VAN HERICK ANGLE OF THE ANTERIOR CHAMBER ESTIMATION METHOD

Angle grade Risk of angle closure Cornea to angle ratio

4 Wide open angle incapable of closure. Anterior chamber depth (shadow) is equal to or Iris to cornea angular separation equals greater than corneal thickness

to 35-45°

3 Moderately open angle incapable of closure. Anterior chamber depth (shadow) is between Iris to corneal angular separation equals to 1/4 and 1/2 of the corneal thickness 20-35°

2 Moderately narrow angle closure possible. Anterior chamber depth (shadow) is equal Iris to corneal angular separation equals to 1/4 of the corneal thickness

to 20°

1 Extremely narrow angle, closure chance Anterior chamber depth (shadow) is equal to less high. Iris to corneal angular separation than 1/4 of the corneal thickness

equals to 10°

0 Basically closed angle. Iris to corneal Anterior chamber depth (shadow) is nil or only

view of the cornea or the crystalline lens. The three dimensional view permits observation of distinguishable details within the crystalline lens “zones of discontinuity”. As with the optic section, the angle between the illumination source and biomicroscope may be varied to expose more corneal epithelium, stroma and endothelium. The whole cornea should be scanned using a parallelepiped. When scanning the cornea, a clear undistorted view must be maintained by positioning the light source to the opposite side when crossing the mid-line of the cornea. Both normal and abnormal findings can be seen when scanning the cornea with varied levels of magnifications and brightness. Look for the following findings:

1. Tear debris is usually related to allergies or occasionally with infections.

2. Corneal nerves are white thread-like structures that bifurcate and trifurcate and are located anywhere within the cornea. 3. Blood filled vessels extend from the limbus onto or into the cornea, and may be diagnostic of chronic or acute insult or inflammation.

4. Ghost vessels extend from the limbus into the cornea. They are empty of blood and diagnostic of past deep corneal inflam- mation.

5. Corneal scars are white in color and diagnostic of some past corneal damage, ulcer, abrasion or foreign body.

6. Corneal striae are white usually vertical thread-like twisting lines found in the Descemet’s membrane and posterior stroma. They are diagnostic of poor fitting soft contact lens and diabetes.

7. Endothelial pigmentation, when heavy and located vertically on the endothelium, is known as Krukenberg’s spindle, it may be diagnostic of iris atrophy and pigmentary glaucoma. Transillumination of the iris may reveal transillumination iris defects (TIDs). Scanty and very fine pigment deposits are commonly seen and are not pathological. Indirect Illumination

Indirect illumination means looking at tissue outside the area which is directly illuminated and can be used in conjunction with most of the above techniques. Corneal opacities, corneal nerves and limbal vessels are easily seen under indirect illumination as glare is reduced. Examine always directly as well as indirectly illuminated areas of the structure. To use this type of illumination place the biomicroscope directly in front of the patient’s eye and the illumination light source at about 45 degrees. Make sure the illumination mirror is in “click” position. Use a parallelepiped beam sharply focused on a given structure like the cornea. The light passes through the cornea and falls out of focus on the iris. The dark area just lateral or proximal to the parallelepiped is the indirect or proximal zone of illumination. This is the area of the cornea which one surveys through the biomicroscope. This type of illumination is helpful in detection of microcystic edema, faint corneal infiltrates and irregularities of the corneal epithelium and tears. Because it utilizes Fig. 3.7: Broad beam (parallelepiped)

direct, indirect and retroillumination simulta- neously, one should consider it to be as important as any other type of illumination. Retroillumination

Retroillumination is another form of indirect viewing. The light is reflected off the deeper structures, such as the iris or retina, while the microscope is focused to study the more anterior structures in the reflected light (Figs 3.8A to D). It is used to study the cornea in light reflected from the iris, and the lens in light reflected from the retina. Structures that are opaque to

light appear dark against a light background (e.g. corneal scars, pigment, and lens opacity). Portions that scatter light appear lighter than the background (e.g. edema of the epithelium, corneal precipitates). This method is useful for examining the size and density of opacities, but not their location.

Retroillumination uses a parallelepiped that bounces unfocused light off one structure while observing the back of another. The alignment and angular separation of the biomicroscope to the illumination source will vary. The light source beam is reflected off another structure like the iris, crystalline lens or retina while the

C _D

Figs 3.8C and D: Retroillumination

Figs 3.8A and B: Retroillumination: This technique allows the observer to view a clear structure with light that has been transmitted through, rather than just bounced off it. A Light from the slit-lamp is shone through the pupil, reflected off the fundus, and transmitted through the lens and cornea. B Light is reflected off the iris and transmitted through the cornea

biomicroscope is focused on a more anterior structure. For retroillumination or transillumi- nation of the iris or crystalline lens a low to medium magnification (X7-X10) is used. A slit- width 1-2 mm wide and 4-5 mm high is used with the biomicroscope and light source placed in direct alignment with each other. They are both positioned directly in front of the eye to be examined. Focus the slit just off the edge of the iris and on the front of the lens. If there are defects or atrophy of the iris they will be seen as a retinal “orange” glow coming back through each defect or hole. Patients who have numerous endothelial pigment deposits must have their iris transilluminated. The cornea is probably the most common structure viewed on retroillumination. Keratic precipitates will appear white in direct illumination but dark by retroillumination. This technique is valuable for observation of deposits on the corneal endothelium and invading blood vessels. Sclerotic Scatter

Sclerotic scatter examination uses the principle of total internal reflection (Fig. 3.9). Slit-lamp is set to a low X6-X10 magnification and a narrow vertical-slit (1-1.5 mm in width) is directed in line with the temporal or nasal limbus. A halo of light will be observed around the limbus as light is internally reflected within the cornea, but scattered by the sclera. Presence of corneal opacities, edema or foreign bodies will be made visible by the scattering light, appearing as bright patches against the dark background of the iris and pupil. Even minute nebular opacities can be picked up.

Specular Reflection

Specular reflection is achieved by positioning the beam of light and microscope in such a position so that the angle of incidence is equal

to the angle of reflection. The light can be reflected from either the anterior or posterior corneal surface. Note that the reflected light should pass through only one eyepiece, and, therefore, this method is monocular. Any roughness or irregularity as induced by the presence of corneal guttata is visible due to irregular reflection of light. A parallelepiped is used to view the endothelial cells of the cornea. The cells are seen only by one eye and they appear in the opposite direction of the illumination light source. A parallelepiped is used for specular reflection. The angle between the illumination source and the biomicroscope should be approximately 60 degrees and a high magnification and high illumination must be used.

Place the biomicroscope directly in front of the patient’s eye and the illumination light source at 45-60 degrees. Just off the limbus, obtain a sharply focused parallelepiped of the Fig. 3.9: Sclerotic scatter: A bright, wide-slit is shone directly at the limbus; most of the light is trapped within the cornea through total internal reflection, and, therefore, the cornea appears dark. When the light hits the opposite limbus or anything abnormal located within the corneal substance, it will scatter; some of the scattered light is directed back to the oculars, the abnormality is visible to the observer

cornea. Slowly advance the parallelepiped across the cornea until a dazzling reflection of the filament is seen within the biomicroscope. This reflection is only seen by one eye. Keeping the reflected light within the field of view of biomicroscope, the focus is moved back toward the endothelial cells. There will be a point where two images of the filament are seen, one bright, and the other ghost-like or copper-yellow in color. When the biomicroscope is focused on the ghost-like filament a mosaic of hexagonal cells are seen. It should be noted that even with X40 magnification the endothelial cells do not look as large as most texts show. They resemble the appearance of the dimpled surface of an orange peel or basketball. When the slit-lamp illumination system and the biomicroscope are at equal angles of incidence and reflection, the endothelium of cornea is viewable. Both front and back surfaces of the crystalline lens can also be viewed by using the specular reflection. Oscillatory Illumination

In oscillatory illumination, a beam of light is rocked back and forth by moving the illuminating arm or rotating the prism or mirror. This method may be used to determine occasional aqueous floaters and the extent of opacities in the crystalline lens.

Tangential Illumination

In tangential illumination iris is examined under very oblique illumination while the microscope is aligned directly in front of the eye. It is useful for examining tumors of the iris.