Glaucoma: Definition, Classification,Pathophysiology And Aetiology

Definition
Glaucoma is not a single disease process but a group
of disorders characterized by a progressive optic
neuropathy resulting in a characterstic appearance
of the optic disc and a specific pattern of irreversible
visual field defects that are associated frequently but
not invariably with raised intraocular pressure (IOP).
Thus, IOP is the most common risk factor but not the
only risk factor for development of glaucoma.
Consequently the term ‘ocular hypertension’ is used
for cases having constantly raised IOP without any
associated glaucomatous damage. Conversely, the
term normal or low tension glaucoma (NTG/LTG) is
suggested for the typical cupping of the disc and/or
visual field defects associated with a normal or low
IOP.
Classification
Clinico-etiologically glaucoma may be classified as
follows:
(A) Congenital and developmental glaucomas
1. Primary congenital glaucoma (without associated
anomalies).
2. Developmental glaucoma (with associated
anomalies).
(B) Primary adult glaucomas
1. Primary open angle glaucomas (POAG)
2. Primary angle closure glaucoma (PACG)
3. Primary mixed mechanism glaucoma
(C) Secondary glaucomas
PATHOGENESIS OF GLAUCOMATOUS OCULAR
DAMAGE
As mentioned in definition, all glaucomas (classified
above and described later) are characterized by a
progressive optic neuropathy. It has now been
recognized that progressive optic neuropathy results
from the death of retinal ganglion cells (RGCs) in a
typical pattern which results in characteristic optic
disc appearance and specific visual field defects.
Pathogenesis of retinal ganglion cell death
Retinal ganglion cell (RGC) death is initiated when
some pathologic event blocks the transport of growth
factors (neurotrophins) from the brain to the RGCs.
The blockage of these neurotrophins initiate a
damaging cascade, and the cell is unable to maintain
its normal function. The RGCs losing their ability to
maintain normal function undergo apoptosis and also
trigger apoptosis of adjacent cells. Apoptosis is a
genetically controlled cell suicide programme whereby
irreversibaly damaged cells die, and are subsequently
engulfed by neighbouring cells, without eliciting any
inflammatory response.Retinal ganglion cell death is, of course, associated
with loss of retinal nerve fibres. As the loss of nerve
fibres extends beyond the normal physiological
overlap of functional zones. The characteristic optic
disc changes and specific visual field defects become
apparent over the time.
Etiological factors
Factors involved in the etiology of retinal ganglion
cell death and thus in the etiology of glaucomatous
optic neuropathy can be grouped as below:
A. Primary insults
1. Raised intraocular pressure (Mechanical theory).
Raised intraocular pressure causes mechanical stretch
on the lamina cribrosa leading to axonal deformation
and ischaemia by altering capillary blood flow. As a
result of this, neurotrophins (growth factors) are not
able to reach the retinal ganglion cell bodies in
sufficient amount needed for their survival.
2. Pressure independent factors (Vascular
insufficiency theory). Factors affecting vascular
perfusion of optic nerve head in the absence of raised
IOP have been implicated in the glaucomatous optic
neuropathy in patients with normal tension glaucoma
(NTG). However, these may be the additional factors
in cases of raised IOP as well. These factors include:
i. Failure of autoregulatory mechanism of blood
flow. The retina and optic nerve share a peculiar
mechanism of autoregulation of blood flow with
rest of the central nervous system. Once the
autoregulatory mechanisms are compromised,
blood flow may not be adequate beyond some
critical range of IOP (which may be raised or in
normal range).
ii. Vasospasm is another mechanism affecting
vascular perfusion of optic nerve head. This
hypothesis gets credence from the convincing
association between NTG and vasospastic
disorders (migranous headache and Raynaud's
phenomenon).
iii. Systemic hypotension particularly nocturnal dips
in patients with night time administration of
antihypertensive drugs has been implicated for
low vascular perfusion of optic nerve head
resulting in NTG.
iv. Other factors such as acute blood loss and
abnormal coagulability profile have also been
associated with NTG.
B. Secondary insults (Excitotoxicity theory)
Neuronal degeneration is believed to be driven by
toxic factors such as glutamate (excitatory toxin),
oxygen free radicals, or nitric oxide which are released
when RGCs undergo death due to primary insults. In
this way the secondary insult leads to continued
damage mediated apoptosis, even after the primary
insult has been controlled.