Paralytic strabismus. Diplopia
Strabismus is the deviation of an eye´s visual axis from its normal position.
Concomitant strabismus
- the deviating eye accompanies the leading eye in every directional movement
- the angle of deviation remains the same
- forms
- monocular → only one eye deviates
- alternating → both eyes deviate alternately
Paralytic strabismus
- results from the paralysis of one or more eye muscles
- the angle of deviation does not remain the same in every direction of gaze
- also referred as incomitant strabismus
The strabismus is the result of the difference in the muscle strength which are responsible for a normal eye movement. The exact action of any muscles depends on the direction of the eye in space.
The MRM and the MRL adduct and abduct the eye respectively, with a little effect on elevation or torsion. The vertical rectus and obliques muscles have vertical movements and torsion as their chief functions.
- Vertical rectus muscles are the main elevators and depressors
- obliques are mostly involved with torsional positioning
Agonist: primary muscle that moves an eye in a given direction/or initiates movement Synergist: muscle in the same eye that moves the eye in the same direction as the agonist Antagonist: muscle in the same eye that moves the eye in the opposite direction of the agonist/ or relaxes
Sheringtons law: When one extraocular muscle receives an impulse from the brain to contract, its opposing muscle in the same eye receives an impuls to relax.
Monocular movements/duction
- movement of one eye seperately
- adduction and abduction
- supraduction (eye moves up) and infraduction (eye moves down)
- incycloduction and exycloduction (rotation around the anteroposterior axis of the globe)
Binocular movements
- simultaneous conjugate movements
- dextroversion and levoversion/ depression
- dextroelevation, levoelevation/depression
- vergences: simultaneous disjunctive movements (convergence, divergence)
Their are different methods to diagnose strabismus
Krimsky test and cover test may also be used
Esotropia is present when one eye is moved medial and is a common type of strabismus in infants. This occurs when focusing at a distance, up close or both.
Exotropia is present when the eye moves laterally and occurs in infants that focus distant objects
Causes
- cerebral palsy
- down syndrome
- hydrocephalus
- brain tumors
- prematurity
- cataract or eye injury
- genetic predisposition
To avoid diplopia or confusion due to the simultaneous transmission of different signals from the eyes the visual system has different mechanisms
- Supression
- anormal retinal correspondence
- motor adaptation (head gets permanently adapted to centralize vision
concomitant strabismus
reasons