Download 8-Strabismus

Strabismus:
Defination:it is misaligment of visual axis. Misaligment may be in any
direction –inward,outward,up, or down.the amount of deviation is angle by
which the deviating eye is misaligned.strabismus present under binocular
viewing condition is manifest strabismus ,hetrotropia ,or tropia.a deviation
present only after binocular vision has been interrupted (ie, by occlusion of
one eye) is called latent strabismus, hetrophoria, or phoria.
Physiology
1.motor aspects
each of the six extraocular muscles plays a role in positioning the eye about
three axes of rotation .the primary action of a muscle is the principal effect it
has on eye rotation .lesser effects are called secondary or tertiary actions.the
precise action of any muscle depends on the orientation of the eye in the orbit
and the influence of the orbital connective tissues,which regulate the direction
of action of the extraocular muscles by acting as their functional mechanical
origins(the active bulley hybothesis).
The medial and lateral rectus muscles adduct and abduct the
eye,respectively,with little effect on elevation or torsion.the vertical rectus
and oblique musclesdebre have vertical rotation and torsional functions.
In general terms,the vertical rectus muscles are the main elevators and
depressors of the eye,and the obliques are mostly involved with torsional
positioning.the vertical effect of the superior and inferior rectus muscles is
greater when the eye is abducted.the vertical effect of the obliques is greater
when the eye is adducted.
Field of action
The position of the eye is determined by the equilibrium achieved by the pull
of all six extraocular muscles.the eyes are in the primary position of gaze
when they are looking straight ahead with the head and body erect.to move
the eye into another direction of gaze,the agonist muscle contracts to pull the
eye in that direction and the antagonist muscle relaxes.the filed of action of a
muscle is the directionof gaze in which that muscle exerts its greatest
contraction force as an agonist,eg,the lateral rectus muscle undergoes the
greatest contraction in abducting the eye.
Synergistic & antagonistic muscles(sherringtion’s law)
Synergistic muscles are those that have the same field of action.thus,for
vertical gaze,the superior rectus and inferior oblique muscles are synergists
in moving the eye upward. Muscles synergistis for one function may be
antagonistMuscles synergistis for one function may be antagonistic for
another .for example , the superior rectus and inferior oblique muscles are
antagonists for torsion ,the superior rectus causing intorsion and the inferior
oblique muscles are antagonists for torsion, the superior rectus causing
intorsion and the inferior oblique extorsion.the extraocular muscles, like
skeletal muscles, show reciprocal innervations of antagonistic muscles
(Sherrington’s law).thus, in dextroversion(right gaze), the right medial and
left lateral rectus muscles are inhibited while the right lateral and left medial
rectus muscles are stimulated.
Yoke Muscles(Hering’s Law)
For movements of both eyes in the same direction, the corrponding agonist
muscles receive equal innervation(Hering Iaw). reciprocal innervations of
antagonistic muscles (Sherrington’s law).the pair of nsieagonist muscles with
the same primary action is called a uoke pair.the right Iateral rectus and the
left medial rectus muscles are a yoke pair for right gaze.the right inferior
rectus and the left superior oblique muscles are a yoke pair for gaze
downward and to the right. The neuromuscular system of an infant is
immature so that it is not uncommon in the first few months of life for ocular
alignment to be unstable.Transient esodeviations are most common and may
be associated with immaturity of the accommodation-convergence system
Gradually improving visual acuity together maturation of the oculomotor
system allows a more stable ocular alignment by age 2 months.Any ocular
misalignment after this age should be investigated
Development of Binocular Movement
2.Sensory Aspects
Binocular Vision
In each eye, whatever is imaged on the fovea is seen subjectively as being
straight ahead.thus, if two dissimilar objects were imaged on the two
foveas,but the dissimilarities would prevent fusion eye,the image in each eye
is actually slightly different from that in the other.Sensory fusion and
stereopsis are the two different physiologic processes that are responsible for
binocular vision.
Sensory Fusion & Stereopsis
Sensory fusion is the process whereby dissimilarities between the two images
are not appreciated. On the peripheral retina of each eye, there are
corresponding points that in the absence of fusion Iocalize stimuli in the
same direction in space. In the process of fusion, the direction values of these
points can be modified.thus each point of the retina in each eye is capable of
fusing that strike sufficiently close to the corrsponding point in the other eye.
Thes region of fusible points is called Panum’s area.
Sensory changes in Strabismus
Up to age 7 or 8,the brain usually develops responses to abnormal binocular
vision that may not occur if the onset of sttabismus is later. These changes
include diplopia, suppression, anomalous retinal correspondence, and
eccentric fixation.
A.DIPLOPIA
If strabismus is present, each fovea receives a different image. The objects
image on the two foveas are seen in the same direction in space. This process
of localization of spatially objects to the same location is called visual
confusion .the object viewed by one of the foveas is imaged on a peripheral
retinol area in the other eye. The foveal image is localized straight ahead, while the
peripheral image of the same object in the other eye is localized in some other
direction. Thus, the same object is seen in two places(diplopia).
B.SUPPRESSION
Under binocular viewing conditions, the images seen by one eye become
predominant and those seen by the other eye are not perceived
(suppressipon). Suppression takes the form of a scotoma in the deviationg eye
Only under binocular viewing conditions. (A scotoma is an area of reduced
vision within the visual field; surrounded by an area of less depressed or
normal vision.)
c.Amblyopia:
prolonged abnormal visual experience in a child under the age of 7 years may
lead to amblyopia (reduced visual acuity in the absence of detectable organic
disease in one eye).the three clinical causes of amblyopia due to visual
deprivation e.g congenital cataract, strsbismus or unequal refractive error
(anisometropia).
D .Anomalous retinal correspondance.
ARC is a sensory adaptation that occurs in strabismus under
binocular viewing conditions.
E.Eccentric fixation:
In eyes with suffeciently severe amblyopia, an extrafoveal retinal area may be
used for fixation under monocular viewing conditions. It is always associated
with severe amblyopia and unstable fixation. The eccentric fixation point is
often not displaced in a dircetion appropriate to the direction of strabismus
(eg, the nasal retina in esotropia). Gross eccentric fixation can be readily
identified clinically by occluding the dominant eye and directing the patient’s
attention to a light source held directly in front. An eye with gross eccentric
fixation will not point toward the light source but will appear to be looking in
some other direction. More subtle degrees of eccentric fixation can be
detected by an ophthalmoscope that projects a small fixation target onto the
retina.
EXAMINATION
HISTORY
A careful history is important in the diagnosis of strabismus.
A.FAMILY HISTORY
Strabismus and amblyopia are frequently found to occur in families.
B.AGE AT ONSET
This is an important factor in long-term prognosis. The earlier the onset of
strabismus, the worse the prognosis for good binocular function.
C.TYPE OF ONSET
The onset may be gradual, sudden, or intermittent.
D.TYPE OF DEVIATION
The misalignment may be in ang direction. It may be greater in certion
positions of gaze, including the primary position for distance or near.
E.FIXATION
One eye may constantly deviated, or alternating fixation may be observed
Visual Acuity
Visual acuity should be evaluated even if only a rough approximation or
comparison of the two eyes is possible. Each eyeis evaluated by itself, since
binocular testing will not reveal poor vision in one eye. For the very young
ptarget. The target should be as small as the child’s age, interest, and level of
alertness allow . fixation is described as being normal if it is
centrally(foveally) fixated and maintained while the eye follows a moving
object. One technique for quantitatively measuring visual acuity in younger
children is forced-choice preferential looking.
By the age of 2.5-3 years, it is possible to perform recognition visual acuity
testing using the Allen pictures By age 4 years, many children will understand
the Snellen tumbling ‘’E’’game and the HOTV recognition test. By age 5 or 6
years, most children can respond to snellen alphabet visual acuity testing. At
this age, single optotype snellen acuity has normally developed fully, but
snelles acuity to a line of multiple optoypes (linear acuity) may not develop
fully for another 2 years.
Determination of Refactive Error.
In children Cyclorefraction using cycloplegic agent to know the refractive
errors by retinoscopy.
Inspection :
Whether the strabismus constanttermi tant, alternating or nonalternating and
variable or constant also asociated ptosis any abnormal head position .
Determination of Angle of strabismus (Angle of
Deviation)
a-prism and cover tests :
cover tests consist of four parts:(1)the cover test, (2)the uncover test, (3)the
alternate cover test, and (4)the prism cover test.
B.Objective Tests
1.Hirschberg method.
2.Prism reflex method (Krimsky test).
Ductions (Monocular Rotations)
Disjunctive Movements
A.Convergence
B.Divergence
Sensory Examination
A.Stereopsis Testing
B.Suppression Testing
C.Fusion Potential
OBJECTIVES &PRINCIPLES OF THERAPY OF STRABISMUS
Timing of treatment in children
Medical treatment
A.Treatment of Amblyopia
1.Occlusion therapy
a.Initial stage
b.Maintenance stage
2.Atropine therapy
B.Optical Devices
1.spectacles
2.Prisms
c.Botulinum Toxin
D.Orthoptics
Surgical Treatment(Figure 12-6)
A.Surgical Procedures
1.Resection and recession
2.Shifting of point of muscle attachment
3.Faden procedure
B.Choice of muscles for surgery
C.Adjustable sutures