint is that your child might close one eye when looking at you, or turns his or her head on one side.
Congenital squints of unknown cause
Congenital squint means that the child is born with a squint, or it develops within the first six months of life. In most cases, the cause is not known. (The eye muscles are not balanced but the reason for this is not known.)In most cases one eye turns inward. This is called congenital esotropia (sometimes called infantile esotropia). This common type of squint tends to run in some families, so there is some genetic component to this type of squint. However, many children with congenital esotropia have no other family members affected. In some cases the eye turns outwards (congenital exotropia). Less commonly, a squint of unknown cause may result in an upward or downward turn of the eye. This type of squint is better corrected by sugery at 3months of age to prevent lazy eyes in early childhood.
Squint related to refractive errors
Refractive errors include: short sight (myopia), long sight (hypermetropia) and astigmatism. An astigmatism is a vision problem where the surface of the eye (the cornea) or the lens, is more oval-shaped, rather than round. This leads to problems with focusing. These are conditions that are due to poor focusing of light through the lens in the eye.When the child with a refractive error tries to focus to see clearly, an eye may turn. This type of squint tends to develop in children who are two years or older, in particular in children with long sight. The squint is most commonly inward looking (an esotropia).
Other causes
Most children with a squint have one of the above types of squint, and are otherwise healthy. In some cases, a squint is one feature of a more generalised genetic or brain condition. Squints can occur in some children with cerebral palsy, Noonan's or Down's syndrome, hydrocephalus, brain injury or tumour, retinoblastoma (a rare type of eye cancer) and several other conditions.
COMPLICATIONS / IMPLICATION
Amblyopia
Amblyopia is sometimes called a lazy eye. It is a condition where the vision in an eye is poor and it is caused by lack of use of the eye in early childhood. The visual loss from amblyopia cannot be corrected by wearing glasses. However, it is usually treatable.If amblyopia is not treated before the age of about 7 years, the visual impairment usually remains permanent.
To understand how amblyopia occurs, it is helpful to understand how vision develops. Newborn babies can see. However, as they grow, the visual pathways continue to develop from the eye to the brain and within the brain. The brain learns how to interpret the signals that come from an eye. This visual development continues until about age 7-8 years. After this time, the visual pathways and the 'seeing' parts of the brain are fully formed and cannot change.If, for any reason, a young child cannot use one or both eyes normally, then vision is not learnt properly. This results in impaired vision (poor visual acuity) called amblyopia. The amblyopia develops in addition to whatever else is affecting the eye. In effect, amblyopia is a developmental problem of the brain rather than a problem within the eye itself. Even if the other eye problem is treated, the visual impairment from amblyopia usually remains permanent unless it is treated before the age of about 7 years.
A squint is the most common cause of amblyopia. In many cases of squint, one eye remains the dominant focusing eye (the one that sees). The other, turned eye (the squinting one) is not used to focusing, and the brain ignores the signals from this eye. The turned eye then fails to develop the normal visual pathways in childhood and amblyopia develops in this eye.
How the squint looks
A squint can be a cosmetic problem. Many older children and adults who did not have their squint treated as a child have a reduced self-esteem because of the way their squint looks to other people.
Impaired binocular vision
With normal eyes, both eyes look and focus on the same spot. This is called binocular vision (bi- means two, and ocular means related to the eye). The brain combines the signals from the two eyes to form a three-dimensional image. If you have a squint, the two eyes focus on different spots. In children with squint this does not usually cause double vision. As described above in the 'Amblyopia' section, in children the brain quickly learns to ignore the signals and images coming from the turned (squinting) eye. The child then effectively only sees with one eye. This means the child does not have a good sense of depth when looking at objects. As a result, he or she cannot see properly in three dimensions.(Adults who develop a squint often have double vision, as their developed brain cannot ignore the images from one eye.)
PATHOPHYSIOLOGY
Strabismus can be caused when the cranial nerves III (oculomotor), IV (trochlear) or VI (abducens) have a lesion. A strabismus caused by a lesion in either of these nerves results in the lack of innervation to eye muscles and results in a change of eye position. A strabismus may be a sign of increased intracranial pressure, as CN VI is particularly vulnerable to damage from brain swelling, as it runs between the clivus and brain stem.More commonly however, squints are termed concominant (i.e. non paralytic). This means the squint is not caused by a lesion reducing innervation. The squint in this example is caused by a refractive error in one or both eyes. This refractive error causes poor vision in one eye and so stops the brain from being able to use both eyes together.
DIAGNOSIS
During eye examinations, orthoptists, ophthalmologists and optometrists typically use a cover test to aid in the diagnosis of strabismus. If the eye being tested is the strabismic eye, then it will fixate on the object after the "straight" eye is covered, as long as the vision in this eye is good enough. If the "straight" eye is being tested, there will be no change in fixation, as it is already fixated. Depending on the direction that the strabismic eye deviates, the direction of deviation may be assessed.Exotropic is outwards (away from the midline) and esotropic is inwards (towards the nose); these are types of horizontal strabismus. "Hypertropia" is upward, and "Hypotropia" is downward; these are types of vertical strabismus, which are less common.A simple screening test for strabismus is the Hirschberg test. A flashlight is shone in the patient's eye. When the patient is looking at the light, a reflection can be seen on the front surface of the pupil. If the eyes are properly aligned with one another, then the reflection will be in the same spot of each eye. Therefore, if the reflection is not in the same place in each eye, then the eyes are not properly aligned.
Laterality
Strabismus may be classified as unilateral if the same eye consistently 'wanders', or alternating if either of the eyes can be seen to 'wander'. Alternation of the strabismus may occur spontaneously, with or without subjective awareness of the alternation. Alternation may also be seen following the cover test, with the previously 'wandering' eye remaining straight while the previously straight eye is now seen to be 'wandering' on removal of the cover. The cover-uncover test is used to diagnose the type of strabismus (also known as tropia) present.
Onset
Strabismus may also be classified based on time of onset, either congenital, acquired or secondary to another pathological process, such as cataract.Many infants are born with their eyes slightly misaligned. The best time for physicians to assess this is between ages 3 and 6 months.
DIFFERENTIAL DIAGNOSIS
Pseudostrabismus is the false appearance of strabismus. It generally occurs in infants and toddlers whose bridge of the nose is wide and flat, causing the appearance of strabismus. With age, the bridge of the child's nose narrows and the folds in the corner of the eyes go away. To detect the difference between pseudostrabismus and strabismus, a Hirschberg test may be used.
MANAGEMENT
As with other binocular vision disorders, the primary therapeutic goal for those with strabismus is comfortable, single, clear, normal binocular vision at all distances and directions of gaze.Whereas amblyopia (lazy eye), if minor and detected early, can often be corrected with use of an eyepatch on the dominant eye and/or vision therapy, the use of eyepatches is unlikely to change the angle of strabismus. Advanced strabismus is usually treated with a combination of eyeglasses or prisms, vision therapy, and surgery, depending on the underlying reason for the misalignment. Surgery does not ALWAYS change the vision; it attempts to align the eyes by shortening, lengthening, or changing the position of one or more of the extraocular eye muscles and is frequently the only way to achieve cosmetic improvement. The procedure can typically be performed in about an hour, and requires about a week for recovery. Double vision can result, and occasionally vision loss can occur. Glasses affect the position by changing the person's reaction to focusing. Prisms change the way light, and therefore images, strike the eye, simulating a change in the eye position.
Early treatment of strabismus and/or amblyopia in infancy can reduce the chance of developing amblyopia and depth perception problems. Most children eventually recover from amblyopia by around age 10, if they have had the benefit of patches and corrective glasses.Eyes that remain misaligned can still develop visual problems. Although not a cure for strabismus, prism lenses can also be used to provide some comfort for sufferers and to prevent double vision from occurring.
Botulinum Toxin (Botox) may also be used in the treatment of strabismus, to improve cosmetic appearance. Most commonly used in adults, the toxin is injected in the stronger muscle, causing temporary paralysis. The treatment may need to be repeated 3–4 months later once the paralysis wears off. Common side effects are double vision, droopy eyelid, over correction and no effect. The side effects will resolve fairly quickly.In adults with previously normal alignment, the onset of strabismus usually results in double vision (diplopia).
PROGNOSIS
When strabismus is congenital or develops in infancy, it can cause amblyopia, in which the brain ignores input from the deviated eye. The appearance of strabismus may also be a cosmetic problem. One study reported that 85% of adult strabismus patients "reported that they had problems with work, school and sports because of their strabismus." The same study also reported that 70% said strabismus "had a negative effect on their self-image."
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