Ординатура / Офтальмология / Английские материалы / Eye Essentials Rigid Gas-Permeable Lens Fitting_Franklin_2006

1.The nipple form has a small central or paracentral ectasia, up to 5 mm in diameter, surrounded by normal cornea. There is sometimes a smaller elevated nodule at the apex. This nodule is rich in fibroblasts and if abraded by lens with heavy apical touch, could result in scarring.

2.The oval form is the most common, in which the corneal apex is displaced inferiorly. This inferior steepening may be associated with a corresponding area above the midline which has normal or even flatter than normal curvature. Some contact lens types feature offset peripheral curves that are designed to provide more clearance over the superior cornea. Oval cones tend to have more breaks in Bowman's membrane, more superior pannus formation and more ruptures of Oescemet's membrane than nipple cones.

3.The globus form involves up to three-quarters of the corneal surface and the mid-peripheral area of normal cornea that surrounds the cone in the other forms is absent.

The best way to find out which form we are dealing with, and indeed to confirm the initial diagnosis, is topographical mapping. If this is not available in-house it is worth seeking elsewhere before attempting fitting contact lenses as it may save a lot of time in the long run. Hospitals often provide such data if they send patients out for fitting in general practice, and most places will have a topographer available locally. In the absence of topographical data the severity of the condition may be indicated by keratometry, and some idea of the type of cone present may be indicated by the use of a Placido disk or similar device:

1.Mild keratoconus will have a corneal curvature that will read up to 480 on the power scale.

2.Moderate stages will read between 48 and 540.

3.Severe stages will read over 540.

Keratoconus

110 Management of keratoconus

The management of keratoconus depends on its severity:

1.Spectacle correction may give a surprisingly good level of vision in early cases, and the wise practitioner is not in too much of a hurry to get the patient into contact lenses. Keratoconic fitting is tricky for a number of reasons and those patients who obtain a marked improvement in vision with contact lenses are likely to be better motivated to cope with what can be a prolonged and frustrating fitting process.

2.Soft toric lenses are rarely fitted, but in early cases may provide adequate vision. Both this option and spectacle correction avoid the increased risk of corneal scarring that RGP lenses may bring.

3.RGP lenses are the most common management strategy for more advance keratoconus. The tear film behind a rigid lens is capable of correcting some 90% of corneal astigmatism, be it regular or irregular.

4.Scleral lenses used to be the first choice for keratoconus but are now generally only used when RGP lenses cannot provide adequate correction.

5."Piggyback" lenses consist of a rigid lens fitted on top of a soft carrier lens. They may be useful on very irregular corneas where conventional lenses fail to center, but the increased combined lens thickness does not help oxygen transmission.

6.Hybrid lenses such as the Softperm (elBA) consist of an RGP center surrounded by a soft lens skirt, which can aid centration in some cases.

7.Corneal surgery may be required by 10-20% of patients eventually.

(a)Epikeratoplasty is a process whereby a lenticule of donor tissue is added to the cornea. The objective is to thicken and flatten the central cornea and reduce astigmatism. This procedure is used on those patients who

(b)Lamellar keratoplasty retains the host endothelium, and graft rejection is reduced. It is a more demanding surgical procedure than penetrating keratoplasty and the optical results are often worse.

(c)Penetrating keratoplasty is undertaken when contact lens options are exhausted, particularly in patients who have significant corneal scarring. Results are usually excellent, as the peripheral cornea is essentially normal, and rejection is rare.

RGP lenses for keratoconus

If we were able to construct an ideal RGP patient, it is unlikely that he or she would bear much resemblance to the average keratoconic. Leaving aside the exotic corneal topograhy, the patient has a good chance of being atopic. Apart from the likelihood of allergic conjunctivitis for at least part of the year, the chances of solution sensitivity and of contact lens-related papillary conjunctivitis (CLPC) are higher. Then there are the personality traits. Anxiety, psychosomatic illness and a tendency to be somewhat "high maintenance" are all relatively common, as they are in other chronic eye diseases. On the credit side, keratoconic patients are often highly motivated, as contact lenses may be their only route to satisfactory vision. To summarize, we may be faced with a patient with a horrible cornea and multiple allergies who is very anxious to succeed, in every sense. The fitting process may be prolonged and involve a certain amount of improvisation on the part of the practitioner so it is important to manage expectations from the outset. This will reassure the patient when things appear to degenerate to trial and error, and may also be beneficial to the long-term mental health and blood pressure of the practitioner.

Keratoconus

The fitting of patients with early keratoconus is likely to be

112similar to that for any astigmatic patient. but once the condition advances the corneal topography will require more specialized lenses. At one time, lenses were fitted with central bearing. in the hope of controlling disease progression. However. the effectiveness of this approach is controversial and there is now evidence (though at present not a huge amount) that this type of fit may initiate or exacerbate corneal scarring. so the desired goal now is to eliminate central bearing by fitting lenses with apical clearance. This is more easily arranged in early keratoconus and in many cases it is impossible to achieve. Alternatively, mid-peripheral bearing may relieve some of the pressure on the apex of the cone. This divided support. or "three-point touch," is characterized by some central bearing

with two mid-peripheral points. roughly 180 0 from each other, also bearing some of the weight. Mid-peripheral bearing may be impractical with the globus form of cone, and here large flat lenses may be the only option.

There are many designs available, though finding them in the ACLM manual is less easy than it might be. They are listed with conventional lenses. though the usual clue is the letter "K" in the title.

Aspheric lenses (e.g. Persecon E Keratoconus, CIBA) may work in early cases, and because peripheral plus power comes built in, early presbyopes may find these lenses useful. However, if they fail to center well the vision will be poor and optics based on spherical curves are more reliable. There are various designs available. of which the most popular in the UK are as follow.

The Soper Cone system uses bicurve lenses and the fitting philosophy is based on sagittal depth, using a combination of BOZRs and BOZOs. The fitting set consists of 10 lenses subdivided into "mild" (7.40 mm TO. 6.00 mm BOZO). "moderate" (8.50 mm TO, 7.00 mm BOZO) and "advanced" (9.50 mm TO,

8.00 mm BOZO) subsets. The aim is to avoid apical touch. but the bicurve construction may provide inadequate peripheral clearance which may cause the lens to seal off tear exchange.

Fitting protocol

There is a terrible old joke concerning a traveler lost in rural Ireland who encounters a farmer leaning on a gate. "Excuse me, my man, but how do I get to Dublin?" enquires the traveler. The farmer, after a long pause for deep cogitation, replies: "Well now, sir, I wouldn't start from here if I was you."

Before commencing the fitting of a keratoconic patient the more accurate information you have to hand, the quicker you are likely to arrive at a satisfactory outcome.

A current and accurate spectacle Rx is essential and by far the best way to ensure this is to do it yourself, even if the last refraction was recent. Keratoconics are difficult to refract accurately and somewhat variable. While it is always nice to have

the fit may change.

The lens may need to be centered manually, using the lower lid, in order to correctly interpret the fit. Air bubbles may collect in the cone area. It is sometimes possible to reduce this by choosing a smaller BOZD. However, to retain sagittal depth the BOZR needs to be proportionally steeper.

Keratoconus

(c)Residual astigmatism may be present. It is always tempting to try to correct this with a bitoric design but on keratoconic patients this rarely works.

Supplementary spectacles may be a better option visually.

Lens fitting after keratoplasty

At one time fitting was delayed until about a year after surgery, when all sutures had been removed. These days some at least of the sutures are left in indefinitely and only removed if complications occur. Lenses are often fitted 3-6 months after surgery. Occasionally, soft lenses may give a satisfactory result, but in most cases the irregular astigmatism induced requires some sort of rigid lens correction, which may take several forms:

1.Conventional RGP lens designs are sometimes successful.

2.Many post-keratoplasty corneas have a flattened central area requiring the use of modified or reverse geometry lenses similar to those used for orthokeratology. Topographical mapping of the cornea is essential to fit these lenses.

3."Piggyback" fitting of an RGP lens on a soft lens, both lenses often specially modified, may help where lenses are reluctant to center.

4.Softperm lenses have a rigid optic zone with a hydrogel "skirt" and these have the same uses as piggyback designs.

5.Scleral lenses, by fitting the sclera and vaulting the cornea, make corneal geometry largely irrelevant. However, they require considerable skill and time to fit well and most scleral fitting is now done in a hospital or specialized practice setting.

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Better sight without glasses: extended wear orthokeratology and

fitting after refractive surgery