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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.

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.

Management of gradual visual loss

The initial management of gradual visual loss depends on the cause. Refractive errors usually require no more than a pair of glasses.

Cataracts can be removed and an artificial lens implanted. Glaucoma requires treatment to lower the intraocular pressure.

A change in the appearance of the lens

If you shine a bright light on the eye the lens may appear brown, or even white if the cataract is more advanced.

Causes

Many conditions are associated with cataracts, but changes within the lens associated with ageing are the most common cause.

Cataracts also occur more often in patients with diabetes, uveitis, or a history of trauma to the eye.

Prolonged courses of steroids, both oral and topical, can also give rise to cataracts.

Children with cataracts need to be investigated to exclude treatable metabolic conditions such as galactosaemia.

Surgery

There is no effective medical treatment for established cataracts.

The treatment is surgical.

Indications for cataract surgery

Whether or not to operate depends primarily on the effect of the cataracts on the patient’s vision.

Many years ago surgeons waited until the cataract was mature or “ripe” (when the contents became liquefied) because this made aspiration of the contents of the lens easier.

With advances in microsurgery, however, there is now no longer any need to wait for the cataract to mature, and cataract surgery can be performed at any stage, with minimal risk.

There is no set level of vision for which an operation is essential, but most patients with a vision of 6/18 or worse in both eyes because of lens opacities benefit from cataract extraction.

Some elderly patients, however, may be perfectly happy with this level of vision.

Simple advice such as the recommendation to use a good reading light that provides illumination from above and behind, may be adequate.

A younger patient, with more exacting visual demands, may opt for an operation much earlier. (The minimum standard for driving is about 6/10; equivalent to a line between 6/9 and 6/12.) With certain types of cataract, such as an opacity at the back of the lens (posterior subcapsular cataract) the vision may be 6/6 in dim conditions when the pupil is dilated. However, in bright sunlight the pupil constricts and most of the light entering the eye has to pass through the opacity, causing glare and a fall in acuity. In this case, surgery would usually be performed even though the tested vision was 6/6.

Generally, the surgeon’s advice is tailored to the individual patient.

Surgical techniques

“Phacoemulsification” method

Most cataract surgery in the United Kingdom is now performed with this method.

A very small tunnel incision (about 3mm wide) is made in the eye and a circular hole (diameter about 5 mm) is made in the anterior capsule of the lens (capsulorrhexis).

A fine ultrasonic probe is then used to liquefy the hard lens nucleus (phacoemulsification) through this hole.

Any remaining soft lens fibres then are aspirated.

A folded replacement lens is then inserted into the empty lens capsular bag and allowed to unfold.

A high viscosity gel substance (viscoelastic) often is used to protect the delicate endothelial cells that line the posterior surface of the cornea during the operation.

This is then washed out at the end of the procedure.

Sutures often are not required as the tunnel incision is self sealing.

These advances in technique have considerably improved the speed of recovery and visual rehabilitation after cataract surgery.

Extracapsular method

This was, until recently, the most popular method of cataract extraction.

An incision is made in the eye (about 10 mm in length) and the anterior capsule is cut open with the tip of a sharp needle.

The large nucleus is then expressed whole and the remaining soft lens fibres aspirated.

A non-folding lens is then inserted into the empty lens capsular bag and the incision closed with fine sutures.

The need for a larger wound in extracapsular surgery may lead to problems with wound security and postoperative astigmatism in some patients.

Intracapsular method

In this method, the entire lens is removed within its capsule, usually with a cryoprobe, after the suspensory ligaments of the lens have been dissolved by the enzyme chymotrypsin.

As there is no remaining lens capsule, the vitreous gel in the eye can move forward and block the flow of aqueous through the pupil.

A hole cut in the iris (iridectomy) allows the aqueous to bypass the pupil. This method is now usually used only in special situations.

Anaesthesia

For most patients, cataract surgery is carried out under local anaesthesia as a day case.

Local anaesthetic can be injected around the eye (peribulbar anaesthesia or sub-Tenon’s anaesthesia), or, with modern, closed system, small incision, cataract surgery, the operation can be carried out safely in selected patients with just topical (eyedrop) anaesthesia.

Occasionally, intraocular (intracameral) local anaesthesia is used. In certain situations general anaesthesia may be needed because of anticipated technical difficulties or because of patient factors (for example, in patients with Down’s syndrome or young patients who are not cooperative).

Intraocular lens implants

The final refractive state of the eye after operation can be chosen by measuring the curvature of the cornea (keratometry) and the length of the eye (ultrasound biometry) and then implanting a lens of appropriate power.

An intraocular lens implant can be more effective in correcting refractive error than glasses and contact lenses, as it is placed in the eye in the same position as the natural lens.

Myopia and hypermetropia can be corrected during cataract surgery by inserting an appropriately powered intraocular lens.

However, patients usually still require glasses for reading or distance, as most implanted lenses have a fixed focus.

Multifocal intraocular lenses have two principal points of focus and in theory enable the patient to have both good distance and reading vision without glasses. However, some patients experience optical aberrations and a reduction in contrast sensitivity with this type of intraocular lens.

Most lenses implanted nowadays are posterior chamber lenses, which are placed in the empty lens capsular bag after the lens contents have been removed from the eye.

With this type of lens the lens implant sits in a natural position.

These lenses can be folded and inserted through a minute incision (2-3 mm).

If the lens capsule is not present or cannot support a posterior chamber lens unaided, the lens can be sutured in place. Alternatively, an anterior chamber lens, which is supported in the anterior chamber angle, can be used. In the past, iris clip lenses were used, but they are not used now.

The pupil should not be dilated if the iris clip type of lens has been used, as the lens may dislocate.

Complications of cataract surgery

Over 200 000 cataract operations are performed annually in the United Kingdom, and although modern surgical techniques have exceptional levels of safety, complications still occur.

Patient expectations of cataract surgery are very high.

All patients should be made aware of the possible risks of the surgery before they give their consent for the operation.

Infective endophthalmitis

This devastating infection occurs very rarely (about 1 in 1000 operations) but can cause permanent severe reduction of vision.

Most cases of postoperative infection present within two weeks of surgery.

Typically patients present with a short history of a reduction in their vision and a red painful eye.

This is an ophthalmic emergency.

Low grade infection with pathogens such as Propionibacterium species can lead patients to present several weeks after initial surgery with a refractory uveitis.

Suprachoroidal haemorrhage

Severe intraoperative bleeding can lead to serious and permanent reduction in vision.

Ocular perforation

Sharp needles are used for many forms of ocular anaesthesia, and globe perforation is a rare possibility.

Modern forms of ocular anaesthesia have replaced many sharp needle techniques.

Retinal detachment

This serious postoperative complication is, fortunately, rare but is more common in myopic (shortsighted) patients after intraoperative complications.

Postoperative refractive error

Most operations aim to leave the patient emmetropic or slightly myopic, but in rare cases biometric errors can occur or an intraocular lens of incorrect power is used.

Despite all efforts to produce accurate biometry, in occasional cases the desired refractive outcome is not achieved.

Posterior capsular rupture and vitreous loss

If the very delicate capsular bag is damaged during surgery or the fine ligaments (zonule) suspending the lens are weak (for example, in pseudoexfoliation syndrome), then the vitreous gel may prolapse into the anterior chamber.

This complication may mean that an intraocular lens cannot be inserted at the time of surgery.

Patients are also at increased risk of postoperative retinal detachment.

Uveitis

Postoperative inflammation is more common in certain types of eyes for example in patients with diabetes or previous ocular inflammatory disease.

Cystoid macular oedema

Accumulation of fluid at the macula postoperatively can reduce the vision in the first few weeks after successful cataract surgery.

In most cases this resolves with treatment of the post-operative inflammation.

Glaucoma

Persistently elevated intraocular pressure may need treatment postoperatively.

Posterior capsular opacification

Scarring of the posterior part of the capsular bag, behind the intraocular lens, occurs in up to 20% of patients.

Laser capsulotomy may be needed.

Postoperative care

Most patients are treated for several weeks with steroid drops to reduce inflammation and with antibiotic drops to prevent infection.

Patients have traditionally been advised to avoid activities that may considerably raise the pressure in the eyeball, such as strenuous exercise or heavy lifting, for a few weeks after the operation.

However, with modern small incision surgery patients can return to normal activities within a few weeks.

If sutures have been necessary, these often need to be taken out before glasses can be prescribed because of the changes they induce in the shape and refractive state of the eye.

Thickening of the lens capsule

The remaining lens capsule may thicken (usually over months or years) and this may need to be cut open. In patients who have had previous cataract surgery, capsular thickening is the most common cause of gradually worsening vision.

Division of this thickened capsule (capsulotomy) is usually done with a special laser (called the Q-switched neodymium yttriumaluminium- garnet or Nd-YAG laser), which creates microscopic focused explosions that dissect tissue rather than burn it.

This avoids the need to open the eye surgically, and it can be performed painlessly (the capsule has no pain fibres) on an outpatient basis, under topical anaesthesia, with the patient sitting at a slit-lamp microscope.

This treatment has given rise in part to patients’ commonly held misconception that cataracts can be removed by laser alone.

Optical correction after surgery

Removal of the crystalline lens results in an eye with a large hypermetropic refractive error.

This refractive error is usually corrected with an intraocular lens implant at the time of surgery.

If the implant results in clear vision for distance, glasses usually will be required for reading fine print, as the new lens has a fixed focus.

If the patient had a cataract extraction before intraocular lenses were used commonly, optical correction has to be achieved with glasses or a contact lens.

Glasses

The natural lens has great refractive power and consequently the glasses required to correct the refractive error after cataract extraction are thick and heavy, even when they are made of plastic.

The corrected image is about 30% larger than that seen by the normal eye.

This means that the image from an eye that has had a cataract removed, with subsequent glasses correction, cannot be fused with the image from the other eye, unless the lens in the other eye is also removed.

Objects are also perceived to be closer than they are, often resulting in accidents for example, pouring tea into the lap rather than into the cup.

The field of vision is restricted, and there is a “blind ring” (scotoma) within this field because of the optical aberrations inherent in such powerful lenses.

These optical problems do not occur with contact lenses or an intraocular lens implant.

Contact lenses

The size of an image with a contact lens is only 10% larger than the image in the normal eye.

The brain can fuse this disparity and thus both an operated eye and an unoperated eye may be used simultaneously. However, most patients with cataracts are elderly and problems may arise in using the contact lens because of an inadequate tear film, difficulties with handling, and infection.

Secondary intraocular lens implantation

If the problems posed by using glasses or contact lenses are too great, secondary implantation of an intraocular lens can be considered.

However, this procedure has associated risks, particularly in patients who have had intracapsular cataract extraction.

Complications may occur, including secondary glaucoma.

The potential advantages and disadvantages of the various options need to be fully considered by the patient and the ophthalmologist before a final decision is made.