Strabismus is an ocular condition in which the visual axes are not aligned towards the same point in space. It can affect all age groups, although there is a higher incidence during childhood and generally before the age of 6.
Depending on the direction of deviation, strabismus can be of the type:
-convergentwhen the deviated eye is inwards;
-divergentif the deviated eye is outwards;
-verticalwhen the eye is deviated downwards or upwards.
In the subject with strabismus, the diplopiai.e. a double vision, and the confusionwhich occurs when a different image is focused in the fovea than that of the other eye. If strabismus occurs in adulthood, diplopia occurs; if, on the other hand, the condition is congenital or arises during childhood, in order to avoid double vision, the brain automatically excludes the off-axis eye, and diplopia develops. amblyopiaor so-called lazy eye. Strabismus is not just an aesthetic problem, but an important visual defect that can lead to serious eye complications, affecting the vision of the affected person. In fact, the patient with strabismus has poor vision overall and if not promptly treated may develop disabling amblyopia and permanent visual impairment.
As well as a reduced visionsquinting involves reduced depth perception, eye fatigue, episodes of headache, photophobia and an abnormal position of the head (ocular torticollis).
There are two types of strabismus:
-not paralytic, most common in childhood, in which the eye muscles are fully functional, but the nervous system that regulates their function is impaired, resulting in impaired synergism of the eye muscles;
-paralytic, which mostly affects the elderly, usually due to paralysis of the eye muscles following a traumatic or inflammatory event or as a result of a nerve injury.
What are the causes of squinting?
The main causes of strabismus include:
-Hereditary factors;
-paresis of cerebral and musculo-ocular origin;
-ocular abnormalities (cataracts);
-diabetic pathology;
-uncorrected refractive defects.
How is it diagnosed?
Early diagnosis is crucial for a total or partial correction of strabismus, for a reduction in the prevalence of amblyopia and also for an improvement in motor co-ordination. However, making an early and accurate diagnosis is not easy as it often requires a very thorough eye examination and one must also consider the patient's age and level of cooperation. Currently, the diagnosis of strabismus is made by making a thorough analysis of the eye and the patient's state of health. The medical specialist first determines the period and cause of onset of the squint, the presence or absence of symptoms and signs related to the condition, such as diplopia and compensatory head tilt, the presence of neurological disorders and the family history.
The diagnosis of strabismus involves the accurate assessment of:
–visual acuity;
-eye motor deviation;
-mono-ocular fixation;
-extraocular and sensorimotor muscle function;
-accomodation;
-refraction;
Locular examination involves measuring the visual acuity of both eyes to determine the presence or absence of amblyopia. For children two years of age or younger, the following are generally used Teller's cards which make it possible to quantify the child's resolution capacity; in children between 3 and 5 years of age, it is preferred to conduct the Lea test or use Cardiff cards. In adulthood, however, the examination is carried out through Snellen's tables.
Ocular motor deviation can be established by carrying out a cover/uncover test, which is useful in the evaluation of neuromotor imbalances and allows differentiation between a manifest or latent squint.
The stereoscopic vision testthree-dimensional vision, is one of the most reliable tests for the diagnosis of strabismus and amblyopia. Monocular fixation is assessed through visuoscopy using an ophthalmoscope with a calibrated fixation target. Extraocular muscle function is important to determine whether the ocular deviation is concomitant (the angle of deviation remains unchanged regardless of the direction of gaze) or incomitant (the angle of deviation changes depending on the direction of gaze). The determination of sensorimotor fusion is established with Worth's four-light test by which the presence of diplopia or suppression (one of the two retinal images is not processed in the brain, resulting in amblyopia) is determined. In addition, an accurate examination of refractive errors is essential because these often represent the aetiological factor in the development of the pathology.
How is strabismus corrected?
Correction of strabismus has as its primary objective visual recovery and restoration of binocular function and the interventional strategy for correcting the pathology is specific to each patient as it must take into account various factors such as the age of the affected person, the patient's state of health, and the presence or absence of amblyopia.
In general, therefore, there are different treatment strategies: in the case of amblyopia, theocclusiontotal or partial (bandage) of the eye that has a visus best in order to stimulate the lazy eye to restore its functionality. If the causes of strabismus are due to refractive errors such as, for example, hypermetropia, correction occurs with theuse of lenses which in most cases completely solve the problem. Pharmacological therapy, on the other hand, involves administration of a cycloplegic eye drop which blocks the accommodative process of the healthy eye and forces the lazy one to work, a strategy that is used to correct, for example, convergent strabismus due to high hypermetropia. When the use of corrective spectacles is not sufficient, surgery may be considered, especially for those subjects in whom the angle of the deviated eye is too wide for binocular comfort and to obtain an improvement in aesthetics. The timing surgery and urgency obviously depend on the type and severity of the squint. Surgery is a treatment option that must be considered very carefully as it can lead to a number of side effects, such as red eye, eyelid swelling and pain, symptoms that can last for several weeks. In addition, several operations may be necessary to realign the visual axes in order to achieve an optimal result.
Bibliography:
–Optometric clinical practice guideline. Strabismus: exotropia and exotropia. American Optometric Association. 2011.
-Guidelines for the management of strabismus in childhood. The royal college of ophthalmologists. 2012.
Dr. Carmelo Chines
Direttore responsabile
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