Ординатура / Офтальмология / Английские материалы / Textbook of Visual Science and Clinical Optometry_Bhattacharya_2009

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TD–Total diameter FVP–Front vertex power BVP–Back vertex power

BC–Base curve—It is the curve of the anterior, i.e. front surface of the contact lens.

Barrer/Dk—It is a unit of oxygen permeability of a contact lens. D stands for diffusion coefficient of oxygen through the material and k stands for the solubility of oxygen in the material. The units of Dk are also known as “Barrer” or “Fatt” units.

Diffusion coefficient—The speed at which oxygen molecules pass through the contact lens material.

Solubility—The number of oxygen molecules which can be absorbed in a given volume of the contact lens material.

Hence, a contact lens with lower water content has lower Dk value, whereas those with higher water content have higher Dk values.

Oxygen transmissibility = Dk/t

Oxygen transmissibility is inversely proportional to the thickness (t in cm) of the contact lens and directly proportional to the oxygen permeability of the contact lens material (Dk or Barrer).

Daily wear contact lens—Contact lenses that are worn daily for 10 to 12 hours. They are kept in a container immersed in a specified solution when not in use. They have to be cleansed daily before wearing. They are usually replaced annually.

Extended wear (EW) contact lens—Contact lenses that can be worn continuously for a period of 1 to 7 days. They have to be cleansed after removal and kept in a container immersed in specified solution before subsequent wearing. Nowadays, silicon hydrogel soft contact lens and RGP contact lens are available for continuous wear upto 30 days (day and night). Silicon hydrogel lenses are safer than conventional soft lenses for extended wear. There is increased incidence of infectious keratitis with conventional soft extended wear contact lenses than conventional soft daily wear lenses.

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Disposable contact lenses—Daily and monthly disposable soft contact lenses are also available.

Cosmetic contact lens—These soft contact lenses change the colour of the eyes and are available in various tints. Such lenses have a clear central optical area (approx. 4 mm) and a clear peripheral area of 1 mm overlying the sclera. Contact lenses with very light blue tint are available for easier handling.

Bifocal contact lens—Various designs of bifocal soft contact lens and multifocal soft contact lens are indicated in presbyopic, aphakic and pseudophakic patients due to reduced or lack of accommodation. Various types of bifocal contact lenses are available:

i.Annular—The central zone is meant for distance vision with a surrounding annular zone intended for near vision.

ii.Segmental—It contains the near addition in the lower segment of the contact lens. Segmental bifocal contact lens is prevented from rotation usually by truncation.

iii.Diffractive—It consists of series of concentric diffraction rings in the centre of the back surface. The diffraction rings are designed to focus equally on the retina from distant and near objects without any movement of the contact lens.

iv.Aspheric—The central area of the multifocal contact lens is intended for distant vision. There is a gradient transition of power from the centre to the periphery to correct all range of vision.

In annular and segmental variety during down gaze, the contact

lens rises relative to the cornea, positioning the near segment in the visual axis. Another method of correction is used earlier, instead of bifocal contact lens, termed “Monovision”. In “monovision” the eye with better vision is fitted with a distance contact lens correction, whereas the other eye is fitted with a near contact lens correction.

ASSESSMENT OF SOFT CONTACT LENS FITTING

•Initial comfort: Initially, both a well-fitting lens and a tight-fitting lens are comfortable. A loose fitting lens gives rise to discomfort

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and awareness of the lens due to excess movement of the lens and poor centration. However, a slight awareness of the lens is considered optimum comfort.

•Vision: Stable good vision indicates good lens centration and good tear film stability.

•Lens centration: If the edge of the lens symmetrically overlaps on the sclera on all sides the centration of the lens is optimum.

•Lens movement: At least 0.3 mm of lens movement is required with each blink for adequate tearfilm lubrication. Tight fitting lenses show little or no movement (less than 0.1 mm) whereas loose fitting lenses show excessive movement (more than 1.0 mm).

•Lag on upgaze and version: A well-fitting lens will decentre between 0.3 to 0.7 mm upon upgaze or lateral version, i.e. abduction and adduction. However, excess of decentration indicates loose fitting lens while absence of decentration indicates tight-fitting lens.

•Push-up test: This test is an indicator of the mobility of the contact lens in response to a push upwards on the inferior edge of the lens through the lower eyelid margin. A well-fitting contact lens moves easily and regains centration quickly.

•Fluorescein stain pattern: It is not done in soft contact lens fitting since it is absorbed by the soft contact lens. However, it can be used to trace lost soft contact lens or fragment of soft contact lens under the upper eyelid or upper conjunctival fornix.

ASSESSMENT OF RIGID CONTACT LENS FITTING

•Comfort: Around 3 weeks are required for adaptation to the lens. The lens edge is at fault if it remains uncomfortable beyond 3 weeks.

•Vision: The vision should be stable during blinking.

•Centration: The rigid lenses are 2.5 mm smaller in diameter than the horizontal visible iris diameter and they should be perfectly centred over the pupil between blinks. Temporal or nasal decentration of the rigid contact lens indicates a flat fit. A high riding lens or a low riding lens also indicates a too flat fit attributed to the back optic zone radius (BOZR).

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DETERMINATION OF CONTACT LENS POWER

Spectacle lenses are placed at a vertex distance of usually 12 mm from the corneal apex, whereas contact lenses are worn at a vertex distance of practically 0 mm. We are also aware that if a concave lens is brought closer to the eye its effective strength increases, whereas converse is true for convex lenses. So, power of a concave lens decreases, whereas power of a convex lens increases for conversion to contact lens power. Hence, the refractive error at the spectacles plan, must be converted to the refractive error at the corneal plane from the vertex conversion table (Table 15-2).

Table 15-2: Vertex conversion table*

*Average 12 mm Vertex Distance. For Minus Powers Read left to right and for

Plus Powers Read right to left.

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vessels are continuous with the limbal vessels (Fig. 15-7C) and involves the superior limbus most frequently. However, deep vascularisation (Fig. 15-7D) is very uncommon and rare with contact lens wearer.

iv.Endothelial changes: Polymegathism, i.e. greater than normal variation in cell size in endothelial mosaic is linked to chronic hypoxia of the cornea of long duration. They are seen in specular reflection of slit-lamp biomicroscopy (Fig. 15-7E). Endothelial blebs are intracellular accumulation of oedema due to hypoxia and acidosis and seen as black dots in specular reflection.

v.Contact lens induced acute red eye (CLARE) syndrome: It is an acute inflammatory response seen in soft extended wear contact lens and extended wear hydrogel lens worn tightly. Large infiltrates are seen in anterior stroma peripherally. The patient complains of severe ocular pain, photophobia, lacrimation and redness. Cellular debris is trapped behind the lens rendering it adherent, i.e. immobile. CLARE syndrome is caused by toxic effects of entrapped cellular debris, dehydration of tearfilm during sleep and toxicity to contact lens solution preservative. Treatment is discontinuation of the contact lens wearing, topical steroid with antibiotic application and switch over to a daily wear lens after remission of the syndrome.

vi.Infection: Pain associated with discharge and redness is a warning sign for possibility of corneal infection. Acanthamoebae keratitis is a rare, serious blinding infection seen more commonly in contact lens wearer. Acanthamoebae is found in soil, chlorinated water, swimming pool, contact lens solutions, etc. Extreme ocular pain disproportionate to the clinical signs is a prominent sign of Acanthamoebae keratitis.

PHYSICAL PROBLEMS OF CONTACT LENS

i.Loss of contact lens

ii.Lens deposits.