Strabismus

Strabismus is the loss of parallelism of the eyes in such a way that they are not aligned in the same direction as the object of visual interest. It affects an estimated 4% of children in the population and is one of the most common diseases found in children.

In normal conditions, the eyes look at the same object and the brain is able to integrate the two images, one in each eye, to obtain a single image with 3D characteristics. 

For this coordination to work, there is a complex system in charge of sending the orders from the brain to each of the extraocular muscles. When one of these circuits fails, strabismus appears. 

Depending on the direction, the strabismuses may be horizontal, vertical, torsional or mixed. Strabismus may affect just one eye or it might altenate, i.e. either one eye or the other deviates. 

The deviation might be constant or intermittent and appear only when an individual is tired, has a fever or is distracted, etc. Furthermore, it may develop at any age: it is congenital if it appears before the age of 6 months, acquired in childhood if it starts between the age of 2 and 3, or an adult strabismus if it develops in adulthood.  

• Visual fatigue: Headache, red eyes, eye pain, stinging... 
• Difficulty reading and writing and poor performance at school. 
• Winking
• Diplopia or double vision: Constant or intermittent in all or some eye positions. This symptom will occur with strabismus acquired in adulthood, given that the child's brain "adapts" by cancelling out the image of the deviated eye.
• Torticollis: It is the anomalous position of the head adopted by the patient to make up for double vision. The head "turns" to the place where the affected muscle has most difficulty in an attempt to substitute its action.
• Sensory changes
• Amblyopia or lazy eye: It develops in children between birth and the age of 8-9, as it is the period when the vision matures and develops. If there is a deviation in one eye, then the child's brain cancels out or disconnects the image of the deviated eye to prevent double vision. As a consequence, the eye's vision will not develop properly and lazy eye will occur. 
• Change in binocular vision development (co-operation of both eyes with the visual cortex of the brain).

The origin of strabismus is mutifactorial. Despite the fact that in certain types the cause is indeed known, in the majority of cases it is unknown or not fully known.

What we do know, however, is that there are different genetic factors (a family history of strabismus) and environmental factors (prematurity, low birth weight, the mother smoking and drinking during the pregnancy, neonatal problems...) and optical factors (moderate to severe hypermetropia, a different prescription in one eye to the other...) that may determine whether the patient develops strabismus. 

Stabismus cannot be prevented, but it can be detected early. For this reason, it is fundamental to have a full eye exam by a specialist at the age of 2-3 even though the child does not have any symptoms, and another at the age of 5-6 before the visual system has fully matured. 

Strabismus treatment includes different therapeutic options that must be considered in a personalised way by a specialist ophthalmologist. For treatment to be effective, it must be administered as early as possible.

Refractive defect correction

First, we must correct the refractive defect, if there is one, using glasses or contact lenses. In some cases, such as accommodative strabismus, optical correction alone may compensate the deviation. 

Visual recovery

Second, if amblyopia (lazy eye) develops in the deviated eye, we must restore the vision to that eye. To do so, the most effective measure is occlusion of the healthy eye using a patch.

The alternatives to occlusion which fall under the term 'penalisation'  involve inducing blurred vision in the good eye. This may be optical using a lens with a filter or incorrect prescription, or pharmacological by putting drops binto the eye that dilate the pupil.

In general, penalisation is usually reserved to cases where it is impossible to use a patch, such as maintaining the vision obtained after the occlusion or in certain cases of nystagmus. 

Surgical treatment

Once the optical defect and amblyopia have been corrected, surgical treatment can go ahead. In addition to the surgery, there are other therapeutic options available such as visual rehabilitation, prisms and botulinum toxin.

The botulinum toxin injection is an alternative in certain cases such as oculomotor paralysis. Orthoptic exercises may be useful as a complement to surgery and in selected cases such as phorias and intermittent divergent strabismus. Prisms are indicated in small strabismuses that cause diplopia.

There are many types of strabismus. The most common in clinical practice are the following:

Convergent strabismus or esotropia

• Congenital esotropia: It appears before the age of 6 months. It is characterised by the presence of a large angle of deviation which is constant and alternating, and there is not usually a significant refractive error. It is frequently associated with vertical deviations, and in some cases, torticollis and nystagmus. 
• Acquired accommodative esotropia usually occurs from the age of 2-3 and is associated with hypermetropia of 2-3 dioptres. It usually only affects one eye and causes amblyopia. We refer to total accommodative esotropia if the hypermetropia optical correction completely neutralises the deviation, and partial accommodative esotropia if is persists despite the use of glasses.
• Acquired nonaccommodative esotropia
• Sensory esotropia is secondary to a loss vision in one or both eyes, hindering biocular vision.
• Consecutive esotropia develops after exotropy surgery.
• Microesotropia is a small deviation of an eye that usually leads to mild amblyopia and that usually involves binocular vision.

Divergent strabismus or exotropia

• Congenital exotropia: It appears before the age of 6 months and has similar charateristics to congenital endotropia.
• Intermitten exotropia: It appears before the age of 3, although it is usually detected in late childhood due to its intermittent character. At the start, it manifests with physical fatigure, lack of focus and a feverish state. It is common to decompensate for the deviation over time, in both frequency and magnitude.

Restrictive strabismus

Restrictive strabismus with a "mechanical" element that stops the eye from rotating and therefore causes limited movement in a certain direction. Children may get restrictive strabismuses, but they are more common in adulthood (acquired). We can categorise them into two large groups: 

• Congenital (present from birth or a few months of age)

1. Duane Syndrome: It is caused by an abnormality in the innervation of the lateral rectus muscle secondary to poor development of the sixth cranial nerve during foetal development. This innervational disorder leads to restricted or limited horizontal movement, which is accompanied by other typical signs including reduction of the palpebral fissure, abnormal vertical movements, eye deviation and torticollis. 
2. Brown's Syndrome: It is characterised by limited upward and inward lifting of the eyeball.
3. Congenital fibrosis of the extraocular muscle: It is characterised by severely limited ocular movements due to the fact that the muscles are substituted with fibrotic tissue that turns into "inflexible bands" which, in addition, may be linked to abnormalities in their insertion. 

• Acquired (they develop in adulthood)

1. Secondary strabismus or thyroid-assoiciated thyroidopathy: It is an autoimmune disease associated with a thyroid gland dysfunction characterised in its first phase by an increase in the size of the intraocular muscles and the orbital fat, followed by a chronic phase causing atrophy and muscle fibrosis, with a resulting loss of elasticity. The most frequently affected muscle is the inferior rectus, which is why the patient will have a hypotrophic eye (vertical deviation downward) with limited lifting.
2. Myopic restrictive strabismus: It happens to patients with myopia magna (more than 15 dioptres). It is characterised by a converging strabismus (inwards) with limited external rotation. It is accompanied by hipotonia (vertical deviation downward) with limited lifting. 
3. Restrictive strabismus secondary to eye surgery: Although the effect varies depending on the type of anaesthesia and surgical technique used, the strabismus may appear after any eye surgery: retinal detachment surgery (particularly sclera surgery), cataract surgery, glaucoma surgery (particularly valve implants), pterygium surgery, orbit surgery or eyelid surgery.