squint

Squint

Basic Knowledge
Eye Movement
Ductions: Monocular eye movement
Adduction (inward movement)
Abduction (outward movement)
Tortional duction; intorsion (inward rotation)
extortion (outward rotation)
Versions: Binocular, conjugate eye movement in the same direction
Dextroversion; both eyes move toward right
Laevoversion; both eyes move toward left
Vergences: Binocular, disconjugate eye movement in opposite directions
Convergence; eyes move inward
Divergence; eyes move outward
Eye movement is achieved with 6 extraocular muscles that move the eye in any direction. Normally both eyes move in a simultaneous and precise action.
Muscle
Nerve supply
Action
Medial rectus
Oculomotor n.( III )
Adduction (inward movement)
Lateral rectus
Abducent n. (VI)
Abduction (outward movement)
Superior rectus
Oculomotor n.( III )
Elevation,
Intorsion (inward rotation), adduction
Inferior rectus
Oculomotor n.( III )
Depression,
Extorsion (outward rotation), adduction
Superior oblique
Trochlear n. (IV)
Intorsion (inward rotation),
depression, abduction
Inferior oblique
Oculomotor n.( III )
Extortion (outward rotation),
elevation, abduction
Binocular Single vision (BSV)
Is the ability to use images from both eyes simultaneously and this feature provide a better Visual Acuity, wider visual field and depth perception ( stereopsis) . prerequisite for this kind of visual performance include
1.clear images from both eyes
2. Images of similar sizes from both eyes
3.acurate and coordinated neuromuscular development so both eyes ( visual axises) directed to the same object of interest
4. normally developed visual cortex .
Amblyopia
Is a unilateral or bilateral decrease in the best corrected visual acuity caused by vision deprivation and / or abnormal binocular interaction, for which there is no pathology of the eye or visual pathway. There are several types of abmbylopia:
Strabismic amblyopia : result from abnormal binocular interaction due to continued suppression of the deviated eye.
Anisometropic amblyopia: caused by difference in the refractive error between the tow eyes.
Stimulus deprivation result from vision deprivation e.g. congenital cataract and sever ptosis
Bilateral Ametropic amblyopia due to high symmetric refractive error usually hyperopia.
Amblyopia is treated basically by
identifying the cause and correcting this cause where ever possible
Occlusion of the normal eye to stimulate the amblyopic eye .



Accommodation
increase in the refractive power of the eye on looking to a near object so that its image is clearly focused on the retina.
Accommodation is part of the near reflex, which is initiated by looking to a near object and includes
1. Accommodation
2. Convergence (inward movement of the eyes)
Miosis (constriction of the pupil)
Normally one dioptre of accommodation is associated with 4 prism dioptres convergence
AC/A = 4/1
Classification of strabismus
Incomitent ( Paralytic): Under action of one or more of the extraocular muscles due to a nerve palsy or muscular disease. The angle of deviation is different with the direction of gaze.
Concomitant ( Non Paralytic): movement of both eyes are full and The angle of deviation is constant unrelated to the direction of gaze.
Esotropia
Accomodative
Refractive
Non Refractive
Mixed
2. Non Accomodative
Essential infantile
sensory
Basic
Convergence excess
Exotropia
Constant exotropia
Intermittent exotropia
Sensory exotropia
Consecutive exotropia


Accomodative esotropia
the relation between accommodation and convergence is attributed to this kind of esotropia. It usually manifest between 6months and 7 years of age.
In Refractive esotropia is there is increased hypermetropia , usually between (+2 and +7). This will provoke an escess convergence to see closer abjects.
Non Refractive esotropia happens due to abnormally high AC/A ratio
Mixed esotropia : both excess hyperopia and high AC/A ratio play role .
Treatment:
1.Refractive esotropia: convex lenses
2.Non Refractive esotropia: Miotic drugs e.g. Pilocarpin which decrease the AC/A ratio
3.Ambylopia treated by occlusion of the better seeing eye to stimulate the poor seeing one
4.Squint surgery .
Non Accomodative Esotrpia
Essential infantile esotropia manifest in the first 6 months of age and characterized by large and fixed angle . treatment is surgical .
Sensory esotropia result from unilateral reduction of visual acuity e.g. cataract, retinoblastoma and macular scaring.

Exotropia

Constant exotropia often present at birth and treatment is surgical
Intermittent exotropia often present at around age of 2 years with exophria which break down to exotropia under conditions of inattention,bright light,fatigue and illness.
Treatment include spectacle correction of refractive errors ( particularly myopia) if present. Ambylopia therapy by occlusion of the better eye. Surgery is needed in some cases.

Incomitent ( Paralytic ) Strabismus

This type of strabismus is the result of underaction of extraocular muscle(s) due to nerve paresis or muscle disease.
Oculomotor (third ) nerve Palsy:
Causes:
1.idiopathic
2. Vascular disease e.g Hypertention and diabetes mellitus .
3. Aneurysm of the posterior communicating artery is very important cause of isolated painful third nerve palsy with pupil involvement.
4. Trauma direct or secondary to subdural haematoma with uncal herniation.
5.Miscellanous e.g. tumors, Giant cell arteritis.
Diagnosis:
Ptosis
Exotropia
Limited elevation
Limited depression
Dilated pupil ( if parasympathetic fibers are involved)
Trochlear Nerve Palsy
Characterized by Hyperdivation of the eye inward position
Impairment of depression in inward position


Abducent nerve palsy:
Characterized by esodeviation in primary position with inability to abduct the affected eye.