Chapter II

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)