Biology Reference
In-Depth Information
2.1. Glaucoma
As mentioned previously, the most prevalent cause of irreversible blindness
in the world is glaucoma. The glaucomas are in reality a group of optic neu-
ropathies that have in common a slow and relentless progressive degenera-
tion of retinal ganglion cells and their axons. The most pathognomonic
feature of glaucoma is the appearance of the optic nerve head (or disc),
the structure within the eye that represents to convergence of retinal gan-
glion cell axons as they turn 90 and exit the eye. In glaucoma, the optic
nerve head becomes excavated as the width of the axons lost within the
neuroretinal rim, baring the lamina cribrosa, a stack of collagenous plates
through which bundles of axons course. This excavation, or enlargement
of the “cup,” occurs without significant pallor of the remaining neuroretinal
rim, as is seen in the other optic neuropathies, and thus serves as a valuable
diagnostic criterion. Consequent to the axonal loss, which likely occurs from
injury at the optic nerve head itself ( Howell et al., 2007; Soto et al., 2008 ),
there are nerve fiber bundle visual field defects. Typical field defect
morphologies include arcuate, wedge-shaped, arising from the temporal
side of the physiological blind spot (corresponding to the optic nerve
head), and “steps” in the nasal field above or below the horizontal
meridian. All of these visual field defects directly correspond to lesions of
specific bundles of axons at different locations within the optic disc,
hence the term “nerve fiber bundle defects.”
Interestingly, symptoms of central visual loss (i.e., decreased acuity) or
dyschromatopsia are uncommon in glaucoma until late in the course of dis-
ease. Recent work has shown that glaucomatous optic neuropathy also
involves centrally located ganglion cells and their axons ( Hood et al.,
2011 ). The fact that this involvement is relatively less clinically apparent than
the more peripheral defects suggests that there is increased redundancy in
this part of the visual system, that the glaucomatous pathophysiology is less
powerful centrally, or some other mechanism.
Our understanding of glaucomatous optic neuropathy is based partly on
pathology and partly on clinical features. These risk factors are informative in
that they give insight into the pathophysiological process underlying what is
still an incompletely understood disease.
A major risk factor for glaucoma is an increase in the pressure within the
eye or intraocular pressure (IOP). In population-based surveys, the preva-
lence of glaucoma geometrically increases with increasing IOP ( Tielsch
et al., 1991 ). Historically, glaucoma was equated with elevated IOP, based
on their concurrence in patients presenting with glaucomatous optic nerve
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