C h a p t e r 3 8
Disorders of Special Sensory Function: Vision, Hearing, and Vestibular Function
971
half-fields in the two eyes is called a
heteronymous loss,
and the abnormality is called
heteronymous hemianopia.
Destruction of one or both lateral halves of the chiasm is
common with multiple aneurysms of the circle of Willis
(see Chapter 37). In this condition, the function of one or
both temporal retinas is lost, and the nasal fields of one or
both eyes are lost. The loss of the temporal fields (nasal
retina) of both eyes is called
bitemporal heteronymous
anopia.
With both eyes open, the person with bilateral
defects still has the full binocular visual field.
Loss of the optic tract, LGN, full optic radiation,
or complete visual cortex on one side results in loss
of the corresponding visual half-fields in each eye.
Homonymous
means “the same” for both eyes. In left-
side lesions, the right visual field is lost for each eye and is
called
complete right homonymous hemianopia.
Partial
injury to the left optic tract, LGN, or optic radiation can
result in the loss of a quarter of the visual field in both
eyes. This is called
homonymous quadrantanopia,
and
depending on the lesion it can involve the upper (superior)
or lower (inferior) fields. The LGN, optic radiation, and
visual cortex all receive their major blood supply from
the posterior cerebral artery; unilateral occlusion of this
artery results in complete loss of the opposite field (i.e.,
homonymous hemianopia). Bilateral occlusion of these
arteries results in total cortical blindness.
The Extraocular Eye Muscles
and Disorders of Eye Movement
For complete function of the eyes, it is necessary that
the two eyes point toward the same fixation point
and that the retinal and central nervous system (CNS)
visual acuity mechanisms are functional. Despite slight
variations in the view of the external world for each
eye, it is important that these two images become
fused, which is a forebrain function. Binocular fusion
is controlled by ocular reflex mechanisms that adjust
the orientation of each eye to produce a single image. If
these reflexes fail,
diplopia
or double vision occurs.
Binocular vision depends on three pairs of extraocular
muscles—the medial and lateral recti, the superior and
inferior recti, and the superior and inferior obliques
16
(Fig. 38-15). Each of the three sets of muscles in each
eye is reciprocally innervated so that one muscle relaxes
when the other contracts. Reciprocal contraction of the
medial and lateral recti moves the eye from side to side
(adduction and abduction); the superior and inferior recti
move the eye up and down (elevation and depression).
The oblique muscles rotate (intorsion and extorsion) the
eye around its optic axis. A seventh muscle, the levator
palpebrae superioris, elevates the upper eye lid.
The extraocular muscles are innervated by three cranial
nerves. The trochlear nerve (CN IV) innervates the supe-
rior oblique, the abducens nerve (CN VI) innervates the
lateral rectus, and the oculomotor nerve (CN III) inner-
vates the remaining four muscles. Table 38-1 describes
the function and innervation of the extraocular muscles.
Normal vision depends on the coordinated action of
the entire visual system and a number of central control
systems. It is through these mechanisms that an object
is simultaneously imaged on the fovea of both eyes and
perceived as a single image. Strabismus and amblyopia
are two disorders that affect this highly integrated
system.
Strabismus
Strabismus, or squint, refers to any abnormality of eye
coordination or alignment that results in loss of binocular
vision. Strabismus affects approximately 4% of children
younger than 6 years of age.
39–41
Because 30% to 50%
of these children sustain permanent secondary loss of
vision, or amblyopia (to be discussed), early diagnosis
and treatment are essential.
Strabismus may be divided into two forms: paralytic,
in which there is weakness or paralysis of one or more
of the extraocular muscles; and nonparalytic, in which
there is no primary muscle impairment. In terms of
Levator palpebrae
superioris
Superior
oblique
Superior
rectus
Medial
rectus
Lateral
rectus
Inferior
rectus
Inferior
oblique
Medial rectus
Superior rectus
Inferior rectus
Lateral rectus
Temporalis muscle
Inferior oblique
Stump of
levator palpebrae
Superior oblique
Optic nerve
FIGURE 38-15.
Extraocular muscles of the right eye.
