1. Epidemiology & aetiology
Because of our dependence on good eyesight, loss of vision is a frightening symptom. As we age, we increasingly rely on glasses to focus, but when poor eyesight cannot be corrected by glasses, it indicates a problem somewhere along the visual pathway.
Loss of vision is caused by opacity preventing light reaching the photo-receptors (for example, cataract) or by an interrupted nervous connection between photoreceptor and brain (maculopathy, glaucoma or stroke).
Good visual acuity requires focused light to fall on a healthy macular retina. Loss of macular vision brings a reduction in image clarity, brightness and colour. Loss of peripheral retina signal causes defects in the visual field. Field defects can be poorly appreciated by patients, especially if change is slow. This explains why even advanced 'progressive open angle glaucoma' (from now on abbreviated to glaucoma, but not to be confused with acute glaucoma, which causes red eye and severe pain) may have no symptoms.
Most conditions that cause loss of vision are typically bilateral, albeit often with asymmetrical presentation.
The most common cause of blindness is cataract (almost half of the 37 million total worldwide). In the UK, where cataract surgery is readily available, age-related macular degeneration (AMD) is the leading cause, followed by diabetic retinopathy, glaucoma and congenital disease.
Classification
When attempting a causal classification of loss of vision, consider sudden, rapid and gradual sight loss. Sudden onset (in seconds to minutes) almost invariably indicates a vascular event somewhere along the visual pathway, for example retinal vessel occlusion, vitreous haemorrhage or 'wet' AMD.
Gradual loss, over months or years, usually indicates a degenerative process such as cataract, glaucoma or 'dry' AMD. Rapid loss (within days) may indicate a number of pathologies - only some of which are discussed here.
An elderly patient with failing sight may have increased incidence of depression, accidents in the home and loss of independence, so it is vital that eyesight checks are part of the systematic health review in this age group.
In young, healthy people, sudden or rapid loss of vision is unusual and in the absence of systemic or congenital disease, the differential diagnosis is short, and includes optic neuritis and central serous retinopathy (CSCR).
2. Making a diagnosis
History
The most important feature of the history is duration of sight loss, and this should guide speed of referral. Sudden sight loss (within the hour) must be referred immediately: vision may be saved if the cause is occluded retinal or posterior ciliary artery. If associated with scalp or jaw pain, or recent lassitude with loss of appetite in an elderly person, occlusion is caused by temporal arteritis and high-dosage systemic steroids may restore sight.
Rarely, non-arteritic retinal artery occlusion (painless sudden loss of vision) may also be resolved by ocular hypotension if ocular massage or paracentesis are instituted within an hour of onset. Sudden sight loss over more than an hour requires less urgent referral (within days).
In wet AMD, sudden vision loss in an elderly patient is due to submacular retinal haemorrhage. In the early stages, distortion of straight lines and reading vision are key symptoms.
Sight loss caused by vascular disease is not usually accompanied by other symptoms but systemic enquiry may indicate the likely aetiology. In vascular events such as retinal vein occlusion, there is usually a history of hypertension, diabetes or thrombo-embolism. If a patient has diabetes, rapid loss of vision may indicate vitreous haemorrhage or, with more gradual loss, diabetic maculopathy.
Non-vascular causes of sudden or rapid sight loss often have key associations. Pain is a feature of rapid sight loss in optic neuritis; pain on eye movements, worse after exercise or heat, is pathognomonic. CSCR causes rapid loss of central vision because of a localised macular subretinal blister.
Retinal detachment is more often found in myopia or those with a recent history of cataract surgery. Patients may complain of increased floaters just prior to the sight loss.
Examination
Without specialist equipment, a diagnostic eye examination is difficult. However, the following tests may enable a tentative diagnosis and indicate an appropriate referral pathway.
In sudden loss of vision caused by retinal vessel occlusion, there may be profound loss of sight, which is unusual in most other causes. This will not improve through a pinhole occluder. Often poor vision caused by cataract will improve when repeated through a pinhole, in contrast to AMD, where pinhole acuity is usually worse.
Using a red target, there is reduced brightness and colour saturation in optic neuritis, ischaemic neuropathy (vascular occlusion of vessels supplying optic disc) and retinal artery occlusion. This should be confirmed by the presence of a relative afferent pupil defect (RAPD, see below).
In unilateral cortical infarct, confrontation fields done with both eyes open will show a vertical cut-off as the target is moved from the unaffected to affected side. Field defects in retinal detachment and glaucoma are less reliably demonstrated by this technique.
In RAPD, when a bright light is moved from the unaffected eye to that with poor vision, the latter pupil dilates. This happens because a diseased optic nerve is not able to relay the electrical signal to the brain as efficiently as the healthy eye.
If you can get a retinal view through an undilated pupil, you can exclude cataract as a cause of poor vision. In symptomatic glaucoma, the optic disc will be obviously cupped. In wet AMD, there may be a macular haemorrhage; in vein occlusion, multiple peripheral haemorrhages with a swollen disc.
3. Management condition
Prophylaxis
Prophylaxis of vision loss in the other eye is vital in the vascular aetiologies.
The GP has an important role investigating and treating associated hypertension, diabetes and lipid abnormality, and in encouraging lifestyle changes to include a healthy diet and regular exercise. This will reverse some visual loss caused by hypertensive retinopathy and may prevent subsequent venous occlusion in the second eye.
In retinal vein occlusion, the benefits of anticoagulation have not been established, however maintenance of normal intraocular pressure (IOP) with glaucoma medication is often necessary when it is high at presentation.
Established treatments
Cataract surgery has developed improved refractive outcomes with astigmatism correction at the time of surgery and use of accommodating and multifocal lens implants where appropriate.
Vitrectomy is indicated to remove long-standing, non-resolving vitreous haemorrhage and is increasingly used to repair retinal detachments.
Lowering IOP is the only effective treatment goal in glaucoma. Recent developments in glaucoma therapies combine agents to reduce the number of times that drops have to be used each day.
The implementation of the national screening programme for sight-threatening diabetic retinopathy encourages hope that there will be a reduced incidence of visual impairment caused by diabetes over the next five years. Retinal laser photocoagulation is still the mainstay of treatment, however.
Emerging treatments
Anti-growth factor drugs (anti-VEGF) technologies have revolutionised the prognosis for possible sight restor- ation in wet AMD. Very small quantities are injected into the vitreous cavity at four- to six-weekly intervals to inhibit leakage and development of submacular blood vessels.
In addition, there has been some reported success with anti-VEGF used to treat vision loss in retinal vein thrombosis, diabetic retinopathy and other maculopathies.
The indications and clinical profile for these drugs have yet to be fully clarified but are eagerly awaited. Gene therapies and stem-cell technology for congenital conditions are still research tools.
This is an exciting time for ophthalmology - for the first time since the widespread introduction of lens-implant surgery for cataract 25 years ago, there is real hope of reducing incident blindness in the UK.
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