PRESBYOPIA

Definition

• Kanski's Clinical Ophthalmology, 10th ed. - "Presbyopia is the gradual loss of the ability of the lens to focus on near objects. It is an inevitable ageing phenomenon which occurs as a consequence of loss of accommodation secondary to reduced lens elasticity."
• Goldman-Cecil Medicine - "Presbyopia typically presents in the fourth to sixth decades of life and manifests as a progressive decrease in the ability to focus clearly on near objects."

Etiology / Pathophysiology

1. Progressive lens growth and thickening - the lens grows throughout life, accumulating more fiber cells
2. Protein denaturation - lens proteins undergo progressive denaturation, causing the lens to lose its elasticity and become rigid
3. Loss of accommodation - normally, the ciliary muscle contracts (via parasympathetic innervation from CN III), relaxing the zonule fibers, allowing the elastic lens to become more convex (increased curvature = increased refractive power) for near vision. As the lens hardens, it can no longer change shape in response to ciliary muscle contraction
4. Quantified progression - accommodation decreases from ~14 diopters in a child, to less than 2 diopters by age 45-50, to essentially 0 diopters at age 70 (Guyton & Hall Physiology)
5. The zonule fibers and ciliary muscle are not primarily at fault - the lens itself is the primary pathology

Clinical Features

• Difficulty reading small print (the most common presenting complaint)
• Eyestrain and asthenopia (eye fatigue) after near work
• Headaches after prolonged reading or close work
• Inability to read material unless it is held farther away (arm's length reading)
• Blurring of near vision, especially in dim lighting
• Need for brighter illumination to read

• Reduced near point of accommodation (near point recedes with age)
• Each eye remains focused at a fixed, permanent distance - the patient cannot voluntarily adjust focus for both near and far
• Distance vision may remain normal (no change in far point unless there is co-existing refractive error)
• Refraction shows a decrease in near-correction power

Treatment

Non-Surgical (First-line)

• Reading glasses - simple plus-power lenses for near work
• Bifocal spectacles - upper segment for distance, lower segment for near tasks (the standard correction for symptomatic presbyopia with co-existing distance refractive error)
• Progressive (varifocal) lenses - gradual transition from distance to near correction without a visible dividing line
• Contact lenses - monovision contact lens correction (one eye for distance, one for near)

Surgical Options

• Lens extraction / Intraocular lens (IOL) implantation - known as Clear Lens Exchange (CLE), Refractive Lens Exchange (RLE), or Presbyopic Lens Exchange (PreLEx); removes the natural lens and implants a prosthetic lens
  • Multifocal / trifocal diffractive IOLs - optically restore reading vision; may cause nocturnal glare, halo, or reduced contrast sensitivity
  • Extended depth-of-focus (EDOF) IOLs - provide a range of clear vision
  • Accommodating pseudo-accommodative IOLs - attempt to mimic natural accommodation
  • Monovision IOL targeting - one eye optimized for distance, the fellow eye for near
• Laser-induced monovision (LASIK/PRK) - one eye corrected for distance, the other for near (requires patient trial with contact lenses first)
• PresbyLASIK - laser ablation creates a multifocal corneal profile by altering corneal asphericity
• Conductive keratoplasty (CK) - uses radiofrequency energy to steepen the peripheral cornea; may impart some multifocal functionality
• Intracorneal inlays - small implants placed within the corneal stroma; two types:
  • Refractive inlays (alter corneal curvature)
  • Small-aperture (pinhole) inlays (use the pinhole effect to extend depth of focus)
• Scleral expansion surgery - inconsistent and unpredictable results; not widely used

AMBLYOPIA

Definition

• Kanski's Clinical Ophthalmology, 10th ed.
• A difference in best-corrected VA of two Snellen lines or more (or >1 log unit) between the two eyes, in the absence of organic lesion, is diagnostic of amblyopia.
• NOTE: Amblyopia is the most common cause of visual loss in children (Wills Eye Manual).

Etiology / Classification

1. Strabismic Amblyopia

• Most common form (along with anisometropia)
• Caused by abnormal binocular interaction - the brain suppresses input from the consistently deviating, non-fixating eye to avoid diplopia
• Vision is worse in the consistently deviating eye

2. Anisometropic Amblyopia

• Most common form (along with strabismus)
• Caused by a significant difference in refractive error between the two eyes (usually ≥1.50 diopters, though as little as 1 dioptre can cause it)
• The more ametropic eye receives a chronically blurred image, a mild form of visual deprivation
• Frequently associated with microstrabismus; may co-exist with strabismic amblyopia
• Can be seen with eyelid hemangioma or congenital ptosis inducing astigmatism

3. Stimulus Deprivation Amblyopia

• Caused by physical obstruction of the visual axis - e.g., unilateral cataract, corneal scar, persistent fetal vasculature (PFV), ptosis covering the pupil
• Can be unilateral or bilateral
• Generally the most severe form because visual input is physically blocked

4. Bilateral Ametropic Amblyopia

• Results from high, symmetrical refractive errors (usually high hypermetropia)
• Both eyes are affected

5. Meridional Amblyopia

• Caused by uncorrected astigmatism (usually >1 D) persisting beyond early childhood
• Produces image blur in one meridian only; can be unilateral or bilateral

6. Occlusion (Iatrogenic) Amblyopia

• Occurs in the fellow (better) eye as a result of excessive patching or over-use of atropine during treatment of the other eye
• Prevented by monitoring at appropriate intervals (1 week per year of age)

Clinical Features

Symptoms

• Usually none - amblyopia is typically asymptomatic and discovered incidentally
• Often detected during routine visual acuity screening of each eye individually (school vision testing)
• History of patching, strabismus, or muscle surgery as a child may be elicited
• Parents may notice a squint or abnormal head posture in the child

Signs

• Poorer best-corrected visual acuity in one eye, not fully correctable with refraction, and not explained by organic lesion
• In anisometropic amblyopia, the involved eye nearly always has the higher refractive error
• Visual loss develops during the first decade of life
• Central vision primarily affected; peripheral visual field usually remains normal

• Crowding phenomenon - individual letters read more easily than a full line of letters; this is more pronounced in amblyopes (must be accounted for when testing preverbal children)
• Neutral density filter effect - in reduced illumination, VA of an amblyopic eye is reduced much less than an organically diseased eye
• When severe, amblyopia may cause a trace relative afferent pupillary defect (RAPD)
• Strabismus may be evident on cover-uncover test
• Normal fundus examination (no organic lesion)

Workup / Diagnosis

1. History - eye problems in childhood (squint, patching, surgery?)
2. Full ocular examination to exclude organic cause
3. Cover-uncover and alternate cover tests to assess eye alignment
4. Cycloplegic refraction of both eyes (critical to identify anisometropia or high refractive errors)
5. VA testing with crowding bars or single optotypes as appropriate for age

Treatment

General Principles

• Treatment is most effective during the sensitive period - usually up to 7-8 years for strabismic amblyopia; may extend into the teens for anisometropic amblyopia where good binocular function exists
• Always exclude and treat any organic disease first
• Correct any underlying refractive error first and allow a period of refractive adaptation (6-12 weeks) before adding occlusion therapy

Step 1: Optical Correction

• Prescribe full cycloplegic refraction with appropriate spectacles as early as possible
• For anisometropia: give the appropriate spectacle correction at the youngest age possible

Step 2: Occlusion (Patching) of the Better Eye

• Gold standard treatment - most effective method
• Occludes the better eye to force use of the amblyopic eye
• Adhesive patches placed directly over the eye are most effective (patches over glasses allow peeking)
• Regimen (full-time vs. part-time) depends on the child's age and density of amblyopia
  • Typically 2-6 hours/day part-time patching
  • Follow-up at 1 week per year of age (e.g., 3-week review for a 3-year-old)
• The younger the patient, the more rapid the improvement, but also the greater risk of inducing occlusion amblyopia in the patched eye
• Monitor VA in both eyes regularly during treatment
• If VA does not improve after 6 months of effective occlusion, further treatment is unlikely to succeed
• Poor compliance is the single greatest barrier to improvement

Step 3: Penalization (Alternative to Patching)

• Atropine 1% eye drops (once daily) instilled in the better eye - blurs near vision in the better eye, forcing use of the amblyopic eye
• Equally effective as patching for mild-to-moderate amblyopia (VA 20/100 or better / 6/24 or better)
• Advantages over patching:
  • Difficult for the child to remove or thwart
  • Less psychosocial stigma, particularly in school-going children
  • Weekend instillation may be adequate
• Weekend-only atropine is an option to reduce impact on schoolwork
• If ineffective alone, the hyperopic lens can be removed from the glasses of the non-amblyopic eye to increase the blurring effect

Other Methods

• Optical degradation - high plus lens (e.g., +9.00 D) or aphakic contact lens over the better eye
• CAM vision stimulator - rotating grating patterns to stimulate the amblyopic visual cortex (limited evidence)
• Dichoptic training / digital therapy - binocular vision therapy using games or screens that present different images to each eye (emerging evidence, especially for older patients)

Special Situations

• Media opacity (cataract, corneal scar): Remove the media opacity urgently, then begin patching the non-amblyopic eye
• Strabismic amblyopia: Delay strabismus surgery until vision in the two eyes is equalized or maximal VA is obtained in the amblyopic eye
• Treatment failure or presentation outside treatment age: Prescribe protective glasses to prevent accidental injury to the non-amblyopic eye ("one-eyed athlete" rule - protective eyewear required for all sports)
• If amblyopia is bilateral (e.g., bilateral dense congenital cataracts), urgent optical rehabilitation is required within the earliest months of life