Slit-lamp biomicroscopy: performance and record-keeping I. Describe the instrumentation and technique

A. Instrumentation 1. Viewing arm (corneal microscope)

a. Magnification depends on eyepieces and objective lens changer settings

i. Magnification numbers on knob apply to one set of oculars

ii. For higher power oculars, magnification increases by proportionate amount

b. Eyepieces i. Adjust dioptric power to compensate for observer’s

refractive error and/or accommodation and for any instrument misalignment

ii. Eyepiece settings and calibration can be confirmed using focusing rod

2. Illuminating arm a. Illumination arm swings in an arc on a co-pivotal axis with the

corneal microscope to allow coaxial alignment with a parfocal and isocentric light beam

b. Beam length generally available with pre-set increments and with continuous-length adjustment; beam width varies from open spot to narrow slit

c. Light filters may include grey filter, cobalt-blue filter, and red-free filter; heat absorption screen often part of lighting system

3. Base a. Allows both corneal microscope and slit illuminator to be

horizontally and vertically mobile, controlled by joystick b. Arresting lock or lever for stabilization

4. Patient positioning frame B. Illumination methods

1. Diffuse and direct illumination a. Diffuse illumination

i. Gives overview of eyelids, ocular surface, and anterior segment of eye

ii. Use full height and broad width iii. Light diffuser may be used for photography

b. Direct focal illumination and slit illumination i. Allows detection and localization of structures, including

differences in refractive index ii. Use medium to narrow beam width to illuminate a

parallelepiped of transparent tissue and use very narrow slit beam to illuminate an optical section

iii. Use shortened beam to evaluate Tyndall flare effect in the anterior chamber and to detect cells in the convection

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currents of the aqueous humor or in the tear film to detect slow tear turnover or presence of inflammatory cells

c. Tangential illumination i. Shine light across anatomical surface, such as cornea or

iris ii. Observe shadowing effects

2. Indirect illumination, retroillumination, and sclerotic scatter a. Indirect, proximal illumination and lateral illumination

i. Purposely focus or reflect the illuminator’s light beam at a different, though adjacent site as the corneal microscope

ii. Useful to highlight abnormalities that have a refractive index similar to their surroundings and that are difficult to discern by direct illumination

iii. Abnormalities visualized by light scattered from its irregular surface or glowing by internal reflection

iv. Three-dimensionality may be enhanced by oscillatory movements of light beam

b. Retroillumination i. Direct retroillumination

i) Used to examine darkened abnormalities against an illuminated background (e.g., keratic precipitates against illuminated iris)

ii. Indirect retroillumination i) Used to examine illuminated abnormalities against

a darkened background (e.g., microcystic epithelial edema against dark pupil)

iii. Fundus retroillumination i) Used to examine darkened (e.g., posterior

subcapsular cataract), illuminated (e.g., keratoconus), or transilluminated (e.g., focal iris atrophy) against “red reflex” background

c. Sclerotic scatter i. Used to detect subtle corneal abnormalities that distort

the total internal reflection property of the normal cornea ii. Detect scattering of light while shining light onto limbus

3. Specular reflection a. Used mainly to examine corneal endothelium (second Purkinje

light reflex), although can also examine corneal epithelium and lens epithelium

b. Monocular viewing c. Estimate endothelial cell density, evaluate endothelial cell

pleomorphism and polymegethism, and detect abnormal areas of nonreflectivity (e.g., guttate changes and pseudoguttata)

C. Devices used with the slit-lamp biomicroscope 1. Linear measurement tools

a. Length and width reticules of the illuminating arm

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b. Eyepiece micrometer of the viewing arm 2. Applanation tonometer

a. Falsely high measurement of intraocular pressure with increased corneal thickness

b. Falsely low measurement of intraocular pressure with corneal thinning

c. Misleading estimation of intraocular pressure with high astigmatism

3. Gonioscopy tools for examining the anterior chamber angle a. Goldmann single-mirror gonioscope b. Koeppe goniolens c. Four-mirror gonioprism d. Zeiss four mirror lens

4. Lenses for examining the vitreous cavity and posterior segment a. Hruby lens b. Goldmann fundus contact lens c. Three-mirror contact lens d. Condensing lens for indirect ophthalmoscopy

5. Optical pachymetry for measuring corneal thickness Additional Resources

1. AAO, Basic and Clinical Science Course. Section 3: Optics, Refraction and Contact Lenses 2013-2014.

2. Leibowitz HM, Waring GO III, eds. Corneal Disorders: Clinical Diagnosis and Management. 2nd ed. Philadelphia: Saunders; 1998:34-81.

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