Structurally the exterior of the eye consists of a clear front lens called the cornea which merges with the white of the eye, which we call the sclera. Seen through the cornea, the iris is the coloured part of the eye (brown in pigmented races, lighter coloured in people with fair skin), with a central aperture called the pupil.
Immediately behind the iris sits the lens, suspended in place by delicate collagen fibres which connect it to the wall of eye. We call these delicate fibres lens zonules. The lens separates the front of the eye (the anterior segment), from the back (the posterior segment) and is the structure which we call a cataract when it becomes hazy as we age.
The retina lines the inner surface of the posterior segment. This is the level at which the optical images of what we are looking at are focused by the cornea and the lens. The retina turns these light stimuli into nerve impulses which are transmitted to the brain via the optic nerve.
The cavity between the retina and the lens is filled with a clear gel like fluid called the vitreous humour. A number of diseases and ageing changes can affect the vitreous and the retina. Some of these can be treated with surgery. Vitreo-Retinal Surgery relates to operations performed on these structures.
Posterior Vitreous Detachment:
The vitreous has a gel like structure in young eyes with a delicate scaffold of extremely fine collagen fibrils containing molecules of mucopolysaccharides and water. As eyes age, the vitreous becomes more liquefied, with the fibrils clumping together to form strands and specks which cast shadows on the retina. We perceive these shadows as floaters. They are mostly harmless but, in some people, especially myopes (people with short sighted eyes), floaters can be quite troublesome. Inflammations, infections, trauma etc. can also give rise to floaters that are more problematic than usual.
There is a common ageing process which affects the vitreous gel called a Posterior Vitreous Detachment (PVD). This typically occurs after the age of 50, but may develop at an earlier age, particularly in myopes or people who have had previous eye surgery such as cataract surgery. In this condition, the vitreous humor separates away from the retina itself and contracts forwards in the eye, remaining attached to the structures immediately behind the lens. Common symptoms of PVD include a sudden increase in floaters and flashing lights which are usually noticeable at the peripheral edges of the vision in dim lighting situations.
Whilst mostly harmless, the separation of vitreous during a PVD can pull on parts of the retina leading to formation of retinal tears or holes in a small percentage of people. These in turn can allow fluid from the interior of the eye to pass through the breaks in the retina and cause it to lift off the back of the eye – a retinal detachment. Once the retina detaches it can no longer function, producing a shadow or a curtain like effect in the vision. Retinal detachments can progress rapidly and if not treated may cause complete blindness. It is therefore important to have the eye checked when symptoms of a PVD (increased floaters or flashing lights) develop. If detected early before the retina has detached, the retinal holes and tears can be treated with laser which will usually prevent retinal detachment. Once the retina has begun to detach it is almost always necessary to undergo surgery.
Vitreo Retinal Surgery:
The most common type of procedure carried out on the vitreoretinal tissues is called a vitrectomy and involves removal of the vitreous gel. Although important when the eye is first forming in a foetus, the vitreous can safely be removed later on in life. Vitrectomy surgery is done under microscopes with special lens attachments and requires a specialised vitrectomy machine with very thin needle like cutting probes that cut and remove tiny pieces of the vitreous at very high cut rates (up to 7500 times a minute). The high cutting rates reduce traction on the vitreous which lessens the chances of damage to the retina. Surgery is mostly performed under local anaesthetic as a day procedure with only a short postoperative stay in the observation ward.
Vitrectomy surgery can be performed to remove troublesome floaters from the eye. It is also performed as part of other vitreoretinal procedures such as macular hole repair and epiretinal membrane surgery. In these operations the vitreous must first be removed to allow access to the retinal surface. The membranes on the retinal surface can then be peeled off using microscopic forceps. In the case of macular holes, removal of the most superficial layer of the retina (the Internal Limiting Membrane or ILM) is followed by replacement of the fluid filling the vitreous cavity by a bubble of gas which helps to close the hole in the macula.
Vitrectomy surgery is also used to fix retinal detachments. The special microscope lenses allow an internal inspection of the retina to identify causative holes or tears. Once located, the tears are treated with laser or a freezing probe to produce an adhesion between the retina and the back of the eye which prevents recurrence of the retinal detachment. The vitreous cavity is then filled with gas (or less commonly a specialised silicon oil) to keep the retina in place while the holes seal. Surgery for retinal detachment is highly effective, with almost 90% of retinal detachments fixed with only one operation. Approximately 1 in a 100 retinal detachments cannot be fixed despite repeated procedures and it is not uncommon for the quality of the eyesight to be impaired to some degree following retinal detachment problems.
Role of post-operative posturing following Vitrectomy:
If gas is used as part of a vitreoretinal procedure, the gas will rise vertically to the highest point in the eye. This property of gas is used to support the diseased part of the retina and may require a person to adopt a particular head posture to ensure that the gas is in the right position. For example, in macular hole surgery patients may be asked to position themselves with their faces down for a few days after surgery to ensure that the gas remains in contact with the back of the eye where the macular is located. The operating surgeon will advise whether posturing may be needed after a procedure.
Cataract after Vitrectomy:
One of the commonest side effects of vitrectomy is the early formation of a cataract so cataract surgery is often combined with vitrectomy surgery to give a clear view of the interior of the eye during surgery and also to remove the necessity for a second operation.
Surgical Macular Problems:
The central part of the retina is called the macula. It has the highest concentration of the photoreceptors called cones which are active in brighter lighting environments and produces our most detailed vision. In later years it is common for the structure of the macula to deteriorate – the condition known as Age Related Macular Degeneration – but there are a number of other conditions that can also impact on the quality of the central vision by causing structural changes to the macula.
Epiretinal membranes are thin layers of cells and fibrous tissue which form delicate membranes on the surface of the retina. Conditions predisposing to epiretinal membrane formation include;
- incomplete separation of vitreous from retinal surface
- retinal vascular occlusions
Epiretinal membranes can contract over time, causing the retina to become thickened and wrinkled. This results in distortion of the vision and at times can also cause images to appear larger or smaller than normal. Although not a blinding condition, epiretinal membranes can severely affect the quality of the central vision.
The very central and most sensitive part of the macula is also the thinnest part of the retina. Called the Fovea Centralis, it is used for performing fine visual tasks such as reading, looking at faces etc. An abnormal interaction between the vitreous and this part of the retina can cause forward and/or outward pulling, causing formation of a macular hole. Commoner in women than men, macular hole formation usually affects people in their 60s-70s. They impair the central vision causing a small blank patch wherever the person looks. Left untreated the visual loss can become permanent, however early surgical treatment is highly successful, with over 95% of holes closing and the vision improving significantly.
Other types of Vitreo-Retinal Operations:
A vitreoretinal surgeon specialises in a number of other procedures that can be performed on the eye. These include:
- Dealing with rare complications of cataract surgery such as lens fragments which have dropped backwards into the vitreous cavity
- Removal of pus and infections from the eye
- Complications of diabetes such as bleeding and scarring
- Complex cataract operations with high complication risk
- Dislocated natural and artificial lenses
- Removal of intraocular foreign bodies
- Injuries to deeper structures of the eye etc.