Refractive errors
The pinhole test is a most useful test for identifying refractive errors. If there is a refractive error, the vision will improve when the pinhole is used. A patient with thick glasses should wear them for the pinhole test. Once other causes of visual loss have been excluded, the patient can be sent to an optometrist for refraction and correction of refractive error (for example, glasses).
Corneal disease
Various disorders can cause gradual loss of the corneal endothelial cells and increasing oedema of the cornea (for example, Fuch's endothelial dystrophy). This leads to a gradual decrease in visual acuity that does not improve substantially with a pinhole. If the damage is advanced the cornea may appear opaque. A corneal graft from a donor may be required.
Cataract
This is probably the most common cause of gradual visual loss. It can be diagnosed through testing the red reflex. The patient should be referred if the visual disturbance interferes appreciably with their lifestyle. If a patient with a cataract cannot project light or has an afferent pupillary defect, however, other diseases such as a retinal detachment must be excluded.
Primary open angle glaucoma
Unfortunately, the patient may not complain of visual disturbance until late in the course of the disease; hence the need for screening. Primary open angle glaucoma should, however, be excluded in any patient complaining of gradual visual loss. Establish whether there is any family history of glaucoma. The vision may still be 6/6, so the visual field should be checked with a red pin. Also check for cupping of, or asymmetry between, the optic discs.
Age-related macular degeneration
This may occur gradually and is typified by loss of the central field. There are usually pigmentary changes at the macula. The disease occurs in both eyes, but it may be asymmetrical, and it is more common in shortsighted people. The gradual deterioration is not treatable, but if acute visual distortion develops this may indicate a leaking area under the retina (choroidal neovascularisation), which may respond to laser photocoagulation or photodynamic therapy.
Macular hole
A macular hole is a full thickness absence of neural tissue at the centre of the macula. Between 10 and 20% of full thickness macular holes (FTMH) will become bilateral. Patients usually present with painless loss of central vision or distortion of the central visual field, although early macular holes may be asymptomatic. Patients with established FTMH can be treated with vitrectomy and instillation of intraocular gas (to provide retinal tamponade), which have a high chance of closing the hole successfully.
Diabetic maculopathy
Diabetic retinopathy occurs in both insulin dependent and non-insulin dependent diabetics and affects all age groups. The patient may or may not give a history of diabetes, although the longer the duration of the diabetes, the more likely the patient is to have retinopathy. Remember that although the patient may describe the onset of visual loss as gradual, sight threatening diabetic retinopathy may still be present. Non-proliferative diabetic retinopathy is typified by microaneurysms, dot haemorrhages, and hard yellow exudates with well defined edges. There also may be oedema of the macula, which is less easily identified but can lead to a fall in visual acuity. Non-proliferative diabetic retinopathy at the macula (diabetic maculopathy) is the major cause of blindness in maturity onset (type 2) diabetes, but it also occurs in younger, insulin dependent (type 1) diabetic patients. Some forms of diabetic maculopathy may be amenable to focal laser photocoagulation. Proliferative retinopathy, typified by the presence of new vessels, requires urgent referral for treatment.
Hereditary degeneration of the retina
These conditions are relatively rare (for example, retinitis pigmentosa) but should be suspected if there is a family history of visual deterioration. Symptoms include night blindness and intolerance to light. Most types of retinal degeneration are not yet treatable, but some are associated with metabolic disorders that can be treated. These patients need to be referred to an ophthalmologist, preferably with a special interest in these conditions, for diagnosis and any possible treatments. Patients with severe visual impairment may develop visual hallucinations and sleep disturbance. It is particularly important for these patients to have an opportunity to discuss their diagnosis and prognosis and to have genetic counselling. Patients can be helped through psychosocial counselling (see below, Management of gradual visual loss).
Compressive lesions of the optic pathways
These are relatively rare, but should always be considered. Clues in the history and examination include headaches, focal neurological signs, or endocrinological abnormalities such as acromegaly. There should not be an afferent pupillary defect in most patients with cataract, macular degeneration, or refractive error. Therefore if an afferent defect is seen, suspect a compressive or other lesion of the optic pathways. Testing of the visual fields may show a bitemporal field defect due to a pituitary tumour. The optic discs should be checked for optic atrophy and papilloedema.
Drugs
Several drugs may cause gradual visual loss. In particular, a history of excessive alcohol intake or smoking; methanol ingestion; or the taking of chloroquine, hydroxychloroquine, isoniazid, thioridazine, isotretinoin, tetracycline, or ethambutol should lead to the suspicion of drug induced visual deterioration. Systemic, inhaled, or topical corticosteroids may cause cataracts and glaucoma.
