Section 1: Aetiology and epidemiology
Good sight depends on an exquisitely complex interaction between tissues in the eye, orbit and brain and we are sensitive to changes in visual perception.
Light from an object must fall on the retina, be translated into electric signal and be transmitted for interpretation to the occipital cortex. Loss of vision occurs when this pathway is interrupted (see diagram).
External environmental hazards associated with poverty (infection, trauma, lack of hygiene) found in the developing world disproportionately harm the beginning of the pathway to produce a profile of causes of blindness largely eliminated or controlled in the West.1
Worldwide, 161 million people live with impaired sight, and a further 153 million are estimated to have poor vision due to lack of corrective glasses, 90 per cent of whom live in the developing world.
In very poor countries, 50 per cent of blindness is estimated to be due to cataract, yet in the West surgery with replacement lens implant is readily available and, although cataract is common, it does not contribute to blindness statistics.
Other conditions that would block light reaching the retina are also routinely treated by eye operations in the developed world: corneal transplantation (often just part of the corneal thickness is replaced) and vitrectomy (microsurgical removal of tissue from the vitreous cavity).
In developed countries, blindness statistics reflect our current inability to cure or prevent degenerative retinal disease.2 Both age-related maculopathy and glaucoma are more common with increasing age and, although we have therapy, we cannot replace damaged tissue effectively, nor reverse scarring.
The main treatment strategies for degenerative disease are to prevent damage and neuronal loss and to treat before irretrievable scarring takes place.
While we have achieved some success with this, there are now increasing fears that the global epidemic of obesity and lack of exercise, with its resultant cardiovascular disease and type-2 diabetes will offer another candidate for top place in the league.
Diabetic retinopathy and retinal vascular disease is currently stabilised by laser treatment, but significant improvement to established sight loss is very unusual.
Although there are hopes of drug treatments, currently we fall far short of effectively restoring vision to pre-disease levels, and damage from this growing disease sector is predicted to become a major cause of vision loss in future statistical tables.
Although affecting fewer people, damage to the vision pathway and brain is similarly only reversible if caught very early, and most neurovascular, trauma and tumour-related vision problems are permanent.
Section 2: Diagnosis
Distinguishing between the many causes of loss of sight depends upon history, examination and investigation. The boxes summarise the common causes.
Sudden loss of vision
It is important to ask whether vision loss was sudden or gradual. Sudden loss indicates a vascular or inflammatory cause.
While vascular causes are otherwise asymptomatic, inflammation is accompanied by redness and pain if at the front of the eye, but shows fewer inflammatory features the more posterior the problem.
As an example, consider sudden vision loss in an elderly patient. As an isolated symptom, this is most likely due to a vascular problem affecting: the macula - neovascular age-related maculopathy (wet AMD) especially with history of smoking and 'dry' AMD; the optic disc - ischaemic optic neuropathy, especially with cardiovascular risk factors; or optic radiation and occipital cortex, where there will be additional features of stroke.
If associated with pain (with or without jaw claudication, scalp tenderness or loss of appetite) temporal arteritis needs to be excluded.
When sudden vision loss is accompanied by pain and redness, the cause is possibly acute glaucoma, corneal infection (herpetic or contact lens related) or intraocular infection if the patient has recently had cataract surgery.
In the young, the same rules apply. Acute vision loss is unusual and is often caused by inflammation. If redness and pain (anterior uveitis) are present it involves the front of the eye; if there are minimal associated symptoms the problem affects the posterior globe or beyond (optic neuritis, central serous retinopathy, macular haemorrhage).
Sometimes there are additional clues. Onset of floaters with sudden vision loss suggest vitreous haemorrhage or pigment release: proliferative retinopathy in a diabetic patient or retinal detachment in myopia, or with history of recent cataract surgery or recent posterior vitreous separation.
Distortion indicates a macular problem and bilateral field symptoms indicate a lesion beyond the optic chiasm.
Gradual loss of vision
Gradual loss of sight in an adult commonly indicates cataract. Other causes such as dry AMD, open angle glaucoma or diabetic maculopathy are often less symptomatic until advanced.
Glaucomatous visual field loss is characteristically asymptomatic, resulting in late diagnosis with a third of all glaucoma remaining undiagnosed even in western societies.
In the young, gradual sight loss is rare and indicates toxic or dystrophic pathology.
Key examination findings
Examination requires magnified focussed light with sophisticated diagnostic lenses (slit-lamp biomicroscopy) not normally found in general practice.
However, basic tests can usefully be carried out in primary care using a vision chart, pinhole occluder, pen torch and a direct ophthalmoscope.
Each eye should be tested using a standardised vision chart, with and without a pinhole occluder to correct for refractive error.
If the pinhole improves vision, the problem is likely to be anterior to the macula (for example, cataract or vitreous haemorrhage), but if vision is worse there is a macular problem (for example, AMD).
The pupil's constriction response to light should be normal unless pathology involves the optic nerve (ischaemic optic neuropathy, optic neuritis) or causes extensive retinal damage (total retinal detachment) when a relative afferent pupillary defect will be found.
If there is anterior inflammation, examining the outer eye will show red conjunctiva, a distorted pupil and hazy cornea, but this will be normal if there is posterior pathology.
Retinal examination with a direct ophthalmoloscope may reveal cupped, swollen or haemorrhagic optic disc and haemorrhage exudates or microaneurysm at the macula.
Gradual visual loss
- Dry AMD
- Intracerebral tumour
Sudden onset loss
- Wet AMD - requires rapid referral after onset of symptoms
- Retinal vascular occlusion - often reported on waking
- Vitreous haemorrhage - diabetic or retinal tear
- Retinal detachment - usually history of myopia or intraocular surgery
- Ischaemic optic neuropathy - exclude temporal arteritis
- Cerebrovascular ischaemia - transient visual loss indicates emboli
Section 3: Management
The role of the GP in management of vision loss is to refer acute pathology promptly and to ensure that chronic sight-threatening problems receive appropriate ongoing care.
Acute loss of vision
In general, speed of referral should reflect symptom duration. Sudden vision loss usually requires urgent referral.
Where arterial circulation is impaired, a few hours may be critical if sight is to be saved.
In temporal arteritis, systemic steroids reverse vision loss if they are given within hours of sight loss. Lowering intraocular pressure with massage or paracentesis may be sight saving in the first hours of retinal artery occlusion.
Retinal detachment involving the macula should be surgically repaired as soon as possible, while laser iridotomy to relieve high intraocular pressure in acute angle-closure glaucoma needs to be done within 24 hours of onset of vision loss.
The new treatments for macular haemorrhage due to neovascular (wet) AMD are more effective the earlier they are given.
Each intravitreal injection of anti-growth factor antibody works by stopping leakage from and inhibiting growth and development of new vessels under the macula for up to four weeks.
Injections are repeated monthly until there is no further leakage.
Vision loss because of vitreous haemorrhage caused by proliferative diabetic retinopathy is an emergency if no previous pan-retinal laser has been given.
Vitrectomy needs to be performed promptly if a poor view still prevents laser treatment. When reasonable, laser treatment can be used, however, it is preferable to wait for spontaneous clearance initially before considering surgical removal.
Where vision loss is caused by vascular disease, the GP has an important role in reducing cardiovascular risk to reduce chances of further thromboembolism and a second retino-vascular event.
Gradual vision loss
The decision to proceed to cataract surgery is made if the doctor believes that surgery will improve vision and the patient needs the improvement.
Cataract surgery is rarely indicated for medical reasons, for example, hypermature lens, phacogenic glaucoma or to enable retinal treatment for diabetic retinopathy, and more usually there is no contraindication to deferring surgery if the patient wishes to wait.
In AMD, glaucoma and diabetic maculopathy, management is a combination of specific therapy and lifestyle advice. In AMD, the most important lifestyle advice is to stop smoking, which will reduce risk of progression to neovascular AMD by two to three times.3
In addition, a good diet including antioxidants and carotenoids, exercise and reduced cardiovascular risk are believed to slow progression.
High-risk dry AMD (large drusen and pigment) may also benefit from specific vitamin preparations formulated to prevent advanced AMD.
In glaucoma, life-long treatment to lower intraocular pressure reduces progressive visual field loss. Treatment is usually in the form of drops, although may involve laser treatment and surgery.
Glaucoma management has been the subject of recent NICE guidelines, which stressed the importance of regular follow up and standardised tests.4
Good diabetes control is the only way to prevent diabetic retinopathy.
Section 4: Prognosis
Gene therapy and stem cell treatment are unfortunately not yet part of mainstream ophthalmic care.
Nevertheless, exciting developments are occurring, aiming to improve outcomes and even prevent blindness.
Notable among these is the recent addition of anti-growth factor antibodies, used as intra-vitreal injections as treatment of neovascular AMD.
Whereas formerly this was a blinding disease, with a 'good' outcome being defined as a less than three line loss on the vision chart, we now routinely salvage workable vision in 90 per cent of cases, with 30 per cent experiencing an improvement on the presenting level.
From this point of view, the updated population blindness statistics are awaited with eager anticipation.
Our success with clearing the ocular media anterior to the retina (corneal tissue replacement, cataract surgery, vitrectomy) is getting better, with improved, less invasive techniques, better instrumentation and intraocular lenses.
In terms of retinal vascular disease and neural damage, there are no dramatic cures and treatment usually stabilises at best. Prevention is still better than any treatment modality and the patient who presents with vision loss due to diabetic maculopathy or vein occlusion is unlikely to experience significant improvement.
In diseases affecting the retina and visual pathway, the goal of treatment is damage limitation.
We await gene therapy for some genetic eye diseases, but this has not yet translated into the ophthalmic clinic where most blindness occurs as a result of degenerative eye disease.
However, the potential that stem cell therapy and genetic engineering holds for repair and regeneration of nervous tissue has engendered much excitement for future therapies.
The revolutionary effect of the AMD intravitreal drug treatments that not only stabilise, but also improve vision if given early enough has rippled through the ophthalmic world, and is anticipated to reduce blindness registration.
Enormous amounts of research time and money is being invested in ophthalmic diagnosis and therapy in recognition of the increasing age of the population and anticipated growth in demand for eye care.
These include improving and refining surgical techniques and prostheses used in eye surgery; refractive techniques to change focus and decrease reliance on glasses and contact lenses, minimising ocular trauma and the immune response in corneal and cataract surgery, using smaller gauge instruments and marker stains to improve surgical success with vitrectomy.
Although these developments will be expensive, blindness and visual impairment are likely to cost society more.5
- 8 October is World Sight Day. For more information visit www.v2020.org
1. Resnikoff S, Pascolini D, Etya'ale D et al. WHO Global data on visual impairment in the year 2002. Bull World Health Organ 2004; 82(11): 844-51.
2. UK blindness http://www.fightforsight.org.uk/statistics-about- blindness-and-eye-disease
3. Thornton J, Edwards R, Mitchell P, Harrison R A, Buchan I, Kelly S P. AMD: smoking and age-related macular degeneration: a review of association. Eye 2005; 19: 935-44.
4. NICE. Clinical Guideline 85: Glaucoma. London: NICE, 2009.
5. Minassian D, Reidy A. An epidemiological and economic model for sight loss in the decade 2010 to 2020. April 2009 www.vision2020uk.org.uk/ukvisionstrategy/
1. Cataract (basic details) www.york.ac.uk/inst/crd/EHC/ehc23.pdf
2. Diabetic retinopathy: www.nhmrc.gov.au/publications/synopses/di15syn.htm
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