Section ONE Preliminaries

Table 5.1  Examples of back vertex powers

5.4 Visual considerations

Ocular refraction

When considering the back vertex power (BVP) of the final contact lens, it is important to convert from the spectacle refraction Fsp to the ocular refraction K. The ocular refraction is theoretically the same as the power of the contact lens since it is in contact with the front surface of the cornea. Spectacles, on the other

hand, are typically 11.00 to 13.00 mm away from the eye (giving the back vertex distance d). With powers higher than ±4.00 DS the ocular refraction

becomes important; below this figure the contact lens BVP can be regarded for practical purposes as the same as the the spectacle refraction (see Table 5.1 for examples and Appendix for a full range of powers and vertex distances). The equation used is:

The correction of ametropia

The back surface of a rigid lens maintains its shape on the eye. A liquid lens of predictable power therefore forms between the back surface of the contact lens and the anterior surface of the cornea. When performing calculations with rigid lenses, it is usual to consider that both elements – the contact lens and the liquid lens – are separated by an infinitely thin ‘air film’. This contact lens–liquid lens system gives rise to the final BVP. If the fitting is not in alignment with the cornea, the final power is no longer the same as the ocular refraction. The liquid lens power depends on the curvature of the back surface of the contact lens and that of the anterior corneal surface. It is positive with a lens fittting steeper than ‘K’, plano with alignment and negative if the lens is flatter than ‘K’ (Figure 5.1A  and B).

A soft lens drapes over the corneal surface and assumes its shape. Any liquid lens is therefore of minimal power.

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Figure 5.1  (A) Steep contact lens – positive tear lens power; (B) flat contact lens – negative tear lens power

Corneal and residual astigmatism

Most modern keratometers, autokeratometers and topographers express the curvature of the cornea’s principal meridians in both diopters and millimetres. Where only millimetres are given, the dioptric equivalent can be estimated from the following rule of thumb:

0.05 mm ≡ 0.25 D

e.g. ‘K’ readings 7.80 along 180 and 7.60 along 90 difference in Ks = 0.2 mm ≡ 1.00 DC

This is most accurate in the region of 7.80 mm. With much steeper corneas (e.g. with keratoconus), the dioptric equivalent is greater; with much flatter corneas (e.g. post refractive surgery), the dioptric equivalent is less.

Two basic assumptions are made when assessing the potential success of patients with astigmatism:

1.Total ocular astigmatism = corneal astigmatism + lenticular astigmatism.

2.Most corneal astigmatism is transferred through a soft lens to its anterior surface.

Patients may therefore be divided into four groups at their initial examination by reference to spectacle correction and ‘K’ readings.

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Section ONE Preliminaries

Spherical cornea with spherical refraction

This is the ideal optical case for contact lens fitting. Vision should be equally good with either rigid or soft lenses.

Spherical cornea with astigmatic refraction

The astigmatism is almost entirely lenticular, so that the visual result is the same with either a rigid or a soft lens. In either case, a front surface toric lens is required to correct the 1.50 D of residual astigmatism. For reasons of comfort a soft lens is the first choice.

Toric cornea with astigmatic refraction

All of the astigmatism is corneal. A spherical rigid lens, which neutralizes 90% of corneal astigmatism, or a toric soft lens should therefore be fitted.

Toric cornea with spherical refraction

There is 1.75 D of with-the-rule corneal astigmatism together with an equivalent degree of against-the-rule lenticular astigmatism, giving a resultant spherical refraction. A rigid lens form would neutralize the corneal but not the lenticular astigmatism. It would therefore leave a residual cylinder of −1.75 × 90. A soft lens should be used because it transfers all of the corneal astigmatism through to its front surface without optically neutralizing it.

GENERAL ADVICE

Where there is an equal choice between fitting either a toric rigid or toric soft, because of the much greater comfort, it is generally better to fit a soft lens.

Near vision

Near vision can often cause problems despite good distance acuity. There are several reasons,2 the main ones being:

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Preliminary considerations and examination 5 Chapter

•Altered accommodation/convergence ratio.

•Low myopes who previously did not wear spectacles.

•Early presbyopes requiring greater accommodation with lenses compared with spectacles. Conversely, hypermetropes are usually delighted with improved near vision.

Intermediate vision

Intermediate visual tasks, such as VDU operation or painting, can also cause problems. Particular difficulties occur with reading music, especially in dim illumination.

Contrast sensitivity and ‘quality of vision’

Contact lenses of all types do not always give the absolute stability of vision achieved with spectacles. Variations may be due to either the lens or environmental factors. The ‘quality of vision’ is very much a subjective interpretation and does not necessarily correlate with Snellen acuity. This can be confirmed by marked differences in contrast sensitivity readings while visual acuity remains the same. These effects may be more apparent with bifocal and multifocal contact lenses.3

Monocular patients

Particular care is necessary when fitting essentially monocular patients. They are much more disturbed by factors such as lens mobility, flare, or unstable vision with toric and multifocal lenses. Similar considerations apply when fitting complex lenses to the dominant eye.

PRACTICAL ADVICE

•Stability of vision is usually as important as acuity.

•Snellen acuity is not always a reliable guide to a patient’s potential success. In some circumstances, a good 6/9 may be more acceptable than a poor 6/6.

•Assess distance, near and intermediate vision separately in relation to work and visual requirements.

•Take particular care with monocular patients.

5.5External eye examination

External examination prior to fitting is essential to allow the practitioner to:

•Confirm the normality of ocular tissues.

•Discover any condition that would preclude contact lens wear.

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Section ONE Preliminaries

•Record for the future any other abnormality.

•Refer for medical treatment any active disease unrelated to contact lens considerations.

•Record for future reference any other abnormality or unusual feature.

Most parts of the examination are carried out using the slit lamp (see Section 2.1.3), looking for any evidence of the following conditions.

Cornea and limbus

•Vascularization or neovascularization.

•Staining.

•Desiccation.

•Infiltrates or other signs of previous infection.

•Scarring or opacification.

•Other signs of injury.

•Central thinning.

•Peripheral thinning (dellen).

•Pterygium.

Bulbar conjunctiva

•Injection.

•Prominent surface vessels.

•Desiccation.

•Pinguecula.

•Naevus.

•Other irregularity.

Lids

•Position and size of palpebral aperture and any tendency towards ptosis.

•Lid tension.

•Irregularity of lid margins.

•Styes and cysts.

•Blepharitis.

•General condition of skin.

•Vesicles on lid margins.

•Patency of meibomian glands.

•Vernal conjunctivitis.

•General condition of the papillary conjunctiva and tarsal plate.

•Contact lens-induced papillary conjunctivitis (CLIPC) in previous lens wearers.

•Concretions.

•Make-up on palpebral conjunctiva.

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Blinking

•Frequency.

•Completeness.

PRACTICAL ADVICE

•Lid eversion to examine the tarsal plate and papillary conjunctiva is an essential preliminary.

•Most patients find it rather unpleasant so a gentle warning is advisable.

•Eversion also gives clues to sensitivity and to how patients will react to having their eyes manipulated.

•Where eversion proves very difficult, it is often possible to examine sufficient of the papillary conjunctiva by instructing the patient to hold the head as far back as possible and look down. The upper lid is gently pulled away from the globe and the light from a pen torch directed towards the upper fornix.

Staining agents (see Section 4.2)

Fluorescein

Fluorescein staining in the horizontal or inferior sectors of the cornea is an indication of tear film dehydration or poor blinking. Staining of the conjunctiva can also be seen at the nasal and temporal positions and in the region of any elevated tissue such as pingueculae.

Under normal conditions, fluorescein blends immediately with the tear film. On fluorescein instillation, the patient should initially be encouraged not to blink. If there is poor mixing, the tear prism and any conjunctival staining may both present a dull orange hue, indicating tear film dysfunction of the lipid layer. This may be either temporary or habitual and, with time, the colour changes to the more usual bright fluorescein green. With blinking, the dye will eventually mix normally with the tears when the lipid layer has been penetrated. Often there is a marked difference between the eyes.

Rose bengal

Rose bengal can be used to assess the severity of corneal or conjunctival disruption and help decide on the future management of the patient. Any significant uptake of the dye prior to fitting suggests that it is unwise to proceed with contact lenses. The dye is viewed with white light and in the examination routine it follows fluorescein observation (see Section 4.2).

Lissamine green

Lissamine green is the preferred alternative to rose bengal as it causes much less stinging. It takes time for the tissues to take up the stain so observation should

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