Retina - thick nerve layer, sensitive to light
At the back of the eye, it is attached to ‘choroid’, a layer with a rich blood supply.
The light that falls on the eye is focused onto the retina with the help of the cornea and the lens. It stimulates the light receptor cells present in the retina to generate an electrical impulse which is then carried to the brain by means of the optic nerve. The optic region of the brain processes the information conducted through these electrical impulses and allows us to understand what we are seeing.
The retina consists of two types of photoreceptor cells, namely the rods and the cones. The cones help us to see the fine details of the objects we view. They are found in dense concentration in ‘macula’, a small area at the centre of the retina. The rods help the eye to see in dim light and provide peripheral vision as they are mainly found in the peripheral parts of the retina.
The retina has a very rich blood supply and is therefore red in color. It is basically supplied by two main arteries:
- The ‘choroidal blood vessels’ which form 65-80% of the total blood supply and provide nourishment to the outer retina which contains the rods and the cones.
- The ‘central retinal artery’ which enters the retina along with the optic nerve and forms the remaining 20-30% of retinal blood supply. It is responsible for supplying the inner retinal tissue comprising the nerve cells.
Retinal detachment is the condition in which the retina gets separated from the choroid and the vitreous leaks into the space between the two. In certain conditions of the eye, the vitreous shrinks. This exerts traction on the retina, especially in the peripheral region, leading to its tearing away from the choroid. The resultant hole in the retina allows the vitreous to seep in behind the retina, further detaching the retina from its base, giving way to a ‘total retinal detachment’.
The retinal detachment is classified into three types. They are:
- Rhegmatogenous - This type of retinal detachment is the most common. In this, the vitreous accumulates in between the retina and the retinal pigment epithelium (RPE), the layer which mainly accounts for the retinal blood supply.
- Tractional - In this, either the vitreous or some scar tissue on the retina shrinks, pulling away the retina from the RPE.
- Exudative - In certain retinal diseases, there is fluid collection between the retina and the RPE due to the inflammation of the retina. There is no hole in the retina in these cases.
Causes of retinal detachment
The main conditions leading to a retinal detachment are:
- High myopia (nearsightedness) of greater than5-6 diopters
- Cataract surgery
- Trauma to the eye
- Age related changes in vitreous
- Inflammations of the eye, like uveitis
- Systemic diseases like diabetes
- Tumors of the eye
- Lattice degeneration of the eye
- Medicines used in glaucoma like pilocarpine
- Family history of retinal detachment
- Similar condition in the other eye
Retinal detachment is commonest in the age between 25-40 years (degenerative myopia) and after 60 years (post cataract surgery).
Signs and symptoms of retinal detachment
Retinal detachment is a painless condition. The first symptom of retinal detachment is the appearance of specks in the field of vision which seem to float in front of the eyes. Besides these floaters, patients also complain of lights flashing in front of their eyes. The appearance of these floaters and flashing lights decreases just before the detachment so the patient may become complacent. However these symptoms should not be taken lightly. As the retinal detachment usually begins at the periphery and then gradually extends towards the centre, the patient complains of obstruction in his peripheral vision first. As the detachment proceeds towards the centre, the vision becomes hazy, undulating and vague. In case of sudden retinal detachment, as in trauma, there is a sudden loss of vision as if a curtain has fallen in front of the eyes. It is a medical emergency and the patient should report to the doctor immediately. No retinal detachment resolves on its own.
Retinal detachment treatment and prognosis
A hole in the retina is generally fixed by means of cryotherapy so that it does not progress to a full blown case of retinal detachment. Here, the edges of the tear are fixed to the adjoining tissue by freezing. In case of complete retinal detachment, there are three modalities of treatment, namely, scleral buckling, pneumatic retinoplexy and vitrectomy.
Scleral buckling: The hole is first identified using an indirect ophthalmoscope. It is then plugged by means of electric current, laser or freezing. This results in scar tissue formation at the margin of the hole which stops further accumulation of fluid behind the retina. Then, a sclera buckle made of plastic, silicone or sponge is put onto the sclera. The buckle applies pressure on the eye and keeps the hole in the retina approximated to the sclera wall.
Pneumatic retinoplexy: It is a cheap method to fix uncomplicated retinal detachments in an OPD procedure. In this technique, an inflatable gas is injected into the vitreous cavity. The gas expands inside, pushing the torn retina out affixing it to the scleral wall. Then the tear in the retina is fixed by means of an electric current, laser or freezing. The gas gets absorbed in two to six weeks. Placing the head in a proper position during the procedure is imperative for the method to succeed. However, in case pneumatic retinoplexy fails to produce the desired results, one can always go for sclera buckling.
Vitrectomy: It is a complex surgery done in large or intricate retinal detachments caused by aberrant blood vessels in the vitreous or on the retina in diabetic retinopathy. It is also performed in cases of major ophthalmic infections, hemorrhages in the vitreous cavity, crusts on the retina or big retinal detachments caused by pulling of scar tissue. After cutting small holes in the sclera to approach the vitreous cavity, the vitreous humor is exchanged with an inflatable gas and the torn retina is relocated. The gas is gradually replaced by fluids produced innately by the eyes. Surgeons prefer to go for a sclera buckling along with vitrectomy for better results.
About 80% of all cases of simple retinal detachment are corrected in the first surgery. Around 15% require a corrective surgery. Certain cases, where the detachment has been there for a long time or involves the macula, cannot be corrected. In these cases, total blindness occurs in about 6 months.