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Disorders of Special Sensory Function: Vision, Hearing, and Vestibular Function
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segment, and a highly modified outer segment. The cell
membrane of the outer segment is tightly folded to form
membranous disks (rods) or conical shapes (cones) con-
taining visual pigment. These disks are continuously
synthesized at the base of the outer segment and shed at
the distal end. Discarded membranes are phagocytized
by the retinal pigment cells. If this phagocytosis is dis-
rupted, as in a condition called
retinitis pigmentosa
, the
sensory retina degenerates.
An area near the center of the retina, called the
macula lutea
(i.e., “yellow spot”), is especially adapted
for acute and detailed vision.
16
This area is composed
entirely of cones. In the central portion of the macula,
the
fovea centralis
(foveola), the blood vessels and inner-
most layers are displaced to one side instead of resting
on top of the cones. This allows light to pass unimpeded
to the cones without passing through several layers of
the retina. Many of these cones are connected one to
one with ganglionic cells, an arrangement that favors
high acuity.
Retinal Blood Supply
The blood supply for the retina is derived from two
sources: the choriocapillaris (i.e., the capillary layer of
the choroid) and branches of the central retinal artery
(Fig. 38-7A). Oxygen and other nutrients are supplied
by diffusion from blood vessels in the choroid. Because
the choriocapillaris provides the only blood supply
for the fovea centralis, detachment of this part of the
sensory retina from the pigment epithelium causes
irreparable loss of vision. The central artery, which is
a branch of the ophthalmic artery, supplies the rest of
the retina. Retinal veins follow a distribution paral-
lel to the arterial branches and carry venous blood to
the central vein of the retina, which exits the back of
the eye through the optic disk.
Funduscopic examination of the eye with an ophthal-
moscope provides the means for examining the retinal
blood vessels and other aspects of the retina (Fig. 38-7B).
Because the retina is an embryonic outgrowth of the
brain and the blood vessels are to a considerable extent
representative of brain blood vessels, the ophthalmo-
scopic examination of the fundus of the eye permits the
study and diagnosis of metabolic and vascular diseases of
the brain as well as pathologic processes that are specific
to the retina.
Retinopathies
Retinopathies, which involve the small blood vessels
of the retina, are characterized by changes in vessel
structure and the development of microaneurysms, neo-
vascularization, hemorrhage, and retinal opacities.
24,25
Microaneurysms
are outpouchings of the retinal vascu-
lature. On ophthalmoscopic examination, they appear
as minute, unchanging red dots associated with blood
vessels. Microaneurysms tend to leak plasma, resulting
in localized edema that gives the retina a hazy appear-
ance. They may also bleed, but areas of hemorrhage
and edema tend to clear spontaneously. However,
they reduce visual acuity if they encroach on the mac-
ula and cause degeneration before they are absorbed.
Neovascularization
involves the formation of new
blood vessels. They can develop from the choriocapil-
laris, extending between the pigment and the sensory
layers of the retina; or from the retinal veins, extending
between the sensory retina and the vitreous cavity and
sometimes into the vitreous. These new blood vessels are
fragile, leak protein, and are likely to bleed. Although
FIGURE 38-7.
(A)
Retinal circulation showing the distribution of the central retinal and posterior
ciliary arteries and the central retinal vein.
(B)
Funduscopic image of the normal retina (From Moore
KL, Daley AF, Agur AMR. Clinically Oriented Anatomy. 6th ed. Philadelphia, PA: Wolters Kluwer Health
| Lippincott Williams &Wilkins; 2010:897.)
Vitreous chamber
(contains vitreous humor)
Retinal blood vessels
Optic disk (blind spot)
Optic nerve
Central retinal
artery
Macula lutea
Fovea centralis
Retina
Choroid
Sclera
Posterior ciliary artery
Central retinal vein
A
Optic disk
Macula
Branches of retinal vessels
B
