Ординатура / Офтальмология / Английские материалы / The Art and the Science of Cataract Surgery_Boyd, Barraquer_2000

disabling glare, which determines the effect of extraneous light on visual performance. Contrast sensitivity evaluation is a measurement of the resolving power of the eye at varied contrasts between image and background (Fig. 3 A-B).

A number of useful contrast and glare sensitivity testing methods have been devised (Fig. 3 A-B). They are accessible and inexpensive. Unfortunately, standardization of these techniques has not yet been achieved. It is essential that the clinician be fully aware of these two factors that may impinge on the patient's real vision or quality of vision, in addition to the Snellen acuity test.

Relation of Glare to Type of Cataract

Neumann et al. have determined that nuclear cataract is more likely to be associated with nighttime glare disability, while cortical cataract formation is associated with daylightglare,andposteriorsubcapsularcataracts may induce glare disability associated with bright, direct sunlight or bright central light sources. Cortical cataracts seem more likely to cause glare symptoms than nuclear cataracts. Masket points out that frequently, patients with dense central posterior subcapsular cataracts frequently retain excellent distance Snellen acuity as measured in the refracting lane, yet they perform poorly on any of the available glare testing devices. Such patients

may have severely lower visual function during daylight driving although they do well with theSnellenacuitychart. Inessence,theSnellen chart evaluates quantity of vision. Contrast sensitivity tests evaluate quantity and quality of vision. The equipment to perform the test is accessible and inexpensive. It is basically a chart about 0.3 meters in size and it costs about US$200.00

Preoperative Considerations

In addition to determining visual acuity by the Snellen chart, contrast sensitivity and glare disability testing as outllined, all patients with cataracts should have a thorough history takenincludinganysystemicorocularmedications being used and any systemic disease for which they receive treatment. A family history is also included. The ophthalmologic examination should include intraocular pressure (IOP)measurements,keratometry,pupilexam, routine motility testing, and dilated slit-lamp and funduscopic examinations including indirect ophthalmoscopy to examine the central and peripheral retina. Ancillary testing such as visual fields, topography, specular microscopy for endothelial cell counts, and fluorescein angiography should be considered in selected cases. There are many causes for decreased vision and ,especially in older patients, these causes may exist concurrently. Age-related macular degeneration is possibly the most important and difficult to detect because of the existing opacity of the cataract.

Evaluation of Macular Function

Themainpreoperativetests todetermine central visual acuity are: 1) the Potential Visual Acuity Meter (PAM) and 2) the Super Pinhole. They permit evaluation of the macular function in patients in whom examination of the macula is difficult due to media opacities. They are more useful when they are integrated into the total evaluation of the patient.

One of the major problems that all of us confront as clinical ophthalmologists is that of patients with cataracts who correct to 20/100 or 20/200 and on whom we are planning to operate but cannot see the fundus, particularly the macula. This is aggravated when the patient has a few old small corneal opacities. The ever-present question is: what is the visual prognosis if we operate, either by a cataract extraction or combined with a corneal transplant? What can we anticipate for the patient or his/her family about future, postoperative vision even if we do not have any significant operative or postoperative complications? Ultrasonography and clinical tests will give us only a partial and limited answer.

Since we cannot see the state of the macula or papilla, we are limited as to the prognosis. Sometimes we have the pleasant surprise of obtaining more vision postoperativelythanwepredicted;inothercases,weface the unpleasant reality of finding macular degeneration or other lesions in the macula or optic nerve that result in poor central vision in spite of a beautifully performed operation.

Any well trained ophthalmologist can diagnose major lesions of the optic nerve or retina preoperatively. The major problem is with the subtle lesions that nevertheless limit the patient's capacity to read or distinguish clear images at distance postoperatively.

One of the most important tests for evaluating macular function in the presence of a lens opacity dense enough to make our clinical examination of the macula unreliable is the

Guyton-Minkowski Potential Visual Acuity Meter (PAM).

The Super Pinhole developed by David McIntyre, M.D., is another highly practical and useful method to evaluate macular function. The Laser Interference-Fringe Method has also been previoulsy used but it is less practical. Most clinical ophthalmologists prefer the PAM test or the Super Pinhole.

The PAM

The Potential Acuity Meter (PAM) is an instrument which attaches to a slit lamp. It serves as a virtual pinhole by projecting a regular Snellen visual acuity chart through a very tiny aerial pinhole aperture about onetenth of a millimeter (0.1 mm) in diameter. The light carrying the image of the visual acuity chart narrows to a fine 0.1 mm beam and is directed through clearer areas in cataracts (or corneal disease), allowing the patient to read the visual acuity chart as if the cataract or corneal disease were not there (Figs. 4 and 5A and B). The PAM is taken from its stand and placed directly onto the slit lamp in the same

manner as the detachable type of Goldmann tonometer. The examination takes from two to five minutes per eye, depending on the density of the cataract.

As pointed out by Guyton, for the PAM to work adequately, there must be some small hole in the cataract for the light beam to pass through. You may find such a hole even in cataracts which have media clouding of up to 20/200 and better. When you find it, then you

can avoid the light scattering produced by the opacities. Itisthislightscatteringwhichwashes out the retinal image and decreases vision behind cataracts. By projecting the image of the visual acuity chart through one tiny area, we avoid that scattering effect, and the patient can see the chart (Figs. 6 A-B and 7 A-B).

How is the instrument operated by the clinicianoranassistant?Thedeviceismounted on a slit lamp so that the operator can see

Figure 5 B (below right): Concept of the

Potential Acuity Meter (PAM) in Cases

of Corneal Opacities and Cataract

In Fig.5-B, by moving the beam to a point between the corneal opacities, the projected Snellenchart canpassonthrough the cornea and onto the retina (arrow) so that the patient can see it and we can determine the visual acuity. The test as shown in Figs. 4-A and 4-B is particularly important if we are considering a combined cataract extraction and penetrating keratoplasty.

Figure 5 A (above left): Concept of the Potential Acuity Meter (PAM) in Cases of Corneal Opacities and Cataract

In Fig. 5-A the tiny beam of light (arrow) of the projected Snellen chart is shown striking a corneal opacity and failing to penetrate the cornea.

exactly where the light beam is passing. The light beam is directed to various parts of the pupil (Fig. 4, 6-A, 6-B, 7-A, 7-B). It can be focused in between lens opacities. It is easy to see when the beam is going in because it practically disappears (Fig. 6-B). When it hits an opacity, you can see the opacity light up (Fig. 6-7). When you move the beam with the slit lamp control to lucent, non-opaque areas, you see the beam pierce through (Figs. 6-B and 7-B). It is valuable to observe this because if you know you are getting the beam through and the patient still reads poorly, you can be fairly confident that there will be a poor result after surgery. If you are not sure whether the beam is penetrating and the patient reads poorly, results of surgery will be uncertain. So, the slit lamp monitoring of the light beam is important.

It is sometimes difficult to find a small hole in a cataract with density greater then 20/200, although holes have been found in counting-fingers cataracts. If you obtain good vision behind any cataract, you have the information you need. As to the visual prognosis behind very dense cataracts, if you cannot obtain a good reading, you still do not know quite where you are.

The instrument is best operated in a darkened room because it is easier to see the light beam. The best results are obtained with a dilated pupil because you have a better chance of finding an appropriate hole in the cataract. Ninety percent of patients whose best correctable vision is 20/200 and better preoperatively, achieve the predicted vision or within two lines

than the predicted vision after surgery. When the preoperativevisualacuityis worsethan20/ 200, only about 60% achieve vision within three lines of the vision predicted by the PAM.

The vision obtained after surgery is generally equal to, or better than the vision predictedwiththePotentialAcuityMeter.False positives occur in 10-15% of cases. When the test is done in cases of cystoid macular edema, the instrument occasionally indicates better

potential vision than the patient can achieve with best refractive correction postoperatively.

No single test of visual function, however, is sufficient to mandate surgery. Instead, it is the visual needs of the patient in combination with careful estimation of the potential for the return of visual function after surgery that finally serves as the basis for the ophthalmologist to decide whether surgery is indicated and useful.

HOW TO PROCEED IN PATIENTS WITH RETINAL DISEASE

The Importance of Pre-Op Fundus

Exam

Thoroughperipheralretinalexamination should be done before cataract extraction. We are all proud to be first class clinical ophthalmologists and not think of cataract surgery only as a mechanical, technical procedure. As patients live longer, they are apt to have more preoperative diseases sometimes difficult to diagnose unless we are on the alert for them. Because the patient with an even moderate degree of cataract has reduced clarity of vision, it is easily possible that recent abnormalities may not have been observed or reported by the patient. This is particularly the case with retinal diseases.

CATARACT SURGERY IN DIABETIC PATIENTS

Because of the increasing importance of diabetic retinopathy, both in incidence and severity, we provide special emphasis to this disease in considering cataract surgery in complex cases. Cataract and retinovascular complications often co-exist in diabetic patients. The combination can present problems in determining the cause of decreased vision. Cataract surgery can also result in rapid progression of diabetic retinopathy that may need treatment with photocoagulation

(Figs. 8 and 9)..

Diabetic patients are very predisposed to developing cataracts. This is especially true of younger diabetic patients, who are also highly predisposedtodevelopingdiabeticretinopathy (diabetes Type I). In a series of diabetic retinopathy and maculopathy patients 15 years after laser treatment, only 22% of the eyes maintained clear lenses (Figs. 10 and 11). Cataracts will often form following vitrectomy surgery for diabetic retinopathy.

Rarely retinopathy can cause cataracts. An example would be prolonged vitreous cavity hemorrhage that results in a partial opacification of the lens. (Very high risk proliferative diabetic retinopathy - Fig. 12)

Evaluating Diabetics Prior to Cataract Surgery

Clinically significant macular edema (CSME) and less obvious macular changes in non-proliferative retinopathy may be the cause of decreased vision in addition to the cataract (Fig. 13).

It is important to listen to the patient's history when evaluating the cause of visual deterioration. This can be helpful in deciding how much of the visual loss may be due to cataract as opposed to visual damage caused by retinovascular conditions.

A good fundus examination through a dilated pupil is essential. In diabetic patients as in all patients, cataract should be removed when a patient's visual function does not meet his/her visual needs and the visual loss is consistent with the cataract. It is very rare that