Retinal structure and function

Marsden, J. (2004) Implications of and treatment options for retinal detachment. Nursing Times; 100: 37, 44–47. The retina is a complex structure that turns light falling onto it via a chemical reaction into nerve impulses, which then travel to the brain for interpretation into images. If the retina is damaged or moved from its correct location, a person’s ability to interpret the world is compromised with all the physical, social and psychological effects this implies. Nurses have a key role in the recognition of signs and symptoms of retinal compromise as well as the care and treatment of patients who undergo therapy for retinal detachment, so that the best possible vision is achieved for each patient. Retinal detachment can have a devastating effect on vision, and early referral to an ophthalmologist is crucial for patients experiencing symptoms if they are to retain good vision. This article discusses the causes, presentation and treatment of retinal detachment. It is important for nurses to understand the condition so that they can give highquality, timely information to patients and others who may ask for their advice. The retina and vision The retina is the innermost of the three ‘coatings’ of the eyeball (the outer two layers being the choroid and the cornea and sclera). It is the layer that enables us to see, although it does not form images itself. The integrity of the retina is crucial and any defect in it has the potential to affect vision. Box 1 lists some of the anatomical markers around the retina. The retina consists of 10 layers, although these can be divided into two main layers: ● The retinal pigment epithelium (RPE) is the outer layer, which lies against the choroid; ● The neurosensory or neural retina. Between the neural retina and the RPE is a space known as the sub-retinal space. The neural retina is firmly attached only at its extreme and the margins of the optic nerve. Without a firm attachment to the underlying RPE there is a potential for these two layers to become separated (Fig 1, p47). Retinal detachment is more common in men, people with myopia (short-sight) and older people (Newell, 1996). Some people have a predisposition to breaks or tears in the retina due to retinal degenerative change. Retinal structure and function In order to recognise the effects of retinal detachment on vision, it is necessary to understand the function of the retina. The neural retina collects the light falling on it from outside the eye that has passed through the cornea, pupil and vitreous gel, then through the neural layers of the retina to hit a layer of photoreceptor cells. A series of chemical changes in the cells transform the light into electrical signals that pass along the optic nerves to the visual cortex within the occipital lobe of the brain. The visual cortex interprets what we see. The retina has two types of photoreceptor cell, rods and cones, which have different functions: rods detect contrast and motion while cones detect light of different colours and fine detail (Box 2, p46). The other parts of the neural retina are concerned with amplifying the electrical signals and, through a series of nerve cells, transmitting them to the visual cortex for interpretation. Types of retinal detachment Retinal detachment is the separation of the retinal pigment epithelium from the neural retina. It will inevitably, if not immediately, affect vision. There are three classifications of retinal detachment: rhegmatogenous, traction and exudative. Rhegmatogenous retinal detachment This is the most common form of detachment and occurs secondarily to the formation of holes, tears or breaks in the retina, which are commonly caused by vitreous traction. The vitreous humour of the eye, which fills the posterior segment of the globe, consists of collagen fibrils and hyaluronic acid molecules within an aqueous gel. Its volume reduces with increasing age (this happens earlier in individuals who are short-sighted) and as the vitreous humour shrinks, it may detach from the face of the retina. This is known as posterior vitreous detachment (PVD), and it does not normally cause problems. However, small adhesions can occur between the retina and vitreous, which may pull holes in the retina as the vitreous moves away from it. Another common cause of retinal breaks is degeneration of the retina. Lattice degeneration is present in approximately seven per cent of the population and in about 40 per cent of eyes with a retinal detachment. People with myopia make up only 10 per cent of the population but have 42 per cent of all retinal detachments due to increased incidence of lattice degeneration in this group and a high frequency of posterior vitreous detachment at a younger age (Taylor et al, 1995). In SP L Implications of and treatment options for retinal detachment NT 14 September 2004 Vol 100 No 37 www.nursingtimes.net RefeRences Adams, Z., Charteris, D.G. (2002) Management of retinal detachment. Ophthalmic Nursing; 5: 4, 24–26. Ciulla, T.A. et al (1996) Postvitrectomy positioning complicated by ulnar nerve palsy. American Journal of Ophthalmology; 122: 5, 739–740. Forrester, J.V. et al (2001) The Eye: Basic Science in Practice. London: WB Saunders. CLINICAL aDvaNCeD