Cataract & Lens Disorders

Cataract is the single largest cause of blindness in India and one of the few causes that is completely reversible by an operation lasting a few minutes. That combination is why it dominates both the national blindness programme and the examination.

The subject looks like a list of opacity types, surgical techniques and complications. It is not. The lens is a simple structure with only three properties, and everything in this chapter is one of those three failing.

1. Three Properties: Clarity, Position and Power

The organising tool is to establish which property has failed before naming anything. A patient who has become more myopic in old age has a power problem caused by a clarity problem. A patient with iridodonesis has a position problem. A patient who cannot read after their cataract operation has a power problem created by treatment.

There is a fourth possibility, and it is the one that produces emergencies. A cataract left long enough stops being a passive opacity and becomes an active cause of glaucoma and uveitis, which is the reason the mature white cataract is not simply a worse version of an immature one.

2. Why the Lens Stays Clear and Why It Fails

The lens is transparent because of an unusual arrangement that is also its weakness.

It has no blood supply and no nerves, and is nourished entirely by the aqueous humour. Vessels would scatter light, so their absence is the price of transparency.

Its fibres are packed with crystallin proteins in a precisely ordered arrangement, spaced closely enough that light passes without scattering. Transparency is therefore a property of protein organisation rather than of the material itself.

Lens cells are never shed. New fibres are laid down over old ones throughout life, so the nucleus of an elderly person's lens contains the cells they were born with, along with every insult those cells have sustained.

That last point explains ageing cataract entirely. Damage accumulates because nothing is discarded, and once crystallins are oxidised they aggregate into particles large enough to scatter light.

The commoner secondary mechanisms follow the same logic. In diabetes, glucose entering the lens is reduced to sorbitol by aldose reductase, and sorbitol cannot leave, so it draws water in osmotically and disrupts fibre architecture. Steroids, ionising radiation and ultraviolet exposure each accelerate oxidative protein damage.

3. Age-Related Cataract and What Each Type Does to Vision

Nuclear cataract raises the refractive index of the nucleus, so the eye becomes more myopic. A presbyopic patient may find they can suddenly read without glasses again, which is called second sight and is a symptom rather than a recovery.

Posterior subcapsular cataract disproportionately affects near vision and bright conditions, because it sits at the nodal point where light rays converge, and because pupillary constriction for near work and in bright light directs light straight through it.

That type has particular diagnostic significance. Posterior subcapsular cataract in a younger patient suggests a cause, most often corticosteroids, diabetes, uveitis or radiation, and its presence should prompt a search rather than a routine listing for surgery.

Maturity is described separately from type. An immature cataract has clear cortex remaining, a mature cataract is completely opaque, and a hypermature cataract has begun to degenerate, either shrinking and wrinkling the capsule or liquefying so that the nucleus sinks within the bag, which is the Morgagnian cataract.

4. Cataract in the Young

Any white pupillary reflex in a child is an emergency until retinoblastoma is excluded. The differential for leukocoria includes congenital cataract, retinoblastoma, retinopathy of prematurity, persistent fetal vasculature and Coats disease.

Congenital cataract is examined mainly on timing, because the threat is amblyopia rather than the opacity. A visually significant unilateral congenital cataract must be operated within the first weeks of life, because the visual cortex will otherwise never develop the pathways for that eye.

The reason unilateral cases are more urgent than bilateral ones is that the two eyes compete. With one clear eye available, the cortex suppresses the deprived one completely, whereas bilateral deprivation produces a lesser degree of loss in both.

Causes worth knowing are galactosaemia, which produces an oil droplet cataract that may reverse on dietary treatment, congenital rubella, Lowe syndrome, and hypocalcaemia, along with a substantial idiopathic and hereditary group.

Complicated cataract is secondary to intraocular disease, most characteristically chronic anterior uveitis, and classically appears as a posterior subcapsular polychromatic breadcrumb opacity.

Traumatic cataract following blunt injury classically forms a rosette or stellate pattern in the posterior cortex, and penetrating injury causes rapid total opacification once the capsule is breached and aqueous enters the lens substance.

5. Assessment Before Surgery

Two measurements determine the outcome, and one of them determines the refractive result entirely.

Intraocular lens power is calculated from keratometry, which measures corneal curvature, and axial length, which measures the length of the eye. These are combined in formulas of which the SRK family is the classical example.

Axial length is the more error-prone measurement and it dominates the calculation. A one millimetre error produces roughly three dioptres of postoperative refractive surprise, which is why optical biometry has largely replaced contact ultrasound, since pressing on the cornea shortens the eye.

A history of corneal refractive surgery makes standard keratometry unreliable, because the cornea's front and back surfaces no longer have their usual relationship, and specific adjusted formulas are needed.

Visual acuity alone is an inadequate indication for surgery. A patient with early posterior subcapsular cataract may read the chart well in a dim room and be unable to drive at night, so glare disability and functional need are assessed alongside acuity.

6. Cataract Surgery

The decisive advance was preserving the posterior capsule, because it supports the intraocular lens in the correct anatomical plane and separates the anterior segment from the vitreous.

Manual small incision cataract surgery matters disproportionately in India, because it produces outcomes close to phacoemulsification in dense cataracts, costs far less, needs no phaco machine, and is faster in high-volume settings. Dismissing it as a lesser technique misreads the epidemiology.

Intraocular lenses are usually placed in the capsular bag. Monofocal lenses give one focal distance, and multifocal or extended depth of focus lenses reduce spectacle dependence at the cost of contrast and glare. Toric lenses correct pre-existing corneal astigmatism.

7. Complications

Posterior capsular opacification is the commonest late complication, occurring in a substantial proportion of eyes over the following years. Residual lens epithelial cells migrate and proliferate on the posterior capsule, forming Elschnig pearls and a fibrotic sheet.

It is not a recurrence of the cataract, and it is treated in seconds with a neodymium-YAG laser capsulotomy rather than by further surgery.

Acute postoperative endophthalmitis is the complication that destroys eyes. It presents within days with pain, falling vision, hypopyon and lid oedema, and the commonest organism is coagulase-negative staphylococcus from the patient's own lid flora.

Intracameral moxifloxacin at the end of surgery roughly halves the incidence, and the evidence base is substantially Indian. A randomised trial across Indian tertiary centres covering 60,000 eyes operated between 2018 and 2024 found endophthalmitis in 0.02 percent of eyes receiving intracameral moxifloxacin against 0.05 percent of controls, with no difference in endothelial cell counts.

Toxic anterior segment syndrome is the important mimic. It appears within 24 hours rather than several days, is painless, shows diffuse limbus-to-limbus corneal oedema, and responds to steroids because it is a sterile inflammatory reaction to a contaminant in the irrigating solution or instruments.

Posterior capsular rupture is the commonest serious intraoperative complication, risking vitreous loss, a dropped nucleus and later retinal detachment or cystoid macular oedema.

8. The Power Limb: Presbyopia, Aphakia and Pseudophakia

The lens is the only adjustable optical element in the eye, and losing that adjustability is universal.

Accommodation works by relaxation, not by pulling. At rest the zonules are taut and hold the lens flattened. When the ciliary muscle contracts, the ciliary ring narrows, zonular tension falls, and the lens is released to assume the rounder shape its own elasticity favours.

Three things happen together during the near reflex: accommodation, convergence and pupillary constriction. The constriction is useful optically, since a smaller pupil increases depth of focus.

Presbyopia is loss of lens elasticity, not weakness of the ciliary muscle. The lens hardens progressively as new fibres are compacted onto old ones, so even a fully contracted ciliary muscle can no longer round it.

This is why presbyopia begins in the fifth decade in everyone, cannot be exercised away, and is corrected with a convex addition rather than by any treatment directed at muscle.

It is also why an uncorrected hypermetrope becomes symptomatic earlier: they have been using accommodation for distance as well as near, so they exhaust their remaining amplitude sooner.

Aphakia is the eye without a lens, and understanding its optics explains why intraocular lenses replaced spectacles so completely.

An aphakic eye is highly hypermetropic and needs around +10 dioptres of correction. Supplying this in spectacles creates severe problems.

Aphakic spectacles magnify the image by roughly 30 percent, so an aphakic patient with one normal eye cannot fuse the two images. They also produce ring scotoma and the jack-in-the-box phenomenon, in which objects disappear at the edge of the field and then leap into view.

Contact lenses reduce magnification to about 7 percent and an intraocular lens to around 2 percent, which is why only the intraocular lens permits binocular vision in unilateral aphakia.

Pseudophakia, the eye with an intraocular lens, is optically excellent but has no accommodation, so a monofocal lens set for distance requires reading glasses.

9. When the Lens Attacks the Eye

Four lens-induced conditions convert a slowly worsening cataract into an emergency, and they are distinguished by mechanism.

The distinction between phacomorphic and phacolytic glaucoma is the mechanical against the inflammatory route to the same pressure. Phacomorphic closes the angle physically; phacolytic leaves the angle open and blocks the drain with protein-laden macrophages.

The definitive treatment in every case is removal of the lens, with medical control of pressure and inflammation first. This is why a mature cataract in a patient who has refused surgery is not a stable situation.

10. Ectopia Lentis and Abnormalities of Lens Shape

Displacement of the lens follows weakness or rupture of the zonules, and the direction is diagnostic.

Marfan goes up, homocystinuria goes down, and this pair is examined constantly because the body habitus of the two overlaps considerably.

Iridodonesis, the tremulousness of the iris on eye movement, indicates lost zonular support and is often the sign that reveals subluxation before the lens edge is seen.

Homocystinuria carries a particular anaesthetic hazard, since these patients are prone to thromboembolism, which makes the distinction from Marfan more than academic.

Shape anomalies

Microspherophakia is a small spherical lens, and because a sphere has greater curvature than a normal lens it produces high lenticular myopia. It is characteristic of Weill-Marchesani syndrome.

Its danger is mechanical. A small round lens can slip forward into the pupil and block aqueous flow, producing pupillary block glaucoma. Miotics worsen this by relaxing the zonules and allowing the lens to move further forward, so cycloplegics are used instead, which is the reverse of standard angle-closure management and is examined for that reason.

Lenticonus is a conical bulge of the lens surface, and the two forms point to different diagnoses. Anterior lenticonus is characteristic of Alport syndrome, alongside sensorineural deafness and hereditary nephritis. Posterior lenticonus is usually isolated or associated with persistent fetal vasculature.

On retinoscopy a lenticonus produces an oil droplet reflex, because the central and peripheral parts of the lens have different powers.

11. Blindness and the Cataract Programme in India

The National Blindness and Visual Impairment Survey of 2015 to 2019 provides the figures that are examined.

Blindness prevalence in those aged 50 and over was 1.99 percent, having fallen from 5.3 percent in 2001. Visual impairment in the same group was 26.68 percent.

Cataract accounted for 66.2 percent of blindness, 80.7 percent of severe visual impairment and 70.2 percent of moderate visual impairment in that age group.

Around 93 percent of blindness and 96.2 percent of visual impairment was avoidable, meaning either preventable or treatable, which is the statistic that justifies the entire programme.

The National Programme for Control of Blindness and Visual Impairment measures its output as the cataract surgical rate, the number of cataract operations performed per million population per year, and its aim is to clear the backlog faster than new cases accumulate.

12. Worked Examples

Example 1. A 68-year-old who has worn reading glasses for twenty years finds he can now read without them, while his distance vision has worsened. Explain.

Nuclear sclerotic cataract. Sclerosis raises the refractive index of the nucleus, increasing the total power of the eye and producing a myopic shift. That myopia partially compensates for presbyopia, so near vision improves, a phenomenon called second sight. It is a symptom of progressing cataract, not an improvement, and distance vision deteriorates in parallel.

Example 2. Three days after uneventful phacoemulsification a patient has pain, vision reduced to hand movements and a hypopyon. What is the diagnosis and how does it differ from the alternative?

Acute postoperative endophthalmitis, usually from coagulase-negative staphylococcus originating in the patient's own lid flora, requiring urgent vitreous sampling and intravitreal antibiotics. The mimic is toxic anterior segment syndrome, which appears within 24 hours rather than days, is painless, shows diffuse limbus-to-limbus corneal oedema, and responds to steroids because it is sterile. Timing and pain separate them.

Example 3. A patient with a long-standing hypermature cataract presents with a red painful eye, pressure of 44 mmHg and an open angle on gonioscopy. Mechanism?

Phacolytic glaucoma. Liquefied cortex leaks through an intact but permeable capsule, and macrophages laden with lens protein obstruct the trabecular meshwork. The angle is open, which distinguishes it from phacomorphic glaucoma, in which an intumescent swollen lens pushes the iris forward and closes the angle mechanically. Both are treated definitively by removing the lens after medical control of pressure.

Example 4. A tall young man with a high-arched palate has bilateral superotemporal lens subluxation. What else must be assessed, and what would inferonasal displacement have suggested?

Marfan syndrome, in which the essential additional assessment is cardiovascular, since aortic root dilatation and dissection determine survival. Inferonasal subluxation would suggest homocystinuria instead, which shares the marfanoid habitus but adds intellectual disability and a strong thromboembolic tendency that becomes critically important around anaesthesia.

Summary

Clarity, position, power are the only three lens properties, and every disorder here is one of them failing.

The lens is avascular and sheds no cells, so damage accumulates lifelong, which is why ageing alone causes cataract.

Nuclear cataract causes myopic shift and second sight; posterior subcapsular cataract worsens near vision and glare and suggests steroids, diabetes or uveitis in a young patient.

Leukocoria in a child is retinoblastoma until proved otherwise, and congenital cataract is urgent because of amblyopia, particularly when unilateral.

Axial length dominates biometry, and a one millimetre error gives roughly three dioptres of refractive surprise.

Preserving the posterior capsule was the decisive surgical advance, and small incision surgery remains appropriate for high-volume Indian practice.

Posterior capsular opacification is the commonest late complication and is treated by YAG laser, not surgery.

Intracameral moxifloxacin roughly halves endophthalmitis, on largely Indian evidence.

Toxic anterior segment syndrome comes within a day and is painless; endophthalmitis comes after days and hurts.

A neglected cataract causes glaucoma and uveitis, and the treatment in every case is to remove the lens.

Marfan subluxates up, homocystinuria down.

Cataract causes 66.2 percent of blindness in Indians over 50, and 93 percent of that blindness is avoidable.