Strabismus is derived from a Greek word, which means “eyes looking obliquely.” Strabismus means misaligned eyes. Often strabismic eyes are referred to as “squinting eyes,” “crossed eyes,” and “wall eyes.” Usually, both eyes fixate equally while focusing on an object with the head held in the primary position. In strabismus or squint, one or both eyes deviate inwards or outwards and appear to be in non-alignment towards the direction of the focused object. It can be due to refractive error or binocular fusion abnormalities or neuromuscular anomalies of ocular movements. If diagnosed and treated early, strabismus has a very good prognosis. Treatment is usually by refractive error correction, orthoptic exercises, occlusive patching, topical medications, and extraocular muscle surgery.
Strabismus or squint is broadly classified as
Causes for Pseudostrabismus
Causes for Heterophoria
Causes of Concomitant Squint:
Causes of Incomitant Squint:
Risk factors
Strabismus is more prevalent with certain syndromes like Down syndrome, cerebral palsy, Apert-Crouzon syndrome, premature infants with low birth weight, and in kids with affected parents or siblings. All siblings of a strabismic child should be screened at an early age for strabismus as sensorimotor anomalies are common in the pedigrees of strabismic probands.
The prevalence of strabismus is 2 to 5 percent in the general population.[1][2]. Five to fifteen million population of the United States of America is affected by strabismus. In a National Health Survey, exotropia is seen in 2.1 percent and esotropia in 1.2 percent of the population between 4 to 74 years of age. This difference is due to the higher prevalence of exotropia in the population between 55 to 75 years of age.[2] In a population-based study done in Los Angeles, California, involving children aged between 6 to 72 months, Hispanic/Latino kids showed 2.4 percent, and African Americans showed 2.5 percent of strabismus. Exodeviations were more common than esodeviations.[3]
Fifty percent of all childhood esotropias are either fully or partially accommodative. Non-accommodative esotropia is seen in 10 percent of all strabismus and is the second most common form of childhood esotropia.[4] Infantile esotropia affects one in every 100 to 500 persons, which accounts for 8.1 percent of cases of esotropia.[5] Intermittent esotropia is seen in one percent of the population and is the most common form of exotropia.[6][7] Exotropia is more prevalent in Asian and African American populations than in Whites.[8] Women are affected in 60 to 70 percent of patients with exotropia.[9]
Etiopathogenesis of strabismus is unclear. The physiology of ocular motility involves extraocular muscles, cranial nerves, supranuclear pathways, and their cerebral controls. All of these have been implicated in the development of strabismus. Following two theories are popular:
In relation to muscle innervation and action, the following two laws of ocular motility govern the extraocular motility:
Although strabismus can occur at any age, its commonly seen before six years with a peak onset around three years. Parents often claim that their child has crossed eyes since birth. Family photographs often help to document the age of onset. Moreover, pictures might show ocular preference in fixation. Alternating fixation denotes an absence of amblyopia in squinting infants. Strong fixation of one eye implies strabismic amblyopia in the other eye. A history of low birth weight indicates retinopathy of prematurity, which might lead to pseudostrabismus from ectopic macula. Acute onset of esotropia in an older child always requires a thorough evaluation to rule out neurological abnormalities. Intermittent strabismus indicates a better recovery of normal binocular vision as it implies that fusion is a present part of the time. Photosensitivity is commonly seen in intermittent exotropia and is caused by a decreased binocular photophobia threshold.[10] History of allergy to dilating drops, familial hepatic porphyria, sensitivity to suxamethonium, and malignant hyperthermia is essential to prevent fatal anesthetic complications during surgical management of strabismus.
Clinical features of the commonly occurring strabismus conditions are as follows:
The evaluation of strabismus starts with good history taking, ocular/physical examination, and with appropriate investigations. The history-taking aspect is already dealt with above. The physical and ocular assessment are as follows:
Visual Acuity:
Stereo acuity: Stereoacuity measures the visual sense of depth. It is the sense of fusion of two simultaneous, slightly dissimilar images with integration by the brain. Titmus stereo-fly test and Lang cards are commonly used. Worth 4 dot test and Bagolini striated glasses are used to assess the fusional reserve while both stereopsis and fusion can be assessed by synoptophore.
Compensator Head Posture (CHP): CHP is a motor adaptation to strabismus to attain binocular single vision (BSV). Loss of CHP in concomitant strabismus may indicate the loss of BSV and warrants surgical intervention. In acquired paretic strabismus, CHP eliminates diplopia and helps to centralize the binocular visual field. Head tilt, face turn and chin up/down are commonly seen as abnormal head postures. A head tilt to the left is seen in right superior oblique palsy. A face turn to the left is seen in the left lateral rectus palsy. Chin up or down is seen in “A” or “V” pattern strabismus.
AC/A ratio: The accommodative convergence/accommodation (AC/A) ratio is defined as the amount of convergence in prism diopters per diopter change in accommodation. Two methods are used to measure the AC/A ratio: the lens gradient method and the heterophoria method. The normative range lies between 3 and 5 to 1.
Measurement of Deviation
Ocular Motility: Extraocular movements involves the assessment of smooth pursuit movements followed by saccades.
Field of BSV: Field of binocular single vision is that area where bifoveal fusion of the object of regard occurs. The field of BSV is assessed by Hess chart to diagnose and monitor patients with incomitant strabismus caused by either extraocular muscle palsy (third, fourth, or sixth cranial nerve palsies) or restriction (thyroid ophthalmopathy, blow-out fracture or myasthenia gravis). Hess chart uses either Hess screen or Lees screen to chart the field by dissociating the ocular movements. Hess screen uses a tangent screen with a red-green goggle, and Lees screen uses two glass screens at right angles to each other.
The following is the interpretation of the Hess chart appearance:
The following sequelae are seen in the muscles in a paretic squint:
Refraction: Assessing the correct refractive error and power is crucial in the management of strabismus. Most commonly, hypermetropia is seen in strabismus patients. Refraction should be done under both non-cycloplegic and cycloplegic conditions. Instilling one drop of 1% cyclopentolate hydrochloride twice at 5-minute intervals followed by retinoscopy thirty minutes later is the standard practice.
Forced duction test (FDT): This test is done to assess if the limitation of movement is due to the mechanical restriction of the muscle (fibrosis/tethering). The anesthetized conjunctiva of the eyeball is held with forceps and moved first in the direction of the muscle action and later in all directions to see if there is a restriction of the movement of the eyeball. This test should be mandatorily performed before any strabismus surgery.
Parks-Bielschowsky 3-step test: This test is done in acquired vertical diplopia to isolate the paretic muscle. Step1: Which eye is hypertropic in the primary gaze? Step 2: Is the hypertropia worsening of right or a left gaze? Step 3: Is the hypertropia worsening with the right head tilt or left head tilt? This test helps in diagnosing superior oblique palsy.
Fundoscopy: Dilated fundus examination is mandatory to rule out intraocular pathologies like optic disc hypoplasia, macular scarring, or retinoblastoma, which might cause squint.
Investigations for strabismus: Neuroimaging is essential, especially in sudden onset adult-onset strabismus to rule out stroke, diabetic mononeuritis, myasthenia gravis, thyroid eye disease, etc. Rarely a primary neurological disorder such as hydrocephalus, optic nerve glioma, medulloblastoma, or craniopharyngioma might cause childhood strabismus and need neuroimaging.
The primary aim of treating strabismus is to restore proper ocular alignment. The secondary aims are to treat amblyopia, maintain binocularity, and eliminate diplopia.
Following are the various methods of treating strabismus:
Observation: Strabismus secondary to myasthenia gravis, diabetic mononeuropathy, and post-traumatic restrictive strabismus mostly improve with time and with the treatment of the primary cause. Some healthy neonates show intermittent deviation of the eyes. These are called ‘neonatal ocular misalignments,’ which improve by two months and resolve in four months. They usually reflect a normally developing vergence system.[11]
Correction of Refractive Errors: The first step in the management of any child with strabismus is to evaluate for a refractive error and correct it fully. The full correction of hypermetropia is the treatment of choice for all forms of esotropia. Full correction without subtracting any lens power for cycloplegia is prescribed. In convergence excess esotropia, executive bifocals are prescribed to relieve accommodation and thereby preventive accommodative convergence. A minimal plus “add” required is prescribed with the flat top of the bifocal segment bisecting the upper border of the pupil.[12] In intermittent exotropia, full myopic correction often controls the exotropia.[13]
Treat Amblyopia: Amblyopia in strabismus is reduced visual acuity in one or both eyes because of misalignment of eyes in the absence of any demonstrable visual pathway abnormality. Refractive correction alone is successful in improving amblyopia in nearly one-third of the patients.[14] Patching of the better eye to force the brain to use the weaker eye is the gold standard treatment for amblyopia. Patching the better eye for 2 to 6 hours in a day is recommended and is most effective with children less than seven years of age. Atropine penalization (atropine 1% eyedrops twice daily in the better eye) is also as effective as occlusion.[15][16] As the effect of atropine actions lasts for 2-3 weeks, frequent follow-ups are necessary to detect occlusion/reversal amblyopia of the better eye.
Orthoptics: Orthoptic exercises are commonly used to treat intermittent exotropia. Fusional exercises like “pencil push-ups” are done with a pencil held at arm’s length and slowly moved towards the nose, thereby inducing accommodation and strengthening convergence, which helps in treating the exotropia.
Prismatic Correction: Ophthalmic prisms move the image closer to the fovea and help in improving the sensory fusion. They are prescribed for deviations of less than 20 PD. Amblyopia, suppression, and anomalous retinal correspondence are contraindications for prism therapy.[17]
Pharmacological Therapy: Miotics act by inducing peripheral accommodation so that the patient uses less of accommodation and thus reducing esotropia. Ecothiopate iodide 0.125 percent once daily is used as a short-term treatment for accommodative esotropia patients. This can be used for kids who are too young or intolerant of glasses.[18]
Botulinum Toxin: Botulinum toxin type A is used as a form of chemodenervation to cause temporary paralysis of the extraocular muscle leading to improvement in the strabismus. It is used as an adjunct to surgical therapy and to assess postoperative diplopia if anticipated following strabismus surgery. This can induce transient ptosis and vertical strabismus.[19]
Extraocular Muscle Surgery: Strabismus surgery should be considered only after conservative treatments have failed to correct the deviation. Esotropias more than 15 PD and exotropia more than 20 PD following full spectacle correction are candidates for surgery. Accommodative esotropias are not ideal candidates for surgery as it induces consecutive esotropia.[20][21] The ideal age for infantile strabismus surgery is before two years of age. A postsurgical deviation of less than 10 prism diopters yields better binocular vision, whereas to achieve stereopsis, the residual deviation should be 4 PD or less.[22] 60 percent reduction of the overall deviation or a deviation of 10 prism diopters or less after six weeks of surgery is considered to be a successful outcome of horizontal strabismus surgery.[23]
The three main types of strabismus surgery are:
Differential diagnoses can be classified on the basis of the type of defect. Following are some important differentials to be considered:
Congenital esotropia
Fully Accommodative esotropia
Intermittent exotropia
Prognosis of strabismus is excellent if detected and managed early in life. Healthy neonates sometimes show intermittent deviation of the eyes, which should not be a concern. At about three months, normal binocular coordination sets in, and any strabismus persistent after this period should be considered abnormal and significant. Amblyopia sets in if the strabismus is not appropriately managed before 6-8 years leading to a permanent decrease in the vision. It is extremely important to know the indications for ophthalmologist referral.
Indications for ophthalmologist referral
Complications of strabismus include the following:
Surgical treatment for strabismus may result in the following:
Parents should be made aware of the prognosis, advantages, and complications of the management of strabismus and its impact on the social, physical, and psychological growth of the child. A treatment plan based on the discussion with the parents should be charted out. In acute onset strabismus, patients and their care providers should be informed about a possible systemic or neurological cause and a possible referral to an appropriate specialist if needed. The parents need to know the risks of developing amblyopia and impaired stereopsis if adherence to treatment is not maintained.
The treatment of strabismus in children involves a multidisciplinary approach and might take place in a variety of locations. The groups involved are the patient and family members, hospital administration, medical staff, orthoptists and optometrists, community pediatricians, general practitioners, and health visitors. In addition to these varied groups, teachers, school nurses, and community optometrists also play an important role in their management. Good communication is very important between staff and parents. A detailed clearly written medical and orthoptic record should be made available with both patients and the doctors during clinic appointments and on surgical admissions. Community pediatricians should always be kept in the loop during all correspondences between the patients and the ophthalmologists.
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