Contact lenses and low vision

In short

Contact lenses are optical devices worn directly on the cornea or sclera to correct refractive error, manage corneal irregularity and deliver cosmetic or therapeutic benefit. Low vision care begins where spectacles and contact lenses stop: it uses magnification, illumination, contrast and training to make residual vision useful. Both are core clinical papers in B.Sc Optometry and B.Sc Allied Health (Optometry) programmes, examined in university practical and viva, and tested in hiring rounds at Lenskart, Titan Eyeplus, Essilor, Bausch + Lomb, Alcon, Cooper Vision, Vision Express and hospital chains such as LVPEI, Aravind, Sankara Nethralaya, Dr Shroff's and Narayana Nethralaya. India's regulatory frame matters: optometry practice is governed by the National Commission for Allied and Healthcare Professions Act, 2021, contact lenses are notified medical devices under the Medical Devices Rules, 2017, and certification of visual disability follows the Rights of Persons with Disabilities Act, 2016.

Explained

Optics and physiology of a contact lens

• A contact lens sits on the tear film, so the tear lens (liquid between lens back surface and cornea) becomes part of the optical system; with a rigid lens the tear layer neutralises corneal astigmatism.
• Vertex distance effect: moving the correction from 12 mm in front of the eye to the cornea changes effective power. Minus powers need less minus at the cornea; plus powers need more plus. Clinically significant beyond about ±4.00 D.
• Retinal image size: contact lenses cause far less magnification/minification than spectacles — the big advantage in high myopia, high hypermetropia and especially aniseikonia after unilateral aphakia.
• Field of view: no frame, no prismatic jack-in-the-box effect, no ring scotoma — important for aphakia and high plus corrections.
• Corneal oxygen: the avascular cornea takes oxygen from the atmosphere through the lens. Transmissibility Dk/t (Dk = permeability, t = centre thickness in cm) determines hypoxia risk. Holden–Mertz criteria: about 24 × 10⁻⁹ units for daily wear without oedema, about 87 for overnight wear.
• Materials: PMMA (obsolete, Dk ≈ 0), rigid gas permeable silicone acrylate/fluorosilicone acrylate, hydrogel (HEMA, water content 38–75%), silicone hydrogel (high Dk, low water, now standard for soft lenses).

Types and classification

• By rigidity: hard (PMMA), RGP/GP, soft (hydrogel, silicone hydrogel), hybrid (GP centre, soft skirt), piggyback (GP over a soft carrier).
• By size relative to cornea: corneal (8.5–9.8 mm, GP), corneo-scleral/semi-scleral, mini-scleral and scleral (15.5–24 mm, vaults the cornea entirely — used in keratoconus, severe dry eye, ocular surface disease, post-graft).
• By wearing schedule: daily wear (removed nightly), extended wear, continuous wear (up to 30 nights, high Dk/t only), orthokeratology (reverse-geometry lens worn at night to flatten the cornea and give daytime freedom from correction; also used for myopia control).
• By replacement: daily disposable, fortnightly, monthly, quarterly, conventional/annual. Daily disposables have the lowest complication rate and no solution cost.
• By design/function: spherical, toric (cylindrical correction; stabilised by prism ballast, peri-ballast, dynamic stabilisation, truncation), multifocal/bifocal (simultaneous vision concentric or aspheric; alternating/translating in GP), monovision (one eye distance, one near), cosmetic/tinted, prosthetic (corneal scars, aniridia — hand-painted iris), therapeutic/bandage lens (bullous keratopathy, recurrent erosion, persistent epithelial defect), drug-delivery and myopia-control soft lenses (peripheral defocus designs).
• Indications: refractive (myopia, hypermetropia, astigmatism, presbyopia, anisometropia, aphakia), therapeutic, diagnostic (gonioscopy, fundus contact lens, ERG lens), occupational and cosmetic.
• Contraindications: active infection or inflammation, severe dry eye (relative), uncontrolled blepharitis, corneal neovascularisation, poor hygiene or motivation, severe allergy, dusty/chemical workplaces, uncontrolled diabetes with epithelial problems.

Fitting routine, step by step

1. History and motivation: purpose, occupation, hours of intended wear, systemic illness, drugs (antihistamines, isotretinoin dry the eye), hygiene and water access.
2. Pre-fit examination: unaided and best-corrected visual acuity, refraction with vertex-corrected powers, slit-lamp (lids, lashes, tear film, cornea, conjunctiva), keratometry or corneal topography, tear film tests — TBUT (normal >10 s), Schirmer I (>10 mm/5 min), NIBUT, phenol red thread.
3. Measurements: horizontal visible iris diameter (HVID), palpebral aperture height, lid tension and position, pupil size in dim light (for multifocals and orthokeratology).
4. Lens selection: soft lens diameter = HVID + 2 mm approximately; base curve about 0.7–1.0 mm flatter than flat K for soft lenses; GP base curve chosen from K readings and corneal astigmatism rules.
5. Trial and assessment after settling (soft 15–20 min, GP 20–30 min): centration, movement on blink (soft 0.2–0.5 mm; GP 1–2 mm with lid attachment), push-up test, over-refraction, comfort, fluorescein pattern for GP (apical alignment vs apical clearance vs apical bearing/ three-point touch).
6. Final power = trial lens power + vertex-corrected over-refraction; order lens, verify with lensometer/radiuscope, check diameter and edge on a measuring magnifier.
7. Patient education: hand washing, insertion and removal, never use tap water or saliva, rub-and-rinse with multipurpose solution, replace case every 3 months, no sleeping/swimming/showering in lenses, replacement schedule, warning symptoms (RSVP — Redness, Sensitivity to light, Vision drop, Pain) and immediate removal.
8. Follow-up: 1 week, 1 month, then 6-monthly; check acuity, fit, cornea with fluorescein, lids everted for papillae, any neovascularisation.

Contact lens complications and care

• Hypoxic: corneal oedema, striae, microcysts, epithelial thinning, limbal hyperaemia, stromal neovascularisation, endothelial polymegethism.
• Mechanical: superficial punctate keratitis, 3- and 9-o'clock staining (GP), superior epithelial arcuate lesion (SEAL, tight silicone hydrogel), corneal abrasion, foreign body track, lens binding.
• Inflammatory/immune: contact lens-induced papillary conjunctivitis (CLPC/GPC), contact lens peripheral ulcer (CLPU), infiltrative keratitis, sterile infiltrates, solution hypersensitivity (thiomersal historically), toxic keratitis.
• Infective: microbial keratitis — the sight-threatening one. Risk highest with overnight wear and poor hygiene; incidence roughly 2 per 10,000 daily wear and about 20 per 10,000 extended wear per year. Pseudomonas aeruginosa is the classic organism; *Acanthamoeba* from water exposure; fungal keratitis is common in Indian field settings.
• Deposits: protein, lipid, mucin, jelly bumps, rust spots, fungal growth in the matrix; rectified by replacement schedule and enzymatic/ surfactant cleaning.
• Dryness and discomfort is the commonest reason for drop-out; manage with material change, daily disposables, lid hygiene, lubricants, humidity and screen-break advice.
• Care systems: multipurpose solution (clean, rinse, disinfect, store), hydrogen peroxide one-step, saline (rinse only — never disinfects), GP conditioning solutions, enzymatic tablets. Discard date and no "topping up" of old solution.
• Regulation in India: contact lenses and care solutions are notified medical devices regulated by CDSCO under the Medical Devices Rules, 2017 (lenses risk Class B/C; solutions as prescribed). BIS has standards for ophthalmic optics and contact lenses aligned with ISO; quote the subject rather than an old IS number if unsure, as several have been revised.

Low vision: definitions and assessment

• Low vision (working definition): a person with an impairment of visual functioning even after treatment and/or refractive correction, with visual acuity less than 6/18 to light perception, or visual field less than 10° from fixation, who is potentially able to use vision for planning and execution of a task (WHO, 1992).
• ICD-11/WHO categories (presenting or best-corrected distance VA): mild <6/12–6/18; moderate <6/18–6/60; severe <6/60–3/60; blindness <3/60 or field <10°. "Economic blindness" in Indian programme usage has been VA <6/60; the RPwD Act, 2016 defines blindness as total absence of sight, or VA worse than 3/60 (10/200 Snellen) in the better eye with best correction, or field of vision limited to less than 10°; low vision as VA 6/18 to 6/60, or field 10°–40°. Certification uses these criteria and generates the UDID card.
• Causes in India: uncorrected refractive error (commonest cause of visual impairment), cataract (commonest cause of blindness), glaucoma, diabetic retinopathy, corneal opacity, age-related macular degeneration, retinitis pigmentosa, albinism, amblyopia, optic atrophy, retinopathy of prematurity in children.
• Assessment differs from routine refraction: take a task-based history (what exactly must the person see — blackboard, mobile, medicine strip, machine dial), measure acuity with a logMAR / Bailey–Lovie or Feinbloom chart at a shortened distance, use trial frame with high-plus/trial clip and retinoscopy at close range, test contrast sensitivity (Pelli–Robson), glare, colour vision, central field with Amsler grid and peripheral field, and eccentric viewing ability for central scotoma.
• Record acuity with the test distance when shortened — 3/60 and 6/120 are not interchangeable notations to be guessed at; convert honestly.

Low vision devices and rehabilitation

• Optical, near: high-plus spectacle magnifiers (half-eye, with base-in prism for binocularity up to about +12 D), hand magnifiers, stand magnifiers (fixed focus, useful for tremor), illuminated magnifiers.
• Optical, distance: Galilean telescope (plus objective, minus eyepiece — erect image, light, short, low power), Keplerian/astronomical telescope (two plus lenses with erect prism — brighter, wider field, higher power), monocular hand-held, bioptic spectacle-mounted telescope; telemicroscope/reading cap for intermediate tasks.
• Electronic: CCTV/video magnifier (desktop and portable), smartphone and tablet magnification with accessibility settings, OCR text-to-speech, screen readers (NVDA, JAWS), electronic travel aids; India-specific: DAISY books, Sugamya Pustakalaya, Bhashini-based reading tools.
• Non-optical: typoscope/reading slit, line and writing guides, bold-line paper, 20W–60W adjustable task lamp with lamp close and to the side, yellow/amber filters and absorptive lenses for glare, high-contrast marking, large-print material, tactile and auditory labels.
• Field defect management: reverse telescope or minus lens to minify (field expansion in tunnel vision), Fresnel prisms and mirrors for hemianopia, scanning training.
• Rehabilitation and training: eccentric viewing and steady-eye strategy, orientation and mobility with white cane, activities of daily living training, Braille where vision is insufficient, school integration, vocational counselling, counselling for the family — delivered through vision centres, district blindness control societies and the National Programme for Control of Blindness and Visual Impairment (NPCBVI), with assistive devices supported under the ADIP scheme and reservation/benefits under the RPwD Act, 2016.

Key points

• Tear lens – liquid layer behind a rigid lens; neutralises corneal astigmatism (one reason GP lenses correct irregular corneas).
• Dk/t – oxygen transmissibility = permeability ÷ thickness; ≈24 units for daily wear, ≈87 for overnight (Holden–Mertz).
• Silicone hydrogel – high Dk with low water content; now the default soft lens material.
• Vertex distance correction – minus powers decrease and plus powers increase when moved from spectacle plane to cornea; matters beyond ±4.00 D.
• Soft lens diameter – roughly HVID + 2 mm; base curve 0.7–1.0 mm flatter than flat K.
• Movement on blink – soft 0.2–0.5 mm, GP 1–2 mm; too little = tight lens, too much = flat/loose fit.
• Fluorescein pattern – judges GP fit: apical alignment is ideal; pooling = steep, central touch = flat.
• Toric stabilisation – prism ballast, peri-ballast, dynamic (thin-zone) stabilisation, truncation; LARS rule corrects axis mislocation.
• Scleral lens – vaults the cornea on a fluid reservoir; the lens of choice in advanced keratoconus and severe ocular surface disease.
• Orthokeratology – overnight reverse-geometry GP lens; temporary corneal reshaping and a myopia-control option.
• Microbial keratitis – uncommon but the most serious complication; overnight wear and water contact are the main risk factors.
• CLPC – giant papillae on the upper tarsal conjunctiva from mechanical and immune reaction to deposits.
• Never water – tap water, well water or saliva on lenses or cases risks *Acanthamoeba* and fungal keratitis.
• Low vision – residual vision usable for tasks after all medical, surgical and refractive treatment; VA 6/18 to 6/60 or field 10°–40° under the RPwD Act, 2016.
• Blindness (RPwD Act, 2016) – VA worse than 3/60 in the better eye with best correction, or field of vision limited to less than 10°, or total absence of sight.
• Magnification sources – relative size, relative distance, angular (telescope), electronic (CCTV); near devices work mainly by relative distance.
• Galilean vs Keplerian – Galilean is shorter and lighter with a narrow field; Keplerian is brighter with a wider field and higher power.
• Illumination and contrast – often give more functional gain than extra magnification; task lamp placement is part of the prescription.

Formulas and examples

Example 1 — vertex distance both ways. A patient has −8.00 D at 12 mm vertex distance. F_c = −8.00 / (1 − 0.012 × −8.00) = −8.00 / (1 + 0.096) = −8.00/1.096 = −7.30 D → order −7.25 D. For a +12.00 D aphakic correction at the same vertex: F_c = 12.00 / (1 − 0.012 × 12.00) = 12.00 / (1 − 0.144) = 12.00/0.856 = +14.02 D → order +14.00 D. Note the direction: minus got weaker by 0.70 D, plus got stronger by 2.02 D.

Example 2 — final soft toric lens power from over-refraction. Spectacle Rx −4.00/−1.50 × 180 at 12 mm. Spherical equivalent of the cyl is small, so take each meridian:

• 180 meridian power −4.00 → F_c = −4.00/(1 + 0.048) = −3.82 D
• 90 meridian power −5.50 → F_c = −5.50/(1 + 0.066) = −5.16 D

Corneal-plane Rx ≈ −3.75/−1.25 × 180 after rounding to 0.25 D steps. Trial lens −3.50/−1.25 × 180 gives over-refraction −0.25 DS, so final order = −3.75/−1.25 × 180. If the lens marking settles 10° left (anticlockwise), LARS (Left Add, Right Subtract) gives an ordered axis of 180 + 10 = 190 ≡ 010.

Example 3 — Dk/t and wear schedule. A silicone hydrogel lens has Dk = 110 and centre thickness 0.08 mm = 0.008 cm. Dk/t = 110 / 0.008 = 13,750 × 10⁻¹¹ = 137.5 × 10⁻⁹ units (hydrogel convention). This exceeds the Holden–Mertz overnight criterion (≈87), so it can be considered for extended wear. A conventional hydrogel with Dk = 20 and the same thickness gives 20/0.008 = 25 units — only just adequate for daily wear and unsafe overnight.

Example 4 — low vision near add by Kestenbaum's rule. Distance VA = 6/60 → decimal 0.10 → initial add ≈ 1/0.10 = +10.00 D, i.e. a working distance of 10 cm and about 2.5× magnification (M = F/4 = 10/4 = 2.5×). If the patient needs N8 print but threshold with +10 D is N8, there is no acuity reserve; doubling the print demand means doubling magnification → try +20.00 D (5×) or a 5× stand magnifier / CCTV. For binocular use at +10 D, add base-in prism of about 2Δ per eye more than the add value in dioptres is the usual rule of thumb (so about 12Δ base-in per eye, 24Δ in total, for a +10 D half-eye) — exact prism depends on the manufacturer's range.

Example 5 — telescope for distance. A student with VA 6/36 must read a blackboard needing 6/9. Required magnification = 36/9 = 4×. A 4× hand-held monocular with objective +25 D needs an eyepiece of F_eye = −4 × 25 = −100 D for a Galilean design, giving a tube length of roughly 1/25 − 1/100 ≈ 0.04 − 0.01 = 0.03 m = 30 mm. Expected field of view is narrow, so pair the telescope with front-row seating and a copy of the notes.

Where it is used

• Retail optical chains and vision centres: Lenskart, Titan Eyeplus, Specsmakers, GKB, Vision Express — contact lens fitting, trial dispensing, aftercare and sales of care solutions; optometrists handle the full fit-to-follow-up cycle.
• Hospital contact lens clinics: LVPEI, Aravind, Sankara Nethralaya, Dr Rajendra Prasad Centre (AIIMS), Narayana Nethralaya — speciality fitting for keratoconus, post-keratoplasty, aphakia, prosthetic shells and therapeutic lenses.
• Industry roles: Bausch + Lomb, Alcon, CooperVision, J&J Vision, Essilor — product training, clinical affairs, professional services, regulatory work under CDSCO and the Medical Devices Rules, 2017.
• Low vision clinics and rehabilitation centres: LVPEI's rehabilitation services, NAB (National Association for the Blind), NIEPVD Dehradun, district blindness control societies under NPCBVI; device trial, prescription, training and ADIP-supported dispensing.
• School and community programmes: school eye screening under NPCBVI, integrated education for children with visual impairment, teacher counselling on seating, print size and lighting.
• Certification and entitlement work: assessing visual acuity and fields for disability certification under the RPwD Act, 2016 and UDID, which drives education and employment reservation, travel concession and pension benefits.
• Myopia control practice: orthokeratology and defocus-incorporated soft lenses, a fast-growing paediatric service in Indian metros.

Interview questions

• Q: Why do you need to apply vertex distance correction before ordering a contact lens? A: Moving the lens from 12 mm in front of the eye to the corneal plane changes its effective power; minus lenses need less power and plus lenses need more, with the error becoming clinically significant beyond about ±4.00 D.
• Q: How does an RGP lens correct corneal astigmatism without a cylinder? A: The tear film fills the space between the spherical rigid back surface and the toric cornea, forming a tear lens that neutralises the corneal astigmatism; only residual lenticular astigmatism remains.
• Q: What is Dk/t and what values matter? A: Oxygen transmissibility — permeability divided by lens thickness; roughly 24 × 10⁻⁹ units avoids daily-wear oedema and about 87 is needed for overnight wear (Holden–Mertz).
• Q: How do you judge whether a soft lens fit is too tight? A: Movement under 0.2 mm on blink, negative push-up test, conjunctival indentation or drag, variable vision clearing just after a blink, and discomfort or injection on removal.
• Q: What fluorescein pattern indicates a steep GP fit? A: Central pooling of fluorescein with mid-peripheral bearing and excessive edge clearance, often with reduced movement and bubbles under the lens.
• Q: Which complication of contact lens wear worries you most, and why? A: Microbial keratitis — it is uncommon but can scar the visual axis or perforate; overnight wear, poor hygiene and water contact raise the risk, so patients are taught the RSVP symptoms and told to remove the lens and attend immediately.
• Q: Why is a scleral lens preferred in advanced keratoconus? A: It vaults the irregular cornea entirely, resting on the sclera, so a smooth fluid–lens interface restores a regular refracting surface with good comfort and stability.
• Q: Define low vision and distinguish it from blindness under Indian law. A: Low vision is usable residual vision after all treatment and correction — under the RPwD Act, 2016 acuity 6/18 to 6/60 or field 10°–40°; blindness is acuity worse than 3/60 in the better eye with best correction, field of vision limited to less than 10°, or total absence of sight.
• Q: How do you choose the initial near add in a low vision patient? A: Start with Kestenbaum's rule — the reciprocal of the decimal distance acuity — then refine by the actual print size the task demands, keeping an acuity reserve of about 2:1 and checking the working distance is tolerable.
• Q: Galilean or Keplerian telescope — which would you give a student for the blackboard? A: A Keplerian for sustained classroom use, because it gives a brighter image, wider field and higher magnification; Galileans are lighter and fine for brief spotting but dim and narrow.
• Q: A patient with low vision gains little from more magnification. What else would you change? A: Illumination and contrast — a close, adjustable task lamp, typoscope, bold-line paper, absorptive filters for glare — and training in eccentric viewing or scanning, which often add more function than extra power.
• Q: What advice prevents most contact lens complications? A: Wash and dry hands, never let water or saliva touch lens or case, rub-and-rinse with fresh multipurpose solution, replace the case quarterly, never sleep or swim in lenses unless specifically prescribed, keep to the replacement schedule and attend follow-up.

Common mistakes

• Applying the vertex distance formula in the wrong direction, or using it on low powers where it changes nothing after rounding to 0.25 D.
• Ordering a soft lens base curve from keratometry as if it were a GP lens — soft lenses are fitted flatter and selected by diameter and sag, not by exact K match.
• Forgetting to let the trial lens settle before judging fit and taking over-refraction, so a transient tight or decentred fit is recorded as the final one.
• Mislabelling low vision assessment as "just a refraction": omitting task history, contrast, glare and field testing, and prescribing magnification without checking the working distance is usable.
• Quoting 6/60 as the legal blindness cut-off; the RPwD Act, 2016 criterion is worse than 3/60 in the better eye with best correction (or field less than 10°), and acuity must be best-corrected.
• Recommending saline as a disinfectant, or allowing patients to "top up" old solution instead of discarding it.
• Prescribing the highest magnification available instead of the lowest that meets the task, which narrows the field, shortens the working distance and causes device abandonment.

Related topics

Refraction and eye examination · Ocular anatomy and physiology of the cornea and tear film · Ocular diseases and the anterior segment · Binocular vision and orthoptics · Dispensing optics and ophthalmic lenses · Public health optometry and NPCBVI