11.6 Strabismus Flashcards
Define Strabismus
- Strabismus is the misalignment of the visual axes.
- I.e. the visual axes of the two eyes do not intersect at the fixation point
At what age should alignment be stable?
8 weeks
Before this, bit of malalignment is normal
Terminology of strabismus
- The deviant eye turns inwards - Esotropia
- The deviant eye turns outwards - Exotropia
- The deviant eye turns upwards - Hypertropia
- The deviant eye turns downwards - Hypotropia
List the consequences of strabismus
- Amblyopia
- Suppression
- Diplopia
Amblyopia ( subnormal vision due to abnormality of visual system)
- Strabismic - If fixing with the same eye, the other eye’s neural pathways regress
- Deprivation - If the visual axis is obscured by any media opacities such as cataract or occluded
- Refractive - A sharp image does not fall on the retina for normal development
Suppression
-the brain does not see the image in the deviating eye as it is so different to the other.
Diplopia
- if deviation occurs after 10y, then the brain perceives both images at the same time
Amblyopia
Def
3 Types
DEFINITION
- Subnormal vision due to abnormal visual experience.
- It is one of the sensory adaptation mechanisms to prevent diplopia.
STRABISMIC AMBLYOPIA
- occurs before the visual system becomes neurologically stable (10 years of age), especially if the child always fixes with the same eye.
- the younger the child the more easily amblyopia can develop, and the more easily it can be reversed by a regime of periodic occlusion of the good eye.
- After the age of 10 years, amblyopia is irreversible.
DEPRIVATION AMBLYOPIA
- Opacities in the media or excessive occlusion of the eye may cause amblyopia.
REFRACTIVE AMBLYOPIA
A high refractive error may mean that a sharp image never falls on the macula.
Suppression
- consequence of strabismus
- Suppression of the image in the deviant eye is another sensory adaptation mechanism to prevent diplopia.
- As in amblyopia, it can only develop in children, but unlike amblyopia, it does not cause a decrease in visual acuity.
Diplopia
- consequence of strabismus
- If strabismus develops after 10 years of age, diplopia occurs, as neither amblyopia nor suppression can develop after 10 years of age.
Concomitance strabismus
CONCOMITANT STRABISMUS
- no impairment of eye movement.
- angle of deviation remains the same in all directions of gaze
Incomitant strabismus
Def
Causes
Characteristics
Def - There is impairment of one or more eye movements.
Causes:
- Neurological: cranial nerves III, IV and VI.
- Myoneural junction: myasthenia gravis.
- Muscular: thyroid orbitopathy.
- Mechanical: blow-out #, orbital mass.
Characteristics:
- angle of deviation will vary with the direction of gaze.
- Impairment of movement and consequently the angle of deviation will be maximal in the test position of the paralysed muscle.
- Strabismus will not necessarily be present in all directions of gaze.
- Binocularity can sometimes be maintained by adopting a head position in which the action of the affected muscle is minimised. Extraocular muscle paralysis with a compensatory head tilt is known as ocular torticollis.
Nerve supply of extraocular muscles + importance of III
Superior oblique - IV Trochlear
Lateral rectus - VI Abducens
All others - III Oculomotor
Remember that III is responsible for 3 other important muscles:
- Levator palpebrae superioris
- Sphincter pupillae via contained parasympathetic fibres which form the efferent pathway for the pupillary reflex.
- Ciliary muscle via contained parasympathetic fibres for accommodation.
Cover test
- singe cover test is used to determine if there is a tropia. (manifest strabismus)
- First eye is covered for 2 seconds. The uncovered eye is observed for a movement to fixate.
- The first eye is uncovered and the newly uncovered eye is observed for a refixation movement.
- test repeated on other side
