Ординатура / Офтальмология / Английские материалы / Medical Contact Lens Practice_Millis_2005

Videokeratoscopes have software programs that create stylish graphics and allow rapid data analysis. The limitations of Placido’s disc to measure an aspheric surface still apply and certain assumptions are made in the calculations. There may be mechanical problems in capturing the image and although the area of cornea measured is greater than in keratometry it is still limited to 8–9 mm in diameter.

Quantitative results are not perfect with current instruments, but they do provide additional helpful information.

The cornea reflects the concentric black and white mires and the virtual image behind the cornea is detected by the video cameras. The patient fixes on an illuminated target, but in current systems the head may need to be turned slightly to achieve correct alignment. Failure to align with the line of sight can lead to errors in the calculations determining the topography.

Some instruments have an image subtraction and enhancement program to locate the pupil, which is critical for centration in cases of refractive surgery.

Clinical factors that affect the use of the instruments include the ability of the patient to maintain fixation and the need to blink frequently to maintain corneal wetting because the image is reflected from the tear film. This may make rapid changes of curvature difficult to detect and it may not be possible to obtain accurate maps in some postsurgical eyes. The presence of tear film debris, particularly make-up particles, can make capturing an image more difficult. Brow or lid obstruction may cause difficulties, and problems may arise from centering the image and focusing the instrument, which require the subjective judgement of the examiner with some instruments. Poor alignment can cause pattern distortions that may mimic keratoconus.6

Szczotka and Thomas7 found axial and instantaneous (tangential) maps differ significantly in apical position and apex curvature in keratoconus. Although the tangential radius may represent corneal shape better, they found that axial curvatures are better to predict base curves for rigid gas-permeable (RGP) lenses in these cases.

Projection-based systems In projection-based systems an image is projected onto the tear film and measurements are made in terms of elevation

above a reference plane; the contours on the ma follow lines of equal height rather than slope. Th image needs to be intensified to be visible. Fluor escein has been used for this, but may interfer with the tear film.

Projection-based systems do, however, have number of advantages. They can record result from irregular and nonreflecting surfaces from the total corneal area, and are as accurate at th periphery as at the center of the cornea. They hav only recently entered into routine clinical us because they require costly computers to rapidl analyse the large amounts of data generated.

Rasterstereography projects a lighted grid ont the cornea that is rendered opaque with sodium fluorescein and viewed obliquely from a know angle. It makes direct point-to-point measuremen of the surface elevation using a stereotriangulatio technique. The advantages of the grid are that:

●it covers the entire cornea and will extend ont the sclera

●it does not need an intact epithelium nor pre cise spatial alignment for accurate imaging

●it is less affected by irregular corneas.

The distortion of the projected grid is converte into true elevation measurements from which cur vature values can be calculated. Pooling of fluo rescein may result in an artificial distortion of th image.

More recently the same principle has been use with a scanning slit beam in which 40 slit section are captured across the cornea.8 This also provide information about the posterior corneal shape an corneal thickness.

The KM-1000 CLAS Corneal Topography Uni (Keratometrics Inc.) uses laser holography t image the corneal surface and fringe detection t assess topographic change.

Color maps

Most current VKS instruments present data a color-coded maps where red represents steepnes and blue flatness. Each color is assigned a dioptric step value, and the larger the step the greater th range of corneal power display, but the less th detail. The pattern displayed therefore depend on the range of the scale used and the step siz

and this should be selected according to the task and be noted carefully when interpreting maps (Fig. 1.3). With a 1-D scale each color represents a 1 D range and so an apparently spherical map may be masking 0.75 D of astigmatism. Sensitivity should be decreased to screen for gross pathology because a map with small increments may detect apparent abnormalities that are not clinically significant.

Many systems also provide an absolute scale that remains constant and displays the entire dioptric range, which the machine measures without reference to the cornea under study (Fig. 1.4). The

same color always represents a specific dioptric interval, which helps to avoid errors of interpretation, if notice has not been taken of the color scale. Reviewing records in the absolute scale makes it simple to monitor changes.

Patterns of normal central and paracentral topography have been defined as round, oval, symmetric bowtie, asymmetric bowtie and irregular9 (Figs 1.5 and 1.6). These patterns are based on power changes across the corneal surface, a “round” indicating flattening in isodioptric steps, but it is the underlying corneal contour that dictates the dioptric pattern.

Maps may be sagittal (axial or global) or tangential (instantaneous or local):

●sagittal maps are highly dependent on fixation and the results obtained vary depending on fixation and corneal asymmetry

●tangential maps are not so axis dependent.

For a particular cornea the sagittal (normalized) and tangential maps may be very similar centrally, but differ considerably in the periphery. It was found that the tangential radius of curvature can provide more accurate shape analysis for the peripheral corneal powers and is best in cases of corneal irregularity due to contact lens wear or diseases such as keratoconus.

The sagittal map is best for corneal power because the sagittal radius is closely related to optical power.10

The orientation of the map in relation to the limbus and the pupil may be confirmed by superimposing the map on an image of the eye. This may be important in checking alignment in cases of strabismus. Errors in fixation or alignment may result in a difference in the relative steepness of the inferior and superior cornea and produce a keratoconus-like pattern.7 Accurate alignment is important. Incomplete mapping inferiorly may be due to an abnormal tear film meniscus and could result in an asymmetric pattern.

FITTING CONTACT LENSES WITH VIDEOKERATOSCOPY

Most corneal topography instruments now have a contact lens fitting program included in the software. These use the information acquired to suggest an initial fit for a contact lens. The program includes simulated fluorescein patterns and, in many instruments, the tear film profile can be displayed and the effect of parameter changes on these may be shown (Fig. 1.7). Although VKS does not result in more successful fits than keratometry it is easier to select the appropriate trial lens and clinical time is saved. A database of lens design information for many brands of lenses can be added to the software, and can also be calculated.

In the author’s opinion the programs have a limited use, particularly in cases of severe keratoconus and in fitting soft lenses, but are helpful in identifying early keratoconus, corneal warpage due to lens wear, and irregular corneas due to other causes that give rise to fitting problems. They will identify decentered ablation zones following excimer laser, and raised graft–host junctions. Distortion maps may show areas of corneal irregularity within the pupil that affects visual acuity.

Caroline and colleagues11 have described a method of fitting rigid contact lenses using the corneal contour map (Table 1.3). They identified

Table 1.3 Using the Caroline technique for fitting rigid lenses using topography

Aim for the widest area of alignment along the horizontal meridian

Maximal bearing should occur 3–4 mm from the center temporally

Aim for tear-film thickness 10–15 m beneath lens

Fit alignment or flatter superiorly to achieve free vertical movement

Obtain clearance between the posterior and midperipheral cornea below

two fundamental steps that are essential to any topographic fitting, notably:

●a bearing area along the horizontal meridian approximately 3–4 cm from the center

●the lens should have free unobstructed movement in the vertical meridian.

Caroline and colleagues noted that a hat is not fitted to a head based on the measurement of the radius of curvature of the top of the head, but on the circumference where the hat will bear on the head. Similarly the contact lens will bear maximally in the midperiphery of the cornea. The location of the bearing area will be determined by the BOZR, peripheral optic zone diameter (POZD) or by the degree of asphericity in aspheric lenses. The peripheral curves are similar to the brim of the hat. They have no function in the alignment fitting of a lens, but serve only to clear the flatter peripheral cornea, and different “brim” widths create more or less interaction between the lid and the lens surface, and so alter the lens position and dynamics.

For a contact lens the maximal area of bearing occurs midperipherally in the horizontal meridian, while maximal clearance occurs along the steepest meridian.

The fitting should aim for the widest area of alignment along the horizontal meridian beneath which is a tear film thickness of 10–15 m. Maximal bearing should be 3–4 mm from the center. To achieve free movement in the vertical meridian it is necessary to have alignment or flatter fit superiorly and clearance between the posterior lens and the midperipheral cornea below.

Horizontal bearing holds the lens in position on an eye with with-the-rule astigmatism, when the cornea is steeper in the vertical than the horizontal meridian, and prevents temporal or nasal decentration (Fig. 1.8).

In cases of against-the-rule astigmatism the midperipheral bearing occurs in the vertical meridian and the lens will follow the path of least resistance and decenter nasally or temporally unless a back toric lens is fitted (Fig. 1.9).

Free vertical movement is often easy to achieve in with-the-rule astigmatism but small amounts of

such astigmatism may cause problems becaus the cornea may flatten rapidly and the lens bear on the cornea above the small astigmatic are unless the BOZR is flattened. Caroline and col leagues11 believe that this corneal flattening ma be due to lid pressure.

Inferior lens clearance occurs as the result o lens movement and corneal shape. As the len moves upward with the blink the superior len tilts towards the eye and the inferior lens is lifte away from the cornea. As the lens moves dow inferior tilting causes an alignment fit. Addition ally the normal inferior cornea is often steeper tha the superior cornea and lens clearance occurs Problems may arise in eyes with an inferior corne that is flatter than the superior cornea, when it i necessary to flatten the BOZR, but only if horizon tal bearing is maintained.

The color map has a grid of 1 mm squares super imposed so that readings of the radius of curvatur

or dioptric power 3.5 mm from the center can be obtained. Readings are taken at the same distance superiorly, inferiorly and temporally. The initial trial lens is selected to fit the temporal keratometry reading, providing that vertical clearance is possible.

The computer program also calculates eccentricity values,10 details of which may be sent to the laboratory or used in designing RGP lenses.

Most videokeratoscopes have a simulated fluorescein program and the chosen lens may be viewed against this.12 Lens parameters may be altered and the fluorescein fit recalculated until a satisfactory fit is obtained. A tear film profile, which illustrates the amount of tears beneath the lens and shows the areas of bearing is visible simultaneously so that the effect of lens alterations on the tear film may be seen.

Programs are available for rigid spherical and toric lenses, and for soft lenses, but are of greatest value in rigid lens fitting.

PHOTOGRAPHY OF THE ANTERIOR EYE

Single lens reflex camera

Good record keeping is an essential part of every practice and one of the best ways of documenting the eye is by photography. The simplest way of recording the external eye is by a single-lens reflex (SLR) camera and a macro lens, with an extension ring between the lens and camera to achieve lifesize magnification. An electronic flash, mounted on the lens, will prevent blur from movement of the subject or the camera.

Polaroid® camera

Polaroid® has designed a camera that is focused by two converging light beams and is very useful for prosthetic contact lens design. The result, in the form of a print, is immediately available, so inadequate definition or a poor color match can be identified while the patient is present, and another picture can be taken. The photograph can then be sent to the artist for handpainted prosthetic lenses. The camera can also be used for external eye lesions of the lids or conjunctiva or for pupil anomalies. The quality of picture obtained is high

enough for transparencies to be made from them for slides.

Clinical slit lamp

General external views may also be taken using a 35 mm SLR camera, or a digital camera (e.g. Nikon Coolpics) with an adaptor that replaces one ocular of a clinical slit lamp. The focus on the slit-lamp eyepiece should be rotated as far as possible to provide the correct optical spacing for the adaptor. The mirror of the slit lamp should be turned to the reverse side to provide diffuse light. The camera flash should be turned off and the manual focus set to infinity. Accurate positioning and focusing are then achieved using the slit-lamp joystick.

Photo slit-lamp biomicrography

The only way to obtain photographs of a slit-lamp view of the eye is with a specialized photo slitlamp biomicroscope. These instruments incorporate a beam splitter to provide a coaxial view, which is shared by the photographer and the film. More than 50% of the light is diverted to the camera by the beam splitter making it more difficult for the examiner, but produces better photographs. If the instrument is used for both clinical and photographic work there must be some compromise in the distribution of the illumination.

An electronic flash provides sufficient exposure so that, even at high magnifications, a short shutter speed can be used to minimize the blur induced by eye movement. The flash system must be coaxial with the light from the slit-beam illuminator to obtain the same effect on film that is seen by the observer.

Exposure settings are based on the subject of the photograph, the magnification used, and the type of illumination (slit, background or diffusion), and a guide to the settings is normally supplied with the instrument. A fill light is necessary to illuminate specific areas of interest against a general background.

Focus

Exact focusing is essential. The slit lamp forms a suspended, aerial image and the correct focus

may therefore be inadvertently altered by accommodation, resulting in blurred photographs. In photo slit lamps, the ocular that shares the image with the camera, usually the right ocular, contains a cross-hair reticle. Before using the instrument the ocular must be adjusted to the individual observer’s refraction. To do this the eye piece is rotated maximally in a plus direction then, with the examiner looking through the ocular and with accommodation relaxed, the eyepiece is rotated to the minus side until the cross hair comes into sharp focus at or near the observer’s refractive error.

Film

The film most commonly used is 35 mm 200 or 400 American Standards Association (ASA) daylight film. “Fast” film needs less light, but does not define detail as well as “slow”, lower ASA, film.

Taking a photograph

The photographer needs to be aware that the circular field seen through the slit lamp will be reduced to a rectangle on the film, so to aid centration and the view of the recorded field some instruments incorporate a rectangular reticle along with the cross hair.

In normal use of the slit lamp constant adjustments are made to the size and position of the slit, the magnification and the illumination. This creates a composite mental picture of the eye, but this is not possible with a still photograph, and care must be taken to adjust these factors so that the resulting picture displays all the relevant information.

Because the view through the microscope differs from the relatively large area captured, the picture is often smaller than expected. Some instruments overcome this disparity by using a 2 optical magnifier, in others the magnification is increased by one setting, or on digital cameras the telephoto setting can be used.

On acquiring a photo slit lamp, time should be spent taking photographs of the ocular structures using different values for illumination, magnification and exposure, and a record kept of these values for each frame taken. This personal record can then be reviewed to ascertain the best values for particular views.

Finally, when taking clinical photographs frame of the patient’s name or number and the dat should be taken to identify images after processing

Photo slit-lamp biomicrography results in goo quality images, but storage may pose a problem and the quality of the image may deteriorate wit time. Digital images are much easier to store.

Image capture

The latest method of recording images is digita imaging. Digital images are formed from pictur elements or pixels, each of which has a color valu selected from a range of colors. The images ar usually viewed on the monitor of a computer, th configuration of which determines the final qual ity of the image and depends on the number o pixels. Each pixel is described in zeros and one and so may be stored on hard drives, floppies o CDs or may be transmitted by telephone line.13

Digital images are obtained using a photo sli lamp with a beam splitter attached to a camera o a video camera. The analog image from a vide camera is in waveform and digitized in th computer by an image capture board, which con verts the signal into a digital signal. The size o the image, and the speed of the incoming signa together with the speed of the computer and th amount of RAM controls the number of frame that can be captured.

Most cameras use a single chip for the red, blu and green components, but newer cameras hav a separate chip for each color, which greatl improves the resolution, but is more expensive More recently, instruments that work well at low light levels have eliminated the need for flash which is more comfortable for the patient, particu larly if a series of pictures is to be taken.

The sharpness and brightness of capture images may be increased or decreased, unwante elements can be removed from the picture and th image can be cropped to emphasize a point. As th image can be viewed immediately after it has bee taken, only satisfactory images need to be saved This reduces the number of pictures taken whe compared to slit-lamp photography and results i less wastage.

Image storage is a problem because image take up a considerable amount of space and th

better the resolution the more space they occupy. They should be stored either on removable optical drives or on writable CD-ROMs, which require a special CD-ROM drive.14

Images can be viewed in real time and can be shown to the patient to demonstrate a condition or illustrate a point.

Transmitting images over telephone lines enables “consultations” to take place between clinicians, but the matter of patient confidentiality must be given serious consideration.

TEACHING APPOINTMENT

Teaching may occur at the initial or a subsequent visit, depending on the availability of the prescribed lenses. Patients must be taught to insert and remove the lenses. They should not be allowed to take lenses away until they are confident that they can safely remove the lens. In a few instances, such as when small children need lenses for medical reasons, an adult may be taught to insert and remove the lens. A starter pack of solutions and a lens case should be provided and their use demonstrated and discussed. Written instructions should be provided. Patients should be asked to contact the practice between visits for advice if any problems arise and advised that if difficulties arise further teaching can be arranged.

FOLLOW-UP EXAMINATION

Follow-up examination should take place 1–2 weeks after commencing lens wear for both soft and RGP lenses. Further examinations should take place after 3 and 6 months and then at least annually (Table 1.4). All patients should be told to attend sooner if problems arise. If lenses are being worn on an extended-wear basis the patient should be reviewed after 24 hours, after 1 week and then at least every 3 months. It is preferable that all extended-wear lens wearers remove and clean their lenses weekly and leave the lens out overnight, reinserting the cleaned lens next day, but this is not always possible for those wearing therapeutic bandage lenses (see Ch. 7).

A follow-up examination should elicit recent history, including both medical episodes and

Table 1.4 Follow-up examination

Recent history

Medical and contact lens Wearing time

Vision Comfort Care regimen Problems

Ocular examination, including slit lamp Examination of lenses

Annual keratometry and topography

Full ophthalmic examination including intraocular pressure measurement annually

contact lens history. The wearing time, comfort, vision and any problems should be discussed. Patients should be asked to describe how they deal with the lenses on removal from the eye. Visual acuity is measured with the contact lenses and an over-refraction performed to ensure that the best vision is obtained.

Slit-lamp examination includes:

●observation of any localized or generalized hyperemia

●examination of the conjunctiva, including eversion of the upper lid

●examination of the quality and quantity of the tear film

●examination of the state of the cornea, including any staining, irregularity, scarring, microcysts and the fit of the lens.

The lens should be assessed for any defects or deposits, which are less common since the advent of disposable and frequently replaced lenses, but for those who do not conform to the parameters for such lenses they may still be a problem. Lenses should be examined both on and off the eye. Deposits are often more readily seen by the naked eye in a good light.

The care regimen should be discussed because the patient may have changed solutions between visits for a variety of reasons.

Keratometry should be performed annually to ensure that there is no corneal warpage, and earlier if symptoms suggest warpage as a possible diagnosis.