Strabismus is a condition in which neither eye lines up in the same direction. Patients with strabismus have difficulty controlling their eye movement and maintaining appropriate ocular alignment.
There are a variety of causes for strabismus in children, some of which can be dangerous to the eyes or in the worst case, life. As a result, strabismus must be recognized and treated effectively and promptly. Amblyopia is usually connected with strabismus and can be a cause or a complication of strabismus. It is important to diagnose and treat amblyopia early to avoid a permanent loss of vision. This article describes how to diagnose strabismus in youngsters correctly.
Examination of Strabismus
A general examination of the child’s eyes should be the beginning point for the assessment. Ptosis, pupil irregularities or asymmetry, and a noticeable strabismus are all things to check for during an examination.
To assess visual function and determine visual activity, and age-appropriate procedure needs to be used. Visual behavior can be examined in extremely young or non-verbal children behavior, such as checking if the child can fix and follow targets and comparing both eyes by occluding one at a time to see if the child has an uneven visual preference (the child may resist an occlusion of the eye with better vision). Various tests can be used to determine quantitative visual acuity: forced-choice preferential looking grating for children under one year, Cardiff cards for children under two years, 3 m uncrowded Kay pictures between 18 months and three years, 3 m crowded Kay pictures for children between two and four years, and 3 m crowded log-MAR letters for children over three years.
Crowded tests are preferred for the detection of amblyopia as they are more accurate, and children should be encouraged to perform these as soon as they are able to. Although each eye’s visual acuity should be measured separately, this can be challenging in young children, thus binocular visual acuity can be measured instead. If the refractive error is present while evaluating visual acuity, it should be corrected (with glasses or contact lenses), and then a pinhole can be used to determine any further visual improvement.
The binocular discrepancy in the images received by both eyes is referred to as stereopsis or depth perception. The level of depth perception is measured by stereoacuity. Various conditions, such as the presence of strabismus, refractive error, or amblyopia, can induce a loss in stereoacuity. The TNO random-dot test, Frisby test, Titmus test, and Lang stereo test, to name a few, are all examples of stereoacuity tests.
Corneal light reflex
The Hirschberg test, commonly known as the corneal light reflex test, is used to diagnose manifest strabismus. It is done by flashing a pen flashlight roughly 50cm from the child’s eyes and directing the child to gaze at the light.
If the Hirschberg test reveals a manifest ocular deviation, the Krimsky test is used to determine the angle of the deviation. Prisms of differing strengths are put over the fixating eye, and the end point is attained when both eyes’ corneal light reactions are symmetrical.
The cover test is useful for detecting manifest strabismus, but it requires the child’s cooperation, which can be difficult to achieve in young children. The youngster is asked to look straight ahead at a target, then one eye is covered while the movement of the other eye is monitored. After that, the procedure is repeated on the second eye. When the fellow fixating eye is covered and manifest strabismus is present, corrective movement of the deviated eye will be seen in the opposite direction of the deviation, as long as the deviated eye has any vision. If the kid has a refractive error, the cover test should be done for both near and far targets, both with and without glasses. In the initial case, a light can be used as a target, but an accommodating target should also be used.
Simultaneous prism cover test
If manifest strabismus is discovered during cover testing, the simultaneous prism cover test is utilized to determine the angle of manifest strabismus. When a prism is placed in front of the misaligned eye and the other fixing eye is blocked, the uncovered eye should be monitored for movement. The intensity of the prism employed is changed, and the angle of strabismus can be observed when it is neutralized by the prism and the deviated eye does not recover movement.
To detect latent strabismus, the uncover test is utilized. The child is told to look directly at a target before having one eye covered for three seconds. After that, the cover is removed, and the movement of that eye is carefully monitored. When latent strabismus is present, the eye deviates when covered; as a result, when the cover is removed, the eye re-fixates in the opposite direction of the latent deviation. This test should be done with both close and far targets.
Alternate cover test
The alternate cover test can be done after the cover and uncover tests have been completed. This test induces a binocularity dissociation in the eyes and determines the total angle of strabismus (manifest plus latent components). To disrupt fusion, each eye is occluded for a few seconds at a time, and the eyes are watched for re-fixation motions that indicate the direction of strabismus. This test should be done with both close and far targets.
Alternate prism cover test
The alternate prism cover test is used to determine the total angle of manifest and latent strabismus. The alternate cover test is performed with prisms in front of one eye. The prism’s strength is increased until no refixation movement is seen; this neutralization point reveals the overall angle of deviation, and the prism strength used to produce it can be recorded.
In children with strabismus, eye movement testing should be undertaken to determine whether the strabismus is comitant or incomitant, that is, whether the size of the strabismus is constant in different positions of gaze (comitant) or fluctuates with gaze position (incomitant). Cranial nerve III, IV, and VI palsies are common causes of incomitant strabismus. Although it may be challenging in young children, an attempt should be made to evaluate all nine gaze locations.
The direct ophthalmoscope is used to perform the red reflex test. A direct ophthalmoscope is used to view the pupillary red reflexes of the child at a distance of roughly 50 cm from the child. Asymmetrical reflexes should be identified (in terms of size, shape, or color). A lack of a red reaction (unilateral or bilateral) could signal a dangerous condition.
Cycloplegic retinoscopy or refraction
Six percent of one-year-old children have a severe refractive error. Strabismus is a condition that develops as a result of refractive defects, astigmatism, or anisometropia. In children with strabismus, cycloplegic retinoscopy in young children or non-cycloplegic refraction in older children should be performed to assist determine the cause. In some instances of strabismus, proper refractive error correction can help to improve the strabismus.
A fundal assessment is necessary when evaluating a child with strabismus because intraocular causes must be eliminated and the optic nerve must be checked for any abnormalities
A general examination of the child might be done to look for any other anomalies as well as identify any linked syndromes or systemic diseases.
In conclusion, we should know the history of a patient with strabismus to help identify the cause and any red flag features. Furthermore, a visual acuity check, the existence of manifest or latent strabismus, a red reflex check, oculomotility testing, and a fundal check should all be included in the examination. As needed, a neurological examination should be undertaken. What is more, strabismus must be identified and investigated as soon as possible. To avoid a permanent visual loss, it is critical to recognize and treat amblyopia if it is present. A referral to an ophthalmology team for a careful examination of the strabismus should be made.
Source: Bommireddy, T., Taylor, K., & Clarke, M. P. (2020). Assessing strabismus in children. Paediatrics and Child Health, 30(1), 14–18. https://doi.org/10.1016/j.paed.2019.10.003