Pre-Fitting Evaluation

A thorough history separates good candidates from poor ones before any lens touches the eye.

Motivation & lifestyle: why the patient wants lenses (cosmetic, sports, occupation), screen time, hobbies, and realistic expectations drive material and modality choice.

Ocular history: prior lens wear and any dropout reasons, dry eye, allergies, giant papillary disease, blepharitis, prior refractive surgery, or corneal disease.

Systemic conditions: poorly controlled diabetes slows epithelial healing; pregnancy and thyroid disease shift corneal curvature and tear film; autoimmune dry-eye disease (Sjögren's) is a relative contraindication.

Medications: antihistamines, decongestants, diuretics, beta-blockers, isotretinoin, and some antidepressants reduce tear production; note them all.

Environment & occupation: dusty, smoky, low-humidity, or chemical workplaces challenge wear, and some jobs restrict lenses entirely.

Contraindications: active infection or inflammation, severe dry eye, poor hygiene, unreliable follow-up, and inability to handle lenses.

Age: children and teens can succeed with parental support (and may benefit from myopia management); presbyopes need multifocal counseling.

Documenting all of this protects the patient and supports informed consent.

The cornea has five layers (anterior to posterior):

- Epithelium — outermost, ~5-7 cell layers; regenerates in days and is the main barrier to microbes. Disrupting it (abrasion, desiccation) opens the door to infection.

- Bowman's layer — acellular; does not regenerate and scars if injured.

- Stroma — ~90% of the ~0.5 mm (about 540 micron) central thickness; regularly arranged collagen lamellae give transparency.

- Descemet's membrane — the tough basement membrane of the endothelium.

- Endothelium — a single cell layer (~3,000 cells/mm² in young adults) that pumps fluid out of the stroma (deturgescence) to keep it clear; it does not regenerate, so cell loss is permanent.

Optics: the cornea supplies about 43 D — roughly two-thirds of the eye's power — with a refractive index near 1.376.

Oxygen supply: being avascular, the cornea draws oxygen from the tear film. With eyes open, atmospheric oxygen is ~155 mmHg; closed (sleep), supply falls to ~55 mmHg from the palpebral conjunctiva — which is why overnight wear demands high Dk/t.

The limbus houses epithelial stem cells vital for healing; soft and scleral lenses land at or beyond it.

Three layers (outer to inner):

1. Lipid — secreted by the Meibomian glands in the lid margins; retards evaporation. Meibomian gland dysfunction is a leading cause of evaporative dry eye and lens discomfort.

2. Aqueous — from the lacrimal gland; the thickest layer, carrying oxygen, nutrients, and antibacterial proteins such as lysozyme.

3. Mucin — from conjunctival goblet cells; coats the epithelium so the watery layer can spread and adhere.

Total tear volume is only ~7 microliters, with pH near 7.4 and turnover about 16% per minute.

Key tests:

- Tear Break-Up Time (TBUT): with fluorescein, time the first dry spot after a blink. Under ~10 seconds suggests an unstable film.

- Schirmer test: a paper strip in the lower fornix; under ~10 mm of wetting in 5 minutes suggests reduced aqueous production, and under 5 mm is significant.

- Phenol red thread and meibomian gland expression add detail.

A stable film is essential for success. A marginal film points toward daily disposables, higher-Dk materials, and rewetting drops, and away from long wear times.

Keratometry (K readings): measures the curvature of the central ~3 mm in the two principal meridians, reported in diopters or millimeters. It selects the trial base curve and quantifies corneal astigmatism.

- Conversion: r (mm) = 337.5 / K (D) (keratometric index 1.3375). Example: 45.00 D → 7.50 mm; 42.00 D → 8.04 mm.

- A steeper cornea = more diopters = smaller radius.

- With-the-rule astigmatism has the steeper meridian near vertical (90); against-the-rule near horizontal (180).

- Distorted, egg-shaped mires suggest irregular astigmatism (keratoconus) — refer for topography.

HVID (Horizontal Visible Iris Diameter): ~11.5-12 mm on average; guides overall lens diameter (soft lenses are typically 2-3 mm larger than HVID).

Pupil size in dim light influences optic-zone choice, especially for multifocals and GPs.

Palpebral aperture & lid position/tension: govern centration and movement — a high, tight lid can lift a GP into a lid-attachment fit.

Vertex compensation: spectacle powers beyond ±4.00 D must be converted to the corneal plane before ordering, because moving the correction ~12 mm closer changes effective power.