Ординатура / Офтальмология / Английские материалы / Contact Lenses in Ophthalmic Practice_Mannis, Zadnik. Coral-Ghanem, Kara-Jose_2003

60P. Bergenske and S. Moreira

7.What is the relationship between base curve and diameter of the contact lens?

Changes in contact lens diameter may alter the choice of radius of the base curve. Increasing the diameter without altering the base curve increases the sagittal depth, effectively steepening the fit. Reducing the diameter of the contact lens without changing the radius of curvature alters the sagittal depth so that the contact lens fit is flatter. To maintain the same sagittal depth, the radius of curvature must be increased (made flatter) as the diameter is increased. For every modification of 0.2 mm in diameter, the radius of the base curve is altered by 0.125 D or 0.023 mm.

8.How does one calculate the power of rigid contact lenses?

The prescription of rigid contact lens power can be made after refraction and keratometry alone, although the ideal method is to make this calculation after overrefraction of a known and well-fitting trial lens. In prescribing the power, one needs to take into consideration the ‘‘tear lens’’ formed between the cornea and the posterior surface of the rigid contact lens. The power of the ‘‘tear,’’ or lacrimal, lens is the difference between the base curve of the contact lens (in diopters ‘‘K’’) and the keratometric reading of the flatter meridian of the cornea. When the refraction is greater than 4 D, the vertex distance must also be taken into consideration.

Power Determination

It is possible to predict the expected power of the final lens based on the refraction in minus cylinder form and on the keratometric readings. The cylindrical component of the manifest refraction is optically eliminated by the lacrimal lens. The spherical component represents the dioptric power of the flatter meridian of the cornea. When the lens is fit with the base curve ‘‘on K,’’ the lacrimal lens power is plano and the power of the lens should be the same as the vertex-corrected spherical component of the refraction in minus cylinder form. When the contact lens is fitted with a base curve steeper than K, a lacrimal lens with a positive dioptric power is formed. To compensate for this added plus power, an equal amount of minus should be added to the spherical power of the spectacle refraction when converting to a contact lens power.

Example

Base curve steeper than the flatter K with a minus refraction.

7. How to Fit Rigid Spherical Contact Lenses 63

11.How does one proceed with evaluation of the trial lens?

A trial contact lens is selected based on the parameters closest to the theoretical calculations of diameter, base curve, and power as described in the preceding sections. Once the patient has adapted to the presence of the lens and there is no further tearing, the fitting can proceed. For patients new to rigid lenses, it may be helpful to first instill a drop of topical anesthetic. The lens should be reasonably comfortable and have appropriate movement to determine the dioptric power adequately. A spherical overrefraction is performed. Fluorescein dye is placed on the eye. At the slit lamp, one should observe:

1.The contact lens position, its mobility during blinking, its location in relationship to the corneal apex, and the fluorescein pattern, both static and dynamic.

2.With rigid contact lenses on K or slightly steeper, the tear film should be uniform with light central pooling and mid-peripheral touch along the horizontal meridian.

3.When there is with-the-rule astigmatism, the tear film should demonstrate fluorescein in the central optical zone with a decrease in the intermediate zone and an increase in the periphery along the horizontal meridian, and greater pooling of fluorescein along the steeper, vertical meridian.

4.In a tight or very steep contact lens, there is concentration of fluorescein in the tear film in the central zone (apical clearance) and absence or minimal fluorescein accumulation in the periphery. A lens that is too steep will not move properly along the vertical meridian.

5.In a loose or flat contact lens, there is an absence of fluorescein pooling at the apex (apical touch) and excessive pooling of fluorescein in the intermediate and peripheral zones.

12.How does one accomplish the final fitting of a trial rigid contact lens?

After placement of the rigid gas permeable contact lens and the cessation of tearing, one evaluates with fluorescein the position of the lens and the lens–cornea relationship.

Position

The position of the contact lens is considered ideal when the border of the contact lens is at or near the superior limbus and is secured by the upper lid throughout the blinking cycle. The most desirable rigid gas permeable contact lens fit is slightly superior or central without excessive movement induced by the lid.

64 P. Bergenske and S. Moreira

Movement

The contact lens should have unrestricted veritcal movement with the blink. When movement is minimal, the contact lens interferes with the lacrimal exchange. When the movement is excessive, it may produce discomfort and interfere with vision.

Visual Quality

The vision should be sharp and stable. With overrefraction, one can arrive at the final refraction. The power of the overrefraction is added to the power of the diagnostic lens. One needs to compensate for the vertex distance only if the overrefraction is greater than 4.00 D.

Evaluation of the Fluorescein Pattern

For rigid contact lenses, the ideal fluorescein pattern should demonstrate a uniform lacrimal film or at least slight central clearance with touch in the mid-periphery of the horizontal meridian. Because many polymers contain an ultraviolet filter it is helpful use a yellow (Wratten or Tiffen) filter designed to enhance viewing of fluorescein patterns.

13.What is the suggested trial set for rigid gas permeable lenses?

It is useful to have a variety of trial contact lenses for the correction of myopia, hyperopia, aphakia, and keratoconus. To achieve the greatest success in fitting, it is helpful to have available contact lens trial sets of different diameters, for example, diameters of 9.2, 9.6, and 9.8 mm. There are many commercially available trial lens sets. It is of greatest value to have trial lenses manufactured by the same laboratory that will be providing the prescribed lenses. Tables 7.3, 7.4, 7.5, and 7.6 are examples of trial sets. Significant confusion and error can occur if trial sets are not carefully maintained. The fitter should routinely verify that trial lenses used in prescribing are correctly labeled.

14. Is trial fitting necessary?

Trial fitting can improve chances of ‘‘first fit’’ success; however, it is time-consuming and dependent on the availability of trial lenses close to correct parameters. In cases of warped or irregular corneas, where keratometry and refraction data are spurious, trial lens fitting may be the only means of determining a lens prescription. For eyes with regular astigmatism and stable refractions, empirical ordering of lenses can be successful, particularly if topography is employed to determine overall corneal shape. An advantage of empirical fitting is that the first lens the patient experiences is one that should fit well and have proper

66 P. Bergenske and S. Moreira

Table 7.4. Trial set for spherical rigid gas permeable contact lenses for myopia with a diameter of 9.2 mm

Table 7.5. Trial contact lens set for spherical rigid gas permeable contact lenses for myopia with a diameter of 9.6 mm

7. How to Fit Rigid Spherical Contact Lenses 67

Table 7.6. Trial contact lens set for spherical rigid gas permeable contact lenses for hyperopia

optical power. Potential confusion caused by mislabeled trial lenses is avoided. In addition, significant chair time can be saved using the empirical method.

Selected Readings

Campbell RC, Connelly S. Rigid gas-permeable contact lens fitting. In: Kast PR, ed. The CLAO Guide to Basic Science and Clinical Practice. IA: Kendall/Hunt, 1995:11–49.

Fletcher LJ, Lupelli L, Rossi AL. Contact Lens Practice. Oxford: Blackwell Scientific, 1994:1772.

Girard LJ, Soper JW, Sampson WG. Follow up visits. In: Girard LJ, ed. Corneal Contact Lenses. St. Louis: CV Mosby, 1970:229–268.

Gordon JM, Hanish SJ. Fitting techniques for gas-permeable rigid lenses. In: Aquavella JV Rao GN, ed. Contact Lenses. Philadelphia: JB Lippincott, 1987: 39–46.

Hales RH. Contact Lenses, a Clinical Approach to Fitting. Baltimore: Williams & Wilkins, 1982:351.

Stein HA, Freeman MI, Stein RM. The CLAO Residents’ Contact Lens Curriculum Manual. Kellner/McCaffery Associates, 15 Fifth Avenue, New York: 1996: 72–86.

Stein HA, Slatt BJ. Fitting Guide for Rigid and Soft Contact Lenses, 3 rd ed. St. Louis: CV Mosby, 1990.

8

Follow-Up After Fitting

Rigid Gas Permeable Lenses

Cleusa Coral-Ghanem

and Lisa Badowski

The most important factor for successful long-term contact lens use is careful attention to the patient after the initial contact lens fit. The eye care professional must clearly indicate to the patient what symptoms might dictate the need for return visits that may result in modification or changing of the lens. By the same token, follow-up is important to verify lens hygiene, to look for toxic or allergic reactions to chemical products in lens care solutions, and to examine the cornea for physiologic changes that may decrease a patient’s comfort or wearing time.

1.How often should follow-up visits for rigid gas permeable lens patients be scheduled?

Follow-up of the contact lens patient actually begins with the contact lens dispensing visit. The dispensing visit is usually scheduled about 1 or 2 weeks after the initial examination/fitting to allow sufficient time for the finishing laboratory to fabricate the ordered contact lens. Following contact lens dispensing, a short-term visit at about 1 to 2 weeks should be scheduled. At this visit, the patient’s adaptation to the new lenses and compliance with instructions are monitored. Changes to lens fit or power may also be made at this time, if indicated. If any changes are made to the contact lens fit or power, then another short-term dis- pensing/follow-up visit should be scheduled. A medium-term visit should then be scheduled 1 to 3 months later. At this point, the patient should be fairly well adapted to the lenses and should be monitored for any physiologic changes secondary to the contact lens wear. After the patient is deemed a successful contact lens wearer, the patient should then be seen for long-term follow-up visits at intervals of 6 to 12 months.

8. Follow-Up After Fitting Lenses 69

2.What clinical tests should be done at a follow-up visit?

There are three primary goals at dispensing/follow-up visits:

1.Evaluating contact lens performance

2.Evaluating ocular physiology

3.Educating the patient

First, one should establish that the newly dispensed lens performs in the same manner as predicted by the diagnostic lens at the fitting visit. In subsequent follow-up visits, the lens should continue to perform in the same manner and meet the patient’s visual needs. Appropriate tasks to achieve this goal include (but are not limited to) taking a case history, testing visual acuity with the contact lens at the appropriate distances to establish the patient’s functional performance, testing the overrefraction to confirm the contact lens power, doing a slit-lamp evaluation of the lens fit with fluorescein, and evaluating the contact lens surface. The case history should include questions about the patient’s subjective evaluation of vision and comfort, average wearing time of the contact lenses, and lens care routine.

The ocular physiology evaluation includes a detailed anterior segment slit-lamp biomicroscopic examination that includes fluorescein staining of the cornea and evaluation of the tarsal conjunctiva with lid eversion. A yellow Kodak Wratten No. 12 or yellow No. 2 camera filter over the objective of the slit-lamp biomicroscope enhances the fluorescence of the fluorescein stain when evaluating rigid gas permeable lens fits or corneal staining. Corneal topography, keratometry, or refraction may be reevaluated if corneal distortion is suspected.

Patient education is an ongoing process. New patients need to understand the initial instructions that were provided. Misunderstandings can often be corrected at early follow-up visits. Established patients can become complacent and start to omit steps essential to proper lens care or otherwise misuse their lenses. The importance of maintaining the prescribed contact lens wearing schedule and following the prescribed lens care routine needs to be reinforced at every patient encounter to ensure patient compliance.

3.What are the common symptoms after rigid gas permeable contact lens fitting?

1.Sensation of ‘‘wet’’ eyes with increased tearing

2.Photophobia

3.Mild lid irritation

4.Incomplete blink (to decrease awareness from the edge of the lens against the lid)

5.Excessive blinking

6.Difficulty with upgaze

7.Intermittent visual blurring