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Squint can be corrected:
It is not a Curse
Human beings are one of the few animals who have two eyes in front of the face instead on the side and can move both eyes in co-ordination over a wide range instead of having to turn the neck to see. The biggest advantage being 'binocular single vision' or 'stereosis/3-D vision'. This improves depth perception or better estimation of distance of objects. The 2-D image seen by each eye has some disparity due to the distance between the two eyes. When these are fused and processed in the brain a three-dimensional (3-D) perception is made. This whole process is a very complex mechanism, and involves constant and controlled movement of the two eye-cameras to maintain focus and keep track of any stationary or moving object. The position of any object in space is defined in three planes and there are six muscles (called extra ocular muscles) in each eye to move the eye in any desired direction. There are always two or more muscles involved in each movement, one or more pulling and the others relaxing, like the reins of a horse. The respective extra ocular muscles that are working in pairs, said to be 'yoked' are called 'synergists or agonists' who may be of the same eye (ipsilateral) or of the other eye (contralateral). The opposing muscles are called antagonists. Like a two-horse carriage, the eyes have to maintain co-ordination for proper functioning with tightening and loosening of appropriate 'reins-muscles' to make the two horses .move and turn together.
EXTRA-OCULAR MUSCLES
Each eye is moved by six extra-ocular (external)
muscles, grouped in three pairs:
- Horizontal Recti: Medial Rectus, Lateral Rectus.
- Vertical Recti: Superior Rectus, Inferior Rectus.
- Obliques: Superior Oblique, Inferior Oblique.
EYE MOVEMENTS-DUCTIONS AND VERSIONS
The eye movements when tested separately (uniocularly) are called 'ductions' and when both eyes are tested together (binocularly) is called 'versions and vergences'.
Normally the two visual axes are parallel when looking at a distance or they meet at the point of regard (the object of attention). In this state the eyes are said to be in alignment (orthophoria or orthotropia). If due to some reason the two visual axes are not aligned to the point of regard, that one eye fixates at the point but the other eye does not, the condition is called strabismus, or heterotropia (squint or cross eyes). The brain does not tolerate double vision or both eyes seeing totally different images and makes all effort to overcome the state of squint by fusional movements and the person may not manifest squint (latent squint or heterophoria). When squint is present at times and controlled at other times it is called intermittent. as against a constant squint.
Classification of Squint (Strabismus): This is a very complex subject; basic types with etiology (cause) and treatment are outlined below:
A. Concomitant Squint
The degree of eye deviation (amount of squint) is equal
in all directions of gaze.
(i) Horizontal
(i) Esotropia (ET) or latent Esophoria (E): In turning of eye(s):
(a) Accommodative: Due to excessive convergence associated with accommodation. Seen in children needing and corrected by plus (+) numbered glasses (hypermetropia).
(b) Partially Accommodative, the squint is partially reduced by wearing glasses and the remaining residual squint is corrected by surgery.
(c) Non-Accommodative: Essential Infantile
(congenital) Esotropia. Generally manifests at birth or before 4 months of age. Treatment is usually by surgery and the earlier the better.
(ii) Exotropia (XT) or latent Exophoria (X)-Outturning of eye(s). Exophoria is very common; especially in people needing minus (-) numbered glasses (myopia) and can usually be controlled (though not completely cured) by appropriate glasses and fusional convergence exercises. This is a frequent cause of eyestrain in many people. Orthoptic treatment on synoptophore in eye hospital or home exercise on special cards helps. Proper exercise has to be done, 'pencil pushups' by simply moving a pencil back and forth in front of the eyes without appropriate guidance will not help. Surgery is indicated for constant Exotropia or if intermittent squint is present for more than 50% of waking time.
(ii) Vertical Concomitant Squint
Hyper- or Hypo-Tropia. (Vertical Rectus Muscle).
(iii) Torsional Squints
In-cyclotropia or Ex-cyclotropia. (Oblique Muscle)
B. Incomitant
The deviations are more in one gaze than in the other.
(i) Paralytic Squint
Due to involvement (palsy) of one or a group of muscles. The paralysis if incomplete or partial is called paresis:
(i) Neurogenic (nerve or brain defect) due to injury, inflammation, neoplasm, ischemia, diabetes, etc. (ii) Myogenic (muscle defect) due to myasthenia, trauma, inflammation in muscle, thyroid disease, congenital, etc.
(ii) Restrictive
Due to entrapment of muscle or fibrosis following injury like blowout fracture of orbit, etc.
(iii) Spastic
Due to over-action of certain muscle(s).
Incomitant strabismus is managed by treating the underlying cause. Usually no surgery is indicated in paralytic squint due to systemic causes like diabetes, etc. Following trauma and appropriate attention to various injuries, the patient is followed-up 6 weekly up to 6 months and surgery is indicated only if two or more consecutive 6 weekly follow-ups reveal no change, including diplopia and Hess/Lee charting. During the waiting period diplopic (double vision) is avoided by covering (occluding) either eye (preferably the nonparalytic eye) or by prisms. Botulinum toxin can be used to temporarily paralyze the antagonist muscle(s). Surgical treatment is aimed at weakening the ipsilateral antagonist and sometimes also the contralateral antagonist, in addition to strengthening the paralyzed muscle and by transposition of muscles.
CONSEQUENCES OF STRABISMUS
Whenever strabismus or squint occurs, only one eye can fixate at the object of regard, the other looks elsewhere so that the two fovea (center of retina) receive two different images, creating 'confusion' in the brain. In addition, because of the misalignment of the eyes the same object is perceived as lying at two different positions in space, causing double vision or 'diplopia'. The brain cannot tolerate confusion or diplopia and it causes the motor system to move the eyes (despite stress) into alignment and convert manifest squint (tropia) to latent squint (phoria). Unusual head posture may be adopted to help the motor system adaptations. The sensory system faces the challenge of squint by sensory adaptations, viz. suppression and anomalous retinal correspondence (ARC). The sensory adaptations are possible only during the stage of plasticity of the neuro-development, that is early childhood, up to 6-8 years of age. If the effect of suppression remains even on closing the other (better) eye amblyopia results.
AMBLYOPIA
Amblyopia or 'Lazy Eye' is a condition where the eye is normal to look at, but it just does not work. It usually affects one eye but may be bilateral. It is medically defined as a decrease of visual acuity (sharpness), caused by form vision deprivation and/or abnormal binocular interaction, that cannot be explained by any disease or disorder in the ocular media or visual pathways (from retina, optic nerve or brain). New research is pointing towards functional disorder in the central visual pathways, but the hallmark of diagnosis of amblyopia remains that no obvious organic cause of decreased vision can be found by simple physical examination. Therefore, the term 'lazy eye' has been coined to explain that this eye is like a person who just refuses to work in spite of having no obvious disease or infirmity. Another important feature of amblyopia is that it is a condition caused by abnormal visual stimulation/experience in early childhood during the critical period of visual development. In simpler words, during the early years of development, due to certain forms of stimulus deprivation or abnormal stimuli, the wiring from the eye to the brain gets disrupted or forms abnormal connections with the better eye taking over from the weaker eye and making it lazy. This plastic period is usually up to 6 -8 years of age and is also the critical period when such process may be reversed or corrected. Amblyopia poses an important socio-economic problem and generally affects 2 to 2.5% of the general population. This in simpler words means that up to 2500 people in a population of one lakh stand to be disqualified from many jobs just because their eyes were not adequately tested or treated in early childhood. This becomes all the more important due to the fact since usually only one eye is involved and the child seems to function normally in school, the parents may not notice anything amiss, and by the time the child himself or herself becomes old enough to realize poor vision it generally is too late. Hence, the importance of proper eye testing of all pre-verbal and pre-school children by a qualified ophthalmologist who has adequate facilities to test such children.
Classification of Amblyopia according to various causes:
- Strabismic Amblyopia
Due to squint or cross-eyes.
- Anisometropic Amblyopia
Due to very different numbers in two eyes, or only one eye needing glasses, causing the weaker eye to become lazy.
- Stimulus Deprivation Amblyopia, Form Vision Deprivation Amblyopia or Amblyopia ex-anopsia
The eye(s) could not see any clear picture since birth due to drooping lids, congenital cataract, high refractive error, any injury to eye in early childhood needing eye bandage, etc.
- Nystagmus-related Amblyopia
Due to shaking or pendular eyes.
- Organic Amblyopia
Due to minute, sub-clinical disorders.
MYTHS AND MISCONCEPTIONS
Children Frequently Out-grow their Squint
Squint and amblyopia need early medical attention.
Early treatment is very important, especially before 6 years of age. These conditions worsen with advancing age. Squint can be operated at any age and the eyes may be made straight, but squint surgery before 6 years of age carries better visual results and long-term stability of effect. Squint can be operated in a few months old child.
A Child's Vision cannot be Tested before he/she Learns to Read
No, check-up of eyes of a child should be done at least once before one year of age and definitely before admission to school (pre-school age, 4-5 years). The child need not be able to read for the eye surgeon to test the need for glasses or to test for many other eye diseases.
Accommodative squint is to be treated by glasses (or contact lenses) alone. Do not insist for surgery for squint simply because you do not want your child to wear glasses. A qualified eye specialist is the one to decide the best time and mode of treatment. Once the child is older (18 years or more) laser treatment can help accommodative squint by correcting the number of glasses.
Eye muscles are not exercised by looking at a burning candle flame, rotating eyes all around or by walking bare-foot on grass. Proper eye muscle exercises needed in your case should be understood from your doctor or orthoptist.
Children should not use glasses full time, they will get dependent on them. This is like saying that we will get dependent on our meals to survive. Glasses prescribed in childhood are meant to be worn full time. Older children may be given the option of contact lenses or laser treatment. Glasses (or contact lenses) may at times be prescribed for children as young as three months old.
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