CHAPTER II
LITERATURE REVIEW
A. Anatomy Of The Extraocular Muscles
There are six muscles that are present in the orbit (eye socket) that attach to
the eye to move it (see figure 1). These muscles work to move the eye up and
down, side to side, and to rotate the eye. (Saunders, 2008)
Figure 1. Extraocular Muscle Anatomy
The superior rectus is an extraocular muscle that attaches to the top of the
eye. It moves the eye upward. The inferior rectus is an extraocular muscle that
attaches to the bottom of the eye. It moves the eye downward. The medial rectus
is an extraocular muscle that attaches to the side of the eye near the nose. It moves
the eye toward the nose. The lateral rectus is an extraocular muscle that attaches to
the side of the eye near the temple. It moves the eye outward. The superior
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oblique is an extraocular muscle that comes from the back of the orbit and travels
through a small pulley (the trochlea) in the orbit near the nose. It then attaches to
the top of the eye. The superior oblique rotates the eye inward around the long
axis of the eye (front to back). The superior oblique also moves the eye
downward. The inferior oblique is an extraocular muscle that arises in the front of
the orbit near the nose. It then travels outward and backward in the orbit before
attaching to the bottom part of the eyeball. It rotates the eye outward along the
long axis of the eye (front to back). The inferior oblique also moves the eye
upward. (Saunders, 2008)
Figure 2. The Muscle Anatomy Associated With Eyeball Movement,
(Anterior view, Right eye)
The optic nerve connects each eye to the brain. It is a structure that sends the
picture seen by the eye to the brain so that it can be processed. The optic nerves
end in a structure called the optic chiasm. In an adult, the optic nerve is about the
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diameter of a pencil. There are over 1 million individual nerve cells in the optic
nerve. (Saunders, 2008)
The optic chiasm is the place in the brain where the two optic nerves meet.
The individual nerve fibers from each nerve are sorted in the chiasm. The sorting
occurs in such a way that the right side of the brain controls the view of objects in
left visual space and the left side of the brain controls the view of objects in right
visual space (see figure 3). (Saunders, 2008)
Visual cortex is an area of the brain in the posterior occipital lobe to which
the neurons in the retina ultimately give visual information. The visual cortex
helps to process information regarding the image such as its color, composition,
and relation in space to other objects. This information is then sent to other parts
of the brain that serve higher visual functions. (Saunders, 2008)
Figure 3. The Optic Chiasm
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B. Congenital Exotropia
1. Definition
Exotropia is a manifest outward deviation of the visual axes of one or
both eyes and may be either constantly or intermittently present. The term is
also used loosely to describe a latent outward deviation that, more
accurately, is termed exophoria. Patients who have intermittent exotropia
compose a spectrum that extends from those that are easy to dissociate to
those that are very difficult to dissociate; thus, there is a continuum of
patients who have a form of exodeviation, as portrayed in figure 4. (Riordan
and John P. Withcher, 2007)
Figure 4. The Continuum Of Exodeviations
The term congenital exotropia is typically reserved for patients
presenting in the first year of life with a large, constant angle. (Bashour,
2014)
However, as Hunter et al (2001) state, no published study provides a
rationale for this restrictive definition. In their study, they evaluated
differences between infants, aged younger than 1 year, with constant
exotropia versus intermittent exotropia at presentation. They found that
"half of infantile exotropia patients may present with intermittent exotropia,
with similar clinical outcomes regardless of presentation." In their study,
surgical intervention resulted in successful alignment in most cases. More
than half the patients developed measurable stereopsis, but none achieved
bifixation.
True congenital exotropia (with a fixed exotropia) is an extremely rare
form of strabismus and may occur with systemic disease in as many as 60%
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of patients. Patients with craniofacial syndromes, ocular albinism, midline
defects, and cerebral palsy may present with congenital exotropia.
(Maconachie, 2013)
2. Epidemiology
Congenital exotropia is extremely rare in the United States. The
worldwide incidence of congenital exotropia is unknown. There is a higher
incidence of amblyopia in congenital exotropia than in other forms of
exotropia. No known racial predisposition to congenital exotropia exists. No
known sexual predilection exists. Congenital exotropia presents in infants
younger than 6 months. Children who are born premature are at higher risk
of developing strabismus; however, congenital exotropia does not occur at a
higher rate in premature children. (Matsuo, 2001)
3. Etiology and Pathophysiology
There is a familial predisposition suggestive of an autosomal
dominant pattern with incomplete penetrance. There is an increased
incidence with cerebral palsy and other neurologic disorders, craniofacial
disorders, and ocular albinism. (Maconachie, 2013)
A high percentage of both exotropia and esotropia patients had a
coexisting ocular or systemic abnormality. Exotropia patients with a
constant strabismus were more likely to have coexisting ocular or systemic
disease than those with an intermittent strabismus. Smaller angles of
exotropia or esotropia were associated with a higher likelihood of coexisting
ocular or systemic diseases. Systemic disorders were found more frequently
than ocular disorders in both the exotropia and esotropia groups.
(Maconachie, 2013; Matsuo, 2001)
The pathophysiology is unknown, although strabismus does occur in
families, suggesting a multifactorial autosomal dominant pattern with
incomplete penetrance. (Maconachie, 2013)
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4. Clinical presentation
Clinical presentation divided in to history and physical examination.
In history, children present congenital exotropia when they are younger than
12 months with a constant outward deviation of the eyes. In physical
examination, unlike other neurologic forms of exotropia, there should be no
pupillary or lid involvement. Although craniofacial syndromes can be seen
with congenital exotropia, there should be no ptosis or pupillary mydriasis.
The eyes should appear diverging with no limitation of adduction. Over
time, a preference may occur with one eye used consistently for fixation;
then, the other eye will develop amblyopia. As many as 60% of patients
may develop oblique muscle dysfunction, dissociated vertical deviation, and
amblyopia. Nystagmus is rare. (Mohney BG and Huffaker, 2003)
Figure 5. Exotropia appearence
5. Work Up
Radiographic imaging is indicated if neurologic signs and/or
craniofacial anomalies are present. High-resolution MRI enables direct
imaging of the ocular motor nerves. In a cohort of 247 consecutive patients
with strabismus, Kim et al demonstrated ocular motor nerve abnormalities
by high resolution MRI in 98/112 (88%) of patients with congenital or
developmental neuropathic strabismus. Chromosomal studies if any other
facial or systemic anomalies are present. (Kim E, 2012)
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6. Treatment
For medical care, the treatment is prevention of amblyopia. For the
surgery treatment, there is a bilateral lateral rectus recession usually
common used. Additional strabismus surgery for oblique muscle
dysfunction, dissociated vertical deviation, and large-angle exotropia.
(Mohney BG and Huffaker, 2003)
7. Follow Up
A child with any form of strabismus is at risk of losing vision
(amblyopia). Since these children present at a nonverbal age, it is imperative
to screen and follow the visual status during the critical years of visual
development. Amblyopia prevention by frequent ophthalmic examinations.
(Bashour, 2014)
8. Complications (Mohney BG and Huffaker, 2003)
a) Loss of depth perception
b) Amblyopia (loss of vision)
c) Neurological consequences if underlying neurologic diagnosis is
undetected
9. Prognosis
Good restoration of binocular vision if detected and treated in time.
Vision maintained if amblyopia is detected and treated while still at the
critical age of visual development. (Mohney BG and Huffaker, 2003)
10. Patient Education (Hunter, 2001)
a) Familial predisposition for siblings and offspring to develop this
or other forms of strabismus
b) Awareness of potential loss of vision, loss of depth perception,
and muscle restriction or shortening
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c) Possible need for amblyopia treatment (patching)
d) Possible need for repeated surgical procedures
C. Exotropia Et Causa Cerebral Palsy
Cerebral palsy is a disorder of movement, muscle tone or posture that is
caused by an insult to the immature, developing brain, most often before birth. It
is the most common cause of severe neurodisability in children. Signs and
symptoms appear during infancy or preschool years. In general, cerebral palsy
causes impaired movement associated with exaggerated reflexes, floppiness or
rigidity of the limbs and trunk, abnormal posture, involuntary movements,
unsteadiness of walking, or some combination of these. Cerebral Palsy may be
diagnosed very early in an infant known to be at risk for developing the condition
because of premature birth or other health problems. (Hoda, 2013)
Interruption of oxygen supply to the fetus or brain asphyxia was classically
considered to be the main causal factor explaining later Cerebral Palsy. However
several ante-, peri-, and postnatal factors could be involved in the origins of
Cerebral Palsy syndromes. Congenital malformations are rarely identified.
Cerebral Palsy is most often the result of environmental factors, which might
interact with genetic vulnerabilities, and could be severe enough to cause the
destructive injuries visible with standard imaging (i.e., ultrasonographic study or
MRI), predominantly in the white matter in preterm infants and in the gray matter
and the brainstem nuclei in full-term newborns. Moreover they act on an
immature brain and could alter the remarkable series of developmental events.
Biochemical key factors originating in cell death or cell process loss, observed in
hypoxic-ischemic as well as inflammatory conditions, are excessive production of
proinflammatory cytokines, oxidative stress, maternal growth factor deprivation,
extracellular matrix modifications, and excessive release of glutamate, triggering
the excitotoxic cascade. Only two strategies have succeeded in decreasing
Cerebral Palsy in 2-year-old children: hypothermia in full-term newborns with
moderate neonatal encephalopathy and administration of magnesium sulfate to
mothers in preterm labor. (Hoda, 2013)
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Cerebral Palsy can be a caused for exotropia congenital. When a patient
diagnosed by a Cerebral Palsy so the patient should get any examine including
eye or visual examination for the possibility of exotropia congenital. (Wright, et
all, 1995)