Page 332 - Color Atlas Of Pathophysiology (S Silbernagl Et Al, Thieme 2000)

Diseases of the Optical Apparatus of the Eye
The optical apparatus of the eye serves to pro- anterior chamber (ca. 4 µl/min) within the cili-
ject a sharp image of outer objects onto the ary body and its outflow from the chamber via
retina. The most common abnormalities of the trabecular network at the edge of the
the image-projecting apparatus are inade- chamber (the iridocorneal angle) into
quate refraction, abnormal regulation of the Schlemm’s canal (→ C). An increase in the in-
Systems internal pressure of the eye (in glaucoma), traocular pressure (high pressure glaucoma)
and lack of transparency of the light-refracting
can be due to impaired outflow of aqueous hu-
system (especially in cataract).
mor (the usual cause) or (more rarely) in-
creased production of aqueous humor. Among
Abnormalities of refraction (→ A). Viewed
Neuromuscular and Sensory ! In myopia the bulb of the eye is usually too ing of the trabecular network or narrowing of
the causes of an impaired outflow are thicken-
objects are not focussed onto the retina.
the chamber angle. The latter is often nar-
long for refraction (axial myopia). Less fre-
quently refraction is too strong (refractive
rowed if the bulb is shallow (marked axial hy-
myopia). As a result, the light that originates
peropia) or by an increase in lens thickness
from distant objects does not converge onto
with age. Widening of the pupil further nar-
rows the angle when the base of the iris is
the retina, and thus distant objects do not pro-
duce sharp images on the retina. The anomaly
broadened, as happens in the dark and
The high intraocular pressure gradually but
! In hyperopia the bulb is either too short (ax-
irreversibly damages the optic nerve, leading
ial hyperopia) or the refraction too low (refrac-
10 can be corrected by means of a concave lens. through sympathetic nervous stimulation.
to visual field defects that start around (Ma-
tive hyperopia). As a result, light that origi-
nates from a near object can no longer con- riotte’s) blind spot and in the nasal periphery
verge on the retina and near objects are not (→ C2). Attempts at treating the defects in-
seen clearly. The abnormality can be corrected volve lowering the intraocular pressure by nar-
by means of a convex lens. rowing the pupil (parasympathetic drugs) and
! The plasticity of the lens deteriorates with reducing aqueous production. Aqueous humor
–
age and thus also its maximal curvature on secretion, like the reabsorption of HCO 3 in the
near accommodation. This results in presbyo- kidney’s proximal tubules (→ p. 96ff.), re-
pia, the inability to see near objects clearly. A quires the action of carbonic anhydrase and
convex lens is necessary for viewing near ob- can be reduced by carbonic anhydrase inhibi-
jects, although it has to be removed when tors. Even without a rise in pressure, damage
looking at distant ones. to the optic nerve typical of glaucoma can oc-
Astigmatism (→ B). The surface of the eye is cur (low-pressure glaucoma), probably due to
not perfectly spherical. In regular astigmatism reduced blood perfusion.
the curvature’s radiuses in the horizontal and Cataract. The transparency of the lens is,
vertical axes are different; and an upright among other factors, dependent on a strictly
square is imaged as a rectangle. This abnor- regulated water content. In diabetes mellitus
mality can be corrected by means of a cylindri- a high glucose concentration brings about gly-
cal lens. A minor form (< 0.5 diopter) of regular cosylation of proteins (advanced glycation
astigmatism, with increased refraction in the end-products [AGE]) (→ C3). Similar products
vertical direction, is normal. In oblique astig- also accumulate with age. In diabetes mellitus
matism the normally horizontal and vertical there is also an accumulation of sorbitol in the
axes are oblique to one another. In irregular lens (→ p. 290). Irregular hydration and a
astigmatism the corneal surface is irregular, change in connective tissue proteins bring
for example, due to a corneal scar, which can about clouding or opacification of the lens
be corrected by a contact lens (more recently (cataract; → C3).
by laser treatment).
Glaucoma. The pressure within the eyeball
322 (ca. 10–20 mmHg) results from the equilib-
rium between the secretion of fluid into the
Silbernagl/Lang, Color Atlas of Pathophysiology © 2000 Thieme
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