Cataracts

Introduction

A cataract is any opacity of the crystalline lens. Cataract is the leading cause of blindness worldwide, responsible for roughly 40% of global blindness, and almost all of it is curable by an operation that takes twenty minutes.¹ The global burden is therefore a problem of surgical access rather than of medicine.

In the UK, cataract surgery is the commonest surgical procedure performed in the NHS, with over 400,000 operations annually. It is also one of the most effective interventions in medicine by any measure of quality-adjusted life years gained, and it has measurable effects beyond vision: cataract surgery reduces the rate of falls and hip fractures, improves depression scores, and allows people to keep driving.

The lens is a transparent, avascular structure suspended behind the iris by zonular fibres from the ciliary body. It has no blood supply and no innervation, deriving nutrition from the aqueous humour. Its transparency depends on the precise arrangement and solubility of crystallin proteins within lens fibre cells, on the absence of organelles in mature fibres, and on active antioxidant defence. Anything that disrupts protein structure - oxidation, glycation, ultraviolet damage, osmotic stress - produces opacity.

Classification

Cataracts are described by their morphology, which is not merely descriptive: the three main types produce recognisably different symptoms and have different associations.

Other classifications

• Immature - the lens is partly opaque and a red reflex is still obtainable
• Mature - the entire cortex is opaque and no red reflex is seen; the pupil appears white
• Hypermature - the cortex has liquefied and the nucleus sinks within the capsular bag (Morgagnian cataract). Leaked lens protein can cause phacolytic glaucoma or phacoantigenic uveitis.
• Congenital - present at birth or in infancy, and an ophthalmic emergency because of the risk of irreversible amblyopia
• Traumatic - after blunt or penetrating injury, classically producing a rosette-shaped opacity
• Secondary (complicated) - from uveitis, retinitis pigmentosa, high myopia or previous intraocular surgery

Risk factors

• Increasing age - the dominant factor; over 70% of people over 80 have visually significant cataract
• Diabetes mellitus - accelerates cataract through sorbitol accumulation in the lens causing osmotic swelling, and produces earlier posterior subcapsular and cortical changes
• Corticosteroids - by any route, including inhaled and topical, causing posterior subcapsular cataract in a dose- and duration-dependent way
• Smoking - roughly doubles the risk of nuclear cataract
• Ultraviolet B exposure - the reason cataract prevalence is higher nearer the equator
• Ocular trauma, including blunt injury, penetrating injury, electric shock and ionising radiation
• Previous intraocular surgery, particularly vitrectomy, which accelerates nuclear sclerosis markedly
• Chronic uveitis and previous intraocular inflammation
• High myopia and retinitis pigmentosa
• Systemic disease - myotonic dystrophy (christmas-tree cataract), atopic dermatitis (shield cataract), Wilson disease (sunflower cataract), hypocalcaemia, and galactosaemia in infants
• Drugs - amiodarone, chlorpromazine, allopurinol and long-term tamoxifen

Clinical features

• Gradual, painless, progressive blurring of vision over months to years, typically bilateral but asymmetrical
• Glare and dazzle - the most disabling symptom for many patients, and the one most likely to stop them driving at night. Light scattering by the opacity is worse against a bright background.
• Haloes around lights, particularly oncoming headlights
• Faded, yellowed or brownish colours, often only appreciated after the first eye has been operated on and the difference becomes obvious
• Reduced contrast sensitivity, which can be marked despite reasonable Snellen acuity - a reason acuity alone is a poor guide to disability
• Monocular diplopia or ghosting - two or more images in one eye, from irregular refraction through an opacity. Binocular diplopia is never caused by cataract.
• Frequent changes in spectacle prescription, often with a myopic shift
• Improved near vision without glasses in nuclear sclerosis
• Falls and loss of confidence - often the real presenting complaint in older people

Examination

1. Visual acuity with glasses and with a pinhole. In cataract the pinhole may improve acuity a little but does not correct it fully, unlike in refractive error.
2. Red reflex with a direct ophthalmoscope held at arm's length. The reflex may be dull, or the opacity may be seen as a dark silhouette against it. This is the single most useful bedside sign and works without dilating the pupil.
3. Direct ophthalmoscopy - the fundal view is progressively obscured as the cataract densifies. Difficulty seeing the fundus is itself a finding.
4. Pupil reactions - these should be normal. A relative afferent pupillary defect is never caused by cataract, however dense, and points to optic nerve or retinal disease.
5. Slit lamp examination after dilatation - to grade and classify the opacity, and to assess the corneal endothelium, zonular stability and pseudoexfoliation
6. Assessment for coexisting disease - macular degeneration, diabetic retinopathy and glaucoma commonly coexist and limit the visual benefit of surgery

Differential diagnosis

The relevant differential is other causes of gradual painless visual loss, and the important point is that they frequently coexist with cataract rather than replacing it as a diagnosis.

• Refractive error - fully corrected by a pinhole or by an up-to-date prescription
• Age-related macular degeneration - central distortion and metamorphopsia with preserved peripheral vision; Amsler grid distortion; the red reflex is normal
• Chronic open angle glaucoma - peripheral field loss with preserved central acuity, and a cupped disc
• Diabetic maculopathy - central blurring with a normal red reflex and characteristic fundal changes
• Corneal opacity or dystrophy - the opacity is anterior to the iris on slit lamp examination
• Vitreous opacities and haemorrhage - floaters and a sudden rather than gradual change
• Optic neuropathy - reduced acuity with an RAPD, reduced colour vision and disc pallor
• Posterior capsular opacification - in a patient who has already had cataract surgery, causing symptoms identical to the original cataract

Investigations

Cataract is a clinical diagnosis made at the slit lamp. Investigation is directed at planning surgery and at identifying coexisting disease that will limit the outcome.

• Biometry - optical measurement of axial length and corneal curvature, used to calculate the power of the intraocular lens to be implanted. This determines the refractive outcome and is the reason patients are asked about their spectacle preferences beforehand.
• Corneal endothelial cell count (specular microscopy) - where Fuchs endothelial dystrophy or a low cell count is suspected, since surgery causes further endothelial loss
• Optical coherence tomography of the macula - increasingly routine, to detect macular disease that would limit the visual result and should be discussed before surgery
• B-scan ultrasonography - if the cataract is too dense to see the retina, to exclude retinal detachment or a posterior segment tumour
• HbA1c and blood pressure - optimising diabetes and hypertension before surgery
• Review of anticoagulants and alpha-blockers - see the note on intraoperative floppy iris syndrome below

Management

Non-surgical measures

There is no medical treatment that reverses or arrests cataract. While waiting, or where surgery is not appropriate, patients can be helped by an up-to-date spectacle prescription, brighter and better-directed reading light, anti-glare measures such as a brimmed hat and tinted lenses for driving, magnifiers, and advice about night driving. Smoking cessation and ultraviolet protection slow progression.⁴

Driving deserves a specific conversation. Glare and reduced contrast sensitivity can make a patient unsafe at night long before their Snellen acuity falls below the DVLA standard of 6/12 with both eyes open, and there is a separate requirement to read a number plate at 20 metres. Ask directly whether they still drive after dark, and advise them of their legal obligation to meet the standard.⁵

Deciding to operate

NICE is explicit that access to cataract surgery should not be restricted by a visual acuity threshold.² The indication is functional: whether the cataract is limiting what the patient needs or wants to do. That means asking about specific activities.

• Driving, including whether they still meet the DVLA standard and whether glare stops them driving at night
• Reading, hobbies, screen use and following faces on television
• Falls, stairs and confidence outdoors
• Work and independence
• Medical indications - where the cataract prevents adequate view of the fundus for diabetic retinopathy monitoring or retinal treatment, or where a lens-induced complication such as phacomorphic glaucoma is developing

Phacoemulsification

1. Local anaesthesia - usually topical or sub-Tenon's, as a day case, with the patient awake
2. Small incisions of around 2.2-2.8 mm at the limbus, usually self-sealing without sutures
3. Capsulorhexis - a circular opening in the anterior lens capsule
4. Hydrodissection to separate the lens from the capsule
5. Phacoemulsification - an ultrasonic probe fragments and aspirates the nucleus and cortex, leaving the capsular bag intact
6. Intraocular lens implantation - a foldable acrylic lens inserted through the small incision and unfolded into the capsular bag
7. Postoperative drops - topical antibiotic and steroid for around four weeks, with advice to avoid rubbing the eye, swimming and heavy lifting

Patients are usually seen at two to four weeks and refracted once the eye has stabilised. The refractive target is normally emmetropia for distance, with reading glasses afterwards, but this is a discussion: some patients prefer to be left myopic so they can read unaided, and monovision or multifocal lenses are options that must be explained honestly, including the loss of contrast and haloes that multifocal lenses can cause.

Complications

Cataract surgery is very safe, with a serious complication rate well under 2%, but the complications are examinable and the counselling for consent depends on knowing them.³

Prognosis

The results of modern cataract surgery are excellent. Around 95% of eyes without coexisting ocular disease achieve 6/12 or better, which is the DVLA driving standard, and most achieve 6/6.³ Patient-reported outcomes improve substantially, and the benefit is greater for the second eye than acuity figures alone suggest, because binocular function and depth perception are restored.

The main determinant of a disappointing outcome is coexisting disease - macular degeneration, diabetic maculopathy, glaucoma or amblyopia - rather than the surgery itself. This is why identifying such disease preoperatively and discussing realistic expectations is part of good consent, and why an OCT scan before listing has become routine in many units. A patient told beforehand that their macular degeneration will limit the result is disappointed; a patient told afterwards feels the operation failed.

Posterior capsule opacification is worth mentioning explicitly during consent, because patients who develop it months or years later frequently believe the cataract has come back. Explaining that the natural lens has been removed permanently, that the capsule holding the new lens can become cloudy, and that this is corrected by a painless laser procedure taking a few minutes, prevents a good deal of anxiety later.