Cataract & Lens Disorders — Practice

Question 1

A 68-year-old finds he can read without his glasses for the first time in twenty years, while distance vision has worsened. Which cataract?

Accepted Answer

C

Question 2

In which direction is the lens typically subluxated in homocystinuria?

Accepted Answer

B

Question 3

Twelve hours after uneventful cataract surgery a patient has painless blurred vision with diffuse limbus-to-limbus corneal oedema. Diagnosis?

Accepted Answer

B

Question 4

A patient with a hypermature cataract has a red painful eye, intraocular pressure of 44 mmHg, and an open angle on gonioscopy. Diagnosis?

Accepted Answer

B

Question 5

An error of 1 mm in axial length measurement produces approximately what refractive error after cataract surgery?

Accepted Answer

C

Question 6

Why is a unilateral congenital cataract a more urgent surgical problem than a bilateral one?

Accepted Answer

Because the two eyes compete for cortical representation during the critical period of visual development.

The visual cortex is not fully formed at birth. Ocular dominance columns and binocular connections develop over the first months and years in response to patterned visual input, and this development is competitive: inputs from the two eyes contest the same cortical territory.

When one eye is deprived by a dense cataract and the other is clear, the clear eye wins that competition decisively. Cortical territory is reallocated to it, and the deprived eye ends up with profound, dense, and largely irreversible amblyopia even if the media are later cleared perfectly.

When both eyes are deprived, neither has an advantage. Development in both is delayed and degraded, but neither is actively suppressed by the other, so the eventual outcome after surgery is better than that of the deprived eye in a unilateral case.

The practical consequence is a timetable. A visually significant unilateral congenital cataract is operated within the first weeks of life, and postoperative optical correction and patching of the good eye must follow immediately, because the surgery alone does not treat the amblyopia.

This is also why any white pupillary reflex demands same-day assessment, both for this reason and because retinoblastoma sits in the same differential.

Question 7

Explain why an intraocular lens, but not aphakic spectacles, permits binocular vision after unilateral cataract surgery.

Accepted Answer

Because of where the correcting lens sits relative to the eye's nodal point, and the image size that results.

An aphakic eye has lost around 20 dioptres of converging power and is therefore very hypermetropic, needing roughly +10 dioptres of correction at the spectacle plane.

A strong convex lens placed about 12 mm in front of the eye magnifies the retinal image substantially, by around 30 percent. It also introduces prismatic effects at its periphery, producing a ring scotoma and the jack-in-the-box phenomenon, in which an object disappears as it enters the scotoma and then abruptly reappears.

Binocular single vision requires the two retinal images to be similar enough in size for cortical fusion. The tolerance is a few percent. A 30 percent difference, called aniseikonia, exceeds it by an order of magnitude, so the patient sees double or suppresses one eye entirely.

Moving the correction closer to the eye's own optical plane reduces the magnification. A contact lens sits on the cornea and gives about 7 percent, which is still often intolerable for fusion. An intraocular lens sits approximately where the natural lens was and gives around 2 percent, which is within fusible range.

This is the optical reason intraocular lens implantation became standard rather than merely convenient, and it explains why aphakic spectacles remain usable only in bilateral aphakia, where both images are magnified equally.

Question 8

Why is a mature cataract in a patient who has declined surgery not a stable situation?

Accepted Answer

Because the lens does not simply remain opaque. Once it matures it begins to change physically and chemically, and each of those changes can injure the eye around it.

An intumescent lens takes up water and swells. The enlarged lens pushes the iris diaphragm forward, crowding and then closing the drainage angle, and the result is phacomorphic glaucoma, which presents exactly like acute angle closure but will not resolve with iridotomy because the mechanism is the lens itself.

A hypermature lens takes the other route. The cortex liquefies, and high molecular weight lens proteins leak out through a capsule that remains grossly intact but has become permeable. Macrophages engulf this material, swell, and obstruct the trabecular meshwork. The angle stays open, the pressure rises, and this is phacolytic glaucoma.

If the capsule ruptures, whether spontaneously in a Morgagnian lens or through trauma or surgery, exposed lens protein provokes an immune response. Phacoantigenic uveitis is granulomatous and can be severe, and retained cortical fragments cause lens particle glaucoma by direct obstruction.

All four are treated definitively by removing the lens, with medical control of pressure and inflammation first.

The clinical consequence is that deferring surgery converts a straightforward elective procedure into an emergency operation on an inflamed eye with a raised pressure, a poorly dilating pupil and a weakened capsule, in a patient whose optic nerve may already have suffered pressure damage that no operation will reverse.

Question 9

What did the National Blindness and Visual Impairment Survey 2015-19 find, and why is the avoidability figure the important one?

Accepted Answer

It found that blindness prevalence in Indians aged 50 and over was 1.99 percent, having fallen from 5.3 percent in 2001, and that visual impairment in the same group was 26.68 percent.

Cataract was responsible 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, making it the dominant cause by a wide margin.

The figure that matters for policy, however, is that around 93 percent of blindness and 96.2 percent of visual impairment was avoidable, meaning either preventable or treatable with existing technology.

That is the statistic which justifies a national programme. If most blindness were degenerative and untreatable, the correct response would be rehabilitation. Because it is overwhelmingly avoidable, and because the dominant cause is corrected by an operation lasting minutes with a very high success rate, the correct response is service delivery at scale.

This is why the National Programme for Control of Blindness and Visual Impairment measures itself by the cataract surgical rate, meaning operations performed per million population per year, rather than by prevalence alone. The programme is in a race: new cataracts accumulate continuously as the population ages, so the surgical rate must exceed the incidence rate for the backlog to fall.

The fall from 5.3 to 1.99 percent over roughly two decades indicates that it has been winning.

Practice — Cataract & Lens Disorders

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