| |
Retinitis Pigmentosa (RP) is a progressive genetic
degeneration of the retina, characterized by loss of night vision,
constriction of side vision ('tunnel vision'), diminution of color
vision and gradual loss of central visions. In RP the rods (the
retinal cells responsible for night vision) degenerate first, then
the cones (the retinal cells responsible for day vision), and secondarily
the retinal pigment epithelium cells, the outermost retinal layer
resembling little cubes stuffed pigment granules ('coffee grounds'),
rupture, and the pigment disperses, giving the condition its name.
RP affects 1 in 4000 people. A small percentage of patients with
RP will develop glaucoma and cataracts at an early age. Macular
edema occurs in approximately 10% of patients with RP. The diagnosed
may be made by ophthalmoscopy (examination of the retina with dilated
pupil) and visual field testing; however the definitive test is
a properly performed and interpreted electroretinogram. Vitamin
A palmitate, 15,000 units per day has been proven to help slow down
the progression of RP.
Top
flight research on this disease is performed by the Berman Gund
Laboratory at Harvard.
Retinal Detachment (RD) secondary to retinal tears
(rhegmatogenous retina detachment) occurs in approximately 1:10,000
in the general population, but is more common in myopes (near-sighted
individuals) and after cataract surgery. Trauma accounts for less
than 2% of all cases of retinal detachment. The classic symptoms
of RD are floaters, flashes, and blurred vision, often described
as a 'curtain' over the field of vision. The confluence of factors
leading to detachment include:1) a congenital tenacious adherence
of vitreous strands to the retina, 2) partial liquefaction of the
vitreous, with incomplete separation of vitreous from the retina
(one of the causes of floaters), 3) traction of vitreous strands
on the retina (one of the causes of flashes), 4) formation of retinal
holes or tears, 5) fluid vitreous passing through the tear and accumulating,
with possibly other fluid, and separating the retinal layers, the
neurosensory retina from the pigment epithelium --"the wallpaper
from the glue layer-(the cause of the blur/curtain).
Binocular indirect ophthalmoscopy, a dynamic exam involving some
pressure on the eyeball (scleral depression) to bring the peripheral
retina into view (that is where the causative tears usually are)
is imperative, because if all the retinal tears are not identified
and sealed, attempts to reattach the retina are doomed to fail.
The standard operation to reattach the retina is a scleral buckle,
perhaps better termed scleral indentation (in essence it is like
pushing in the wall to reattach to the separated wallpaper). It
consists of 1) identifying all the causative retinal holes and the
extent of detachment (indirect ophthalmoscopy with scleral depression),
2) applying a stimulus through the sclera (the outer coat of the
eyeball) to create a 'sticky surface' on that area inside from which
the retina is separated, 3) perforating the eyeball to remove the
fluid from under the retina (it flows out), 4) creating an indentation
by suturing a silicone band under the tears and encircling the globe
with a silicone 'belt'.
If retinal tears are diagnosed sufficiently early, before fluid
accumulates under the retina, they may be successfully sealed by
laser or cryotherapy, thus avoiding the detachment and more extensive
surgical procedure. In some selective cases, if the detachment is
limited and involving the superior part of the retina, pneumatic
retinopexy may be considered. It involves intravitreal injection
of expansile gas, which by expanding 'pushes' the retina back in
place. Laser or cryotherapy is then used to seal the retinal tears.
The success rate of retinal surgery is in the upper 90s%. In those
cases where scleral buckle is not successful or if the detachment
is more complex (because of extensive scarring), vitrectomy may
be necessary. Such a procedure involves removing the vitreous, peeling
the scar tissue from the retina, reattaching the retina by aspirating
the subretinal fluid and infusing gas in the eye, and applying laser
to seal the tears. The gas, which helps keep the retina in place
while laser takes effect ("while the glue hardens"), gradually
is absorbed.
Diabetic Retinopathy refers to abnormal changes
in the small blood vessels (capillaries) of the retina in patients
with diabetes, whether type I or type II. Elevated levels of sugar
(glucose) in the blood is the primary cause of vessel damage, which
consists of thickening or thinning of the capillary wall. The result
of such damage is blockage of flow in the region of thickening (capillary
closure) and accumulation of fluid (edema) due to leakage where
the capillary wall is thinned and bulging out (microaneurysm). In
either case the effect is starvation of those retinal cells deprived
of nourishment (ischemia). Depending on which region of the retina
is involved, vision may be directly or indirectly affected. If the
macula is affected (macular edema), central vision may be mildly
to moderately reduced; in more peripheral involvement, the patient
may be totally unaware until more severe complications occur.
Diabetic retinopathy is divided into two stages: non-proliferative
and proliferative. Non-proliferative diabetic retinopathy (NPDR),
also known as 'background diabetic retinopathy (BDR)' refers to
the early stage of capillary closure. The retinal changes include:
red dots (microaneurysms), white fluffy spots (ischemic infarcts,
'cotton wool' exudates), and small red blots (microhemorrhages).
Unless the macula is involved at this stage, the patient is without
symptoms.
Proliferative diabetic retinopathy (PDR) is the more advanced stage
with potentially serious effect on the vision. Characterized by
abnormal growth of blood vessels out of the retina into the vitreous
cavity (neovascularization), it is the 'body's response' to starving
tissue-an unfortunately misdirected attempt to replace or by-pass
the closed or leaking capillaries. The neovascular vessels are poorly
formed, fragile, and combined with traction from adherent vitreous,
inevitably bleed. The visual effects of vitreous hemorrhage vary
from 'dark floaters' to 'clouds' to loss of vision. The horror stories
and fears related to diabetic retinopathy are related to vitreous
hemorrhage. However, gradually vitreous hemorrhages clear. More
devastating is the 'scar tissue' (fibrous proliferation) which accompanies
the neovascularization and proliferates with each successive bleed.
With time, the scar tissue contracts (as does all scar tissue in
the body). The traction on the retina, to which it is attached,
results in retinal detachment (traction retinal detachment), severe
vision loss (if the macula is involved), and in a few cases a type
of glaucoma virtually unresponsive to any treatment (neovascular
glaucoma) .
A simplistic way to think of diabetic retinopathy, if untreated,
is that it progresses in 3 steps:
1) blots and dots 2) new blood vessels and hemorrhage, and 3) traction
detachment,--with not much vision loss in #1, severe vision loss
in #2, and hopeless vision loss in #3.
Treatment of diabetic retinopathy should be directed first and foremost
to tight blood glucose control and other systemic conditions which
aggravate it (blood pressure, anemia, heart failure, renal failure),
with proper emphasis on diet, weight and exercise and medications.
For diabetic retinopathy, laser remains the only proven treatment,
based on nationwide multicenter trials (The Early Treatment Diabetic
Retinopathy Study-'ETDRS' for macular edema, and the Diabetic Retinopathy
Study -DRS for neovascularization.) In non-clearing vitreous hemorrhages
vitrectomy has been successful in improving vision by surgically
removing the blood. Vitrectomy,
with peeling of the fibrous membranes and adjunctive use of special
gases or silicone oil, has been moderately successful in re-attaching
the retina and salvaging some vision in eyes previously relegated
to blindness.
Retinal Artery or Vein Occlusions not only are responsible for a
sizeable percentage of vision loss but also are indicative of significant
systemic vascular disease. The retinal circulation, though complex,
may be thought of as like a tree, with various branches progressively
smaller. Obstruction of flow may involve any of the branches or
the central 'trunk'. The cause is seldom clear-cut, though association
with various other conditions is inescapable. Branch retinal artery
occlusion (BRAO), presenting as loss of vision in a portion of the
visual field, occurs virtually always in patients with atherosclerotic
disease or secondary to blockage from emboli (platelets, calcium,
cholesterol). Central retinal artery occlusion (CRAO) most commonly
is caused by atherosclerosis or emboli similar to BRAO; however,
vision loss is more severe, involving the entire visual field. A
rarer cause of abrupt vision loss is Giant Cell Arteritis (GCA)or
Temporal Arteritis (TA). It is an auto-immune inflammation of the
arteries that may affect one eye and then the other and lead to
total blindness in a matter of days. In BRAO and CRAO, the diagnostic
evaluation focusing on the carotid arteries and the heart may be
performed over a few days, but in suspected giant cell arteritis
it must be done in hours. Blood tests-sedimentation rate and C-reactive
protein are helpful in establishing the diagnosis, but the 'gold
standard' is a temporal artery biopsy. Treatment with high dose
steroids is instituted even before the test results are obtained.
Vision loss from retinal vein occlusion may be less abrupt than
what occurs with arterial occlusion and generally less severe. Branch
retinal vein occlusion is virtually always associated with elevated
blood pressure. The resultant partial visual loss, corresponding
to the segment of retina affected, is explained by the retinal hemorrhages,
edema and possibly ischemia in the involved retina. The latter may
be documented with fundus fluorescein angiography (FFA) and optical
coherence tomography (OCT). If edema is documented and does not
clear within 4-6 months, laser may be of some help in decreasing
it. Visual loss from central retinal vein occlusion (CRVO) is a
general obscuration of the entire visual field. The appearance of
the retina is described as "pizza-pie" or "blood
and thunder". Though the cause is not known, frequently associated
conditions include: atherosclerosis, glaucoma, diabetes, hypertension,
and a variety of blood diseases which increase its viscosity. In
addition to visual loss, a significant complication of all vascular
occlusions is the development of new blood vessels (neovascularization)
which grow out of the retina into the vitreous cavity, where they
can bleed and further aggravate visual loss. A more devastating
complication is neovascular glaucoma, caused by growth of new blood
vessels over the iris and the angle between it and the cornea, thus
closing the channels through which aqueous fluid normally flows
out of the eye. Neovascular glaucoma can be so painful and disfiguring
as to require enucleation (removal of the eye). With timely intervention
with laser or cryotherapy, such a radical procedure is now extremely
rare.
AGE RELATED MACULAR DEGENERATION (AMD) is the leading
cause of blindness in people over 60 years of age. It robs patients
of their central vision, but the peripheral vision is retained,
so that patients do not go totally blind. The cause(s) of AMD is
(are) not known. At present the condition is like a puzzle with
only a few pieces and not a clear idea of where, and if, they belong.
In a descending order of importance one might list:
genetics, age, caucasian race, smoking, hypertension, hypercholesterolemia,
blue iris, body fat, and possibly, exposure to light.
AMD is generally subdivided into "dry" type and "wet".
In dry AMD there is a disintegration (atrophy) of the outermost
layer of the retina (the retinal pigment epithelium) which is analogous
to the glue layer under wallpaper (the wallpaper being analogous
to the rest of the retina). As patches of pigment epithelium atrophy,
degeneration of the overlying retinal cells follows, often forming
a geographic pattern. Vision loss is dependent on the extent of
atrophy. Approximately 15% of patients with dry AMD will develop
wet AMD which can lead to more severe vision loss.
Wet AMD is the result of new blood vessels 'growing' under the retina,
where they leak or bleed causing progressive enlargement of a central
blind spot.
Presently available treatment for AMD includes: vitamins (anti-oxidants
with zinc) for a small specific group of patients with dry AMD,
and laser for some of the patients with the wet type (the thermal
- hot - laser or photodynamic therapy ). Unfortunately, none of
the treatments is curative. There is promising research but nothing
yet proven. For now low vision aids are the most helpful approach
to some visual rehabilitation.
|
|
