Ординатура / Офтальмология / Английские материалы / Notes on Veterinary Ophthalmology_Crispin_2005

SECTION 1

OPHTHALMIC EQUIPMENT AND EXAMINATION

INSTRUMENTS AND EQUIPMENT

BASIC INSTRUMENTS FOR OPHTHALMIC EXAMINATION

•Penlight (focussing or narrow beam)

•Condensing lens (20D or 2.2 pan retinal®)

•Magnifying loupe or otoscope with speculum removed

•Direct ophthalmoscope

•Schiøtz tonometer

ADDITIONAL INSTRUMENTS

•Finoff ocular transilluminator

•Slit lamp biomicroscope

•Monocular indirect ophthalmoscope

•Binocular indirect ophthalmoscope

•Tonometer (e.g. TonoPen®)

•Gonioscopy lens (e.g. Barkan or Koeppe)

BASIC EQUIPMENT FOR DIAGNOSTIC PROCEDURES

•Ophthalmic stains – fluorescein sodium (1% and 2%) and rose bengal 1%

•Sterile water, sodium chloride 0.9%, Hartmann’s solution

•Local anaesthetic for topical use (e.g. proxymetacaine hydrochloride 0.5%, amethocaine hydrochloride 0.5% and 1%)

•Combined local anaesthetic and ophthalmic stain (e.g. proxymetacaine hydrochloride 0.5% and fluorescein sodium 0.25%)

•Local anaesthetic for injection (e.g. lignocaine [lidocaine] hydrochloride 1% and 2%, prilocaine hydrochloride 1%, mepivacaine hydrochloride 2%)

•Mydriatic (e.g. tropicamide 1%)

•Miotic (e.g. pilocarpine hydrochloride 1%)

•Schirmer Tear Test papers

•Kimura spatula or other sterile scraper, e.g. disposable scalpel blade

•Sterile swabs or cytobrushes for sample collection for culture

•Clean, dry, grease-free microscope slides

•Suitable stains for smears, e.g. Gram and Giemsa

•Cotton wool

•Calipers/ruler

•Nasolacrimal cannulae (re-usable sterilisable metal or disposable sterile plastic)

•Sterile syringes and needles

Figure 1.2 (a) Penlight light source. (b) A Finoff ocular transilluminator (Welch Allyn) can be used to provide a white light source or a cobalt blue filter can be attached to provide a blue light source.

Technique

•The light source is used on its own or in conjunction with a magnifying lens

•It is better to conduct the examination in the dark, so as to minimise distracting reflections

•The information obtained from light source examination is enhanced when the light is shone from a number of different angles

Magnification

Magnification can be achieved using an otoscope with the speculum removed (Figure 1.3(a)), a convex lens, a loupe (e.g. simple monocular magnifying glass or binocular loupe), a slit lamp biomicroscope or a direct ophthalmoscope (see below). Magnification, combined with a light source, is ideal for detailed examination of the adnexa (eyelids, lacrimal apparatus, orbit and para-orbital areas), anterior segment (anterior part of the globe, up to and including the lens) and posterior segment (lens, vitreous and fundus).

•A slit lamp biomicroscope (Figure 1.3(b)) is an essential purchase for those with a serious interest in ophthalmology, but cost is the major limiting factor for those in general practice

•Both portable and table mounted types of slit lamp are available. The advantages of portable, cordless models for veterinary ophthalmology are obvious

Lens

•A lens is a device that refracts (bends) light, and the vergence (bending) power is measured in dioptres (D). A convex, magnifying (plus) lens causes convergence of incident light, whereas a concave, reducing (minus) lens causes divergence of incident light.

•The focal length of a lens is directly proportional to its curvature radius, so that a lens of high curvature has a shorter focal length, and is more powerful, than a lens of longer focal length. A strong convex lens (high plus) produces a small, bright image whereas a weaker convex lens (lower plus) produces a larger, less bright image.

•Lenses of 10D or 15D can be used to aid examination of the adnexa and anterior segment. Lenses of 20D–40D can be used for indirect ophthalmoscopy. Lenses of 60D–90D are also available and usually used in conjunction with a slit lamp for fundus examination.

Indirect ophthalmoscopy

Commercially-manufactured monocular and binocular types are available, but are expensive. A penlight, or Finoff transilluminator, and condensing (convex) lens may be combined to provide an inexpensive means of performing monocular indirect ophthalmoscopy and this is probably the technique of choice for those in general practice.

Monocular indirect ophthalmoscopy

Optics

•If a convex (plus) lens is held in front of the eye, an aerial image of the fundus can be seen. This technique is known as indirect ophthalmoscopy, because the examiner is not observing the fundus directly.

•The image obtained is virtual, inverted, reversed left-to-right and magnified. The field of view (the area of the fundus that can be seen at any one time) is greater, albeit with less magnification, than can be obtained with a direct ophthalmoscope. Dilating the pupil will enlarge the field of view, irrespective of whether indirect or direct ophthalmoscopy is used.

•Indirect ophthalmoscopy is useful when the ocular media are opaque as it provides better penetration of partially-opaque eyes than direct ophthalmoscopy. It is not affected by major refractive errors in the patient’s eyes.

Equipment

•Penlight: Any type of penlight or transilluminator, or even a small torch with narrow beam of light, will be adequate. The light source should be bright. Focussing ability is valuable, but not essential.

•Indirect lens and penlight (Figure 1.4(a)): In most domestic mammals, a 2.2 pan retinal® lens (Volk) or a 20D convex lens are the most versatile. The pan retinal® 2.2 is particularly valuable as it combines magnification of 2.68¥ – nearly that of the 20D lens (image magnification 3.13¥, field of view 46°), with a field of view of 56° – almost that of a 30D lens. A 30D lens (image magnification 2.15¥, field of view 58°) or 40D lens (image magnification 1.67¥, field of view 69°) may be used if the eye or pupil is relatively small (e.g. puppies).

•Indirect lens and slit lamp biomicroscope: A 60D lens (image magnification 1.15¥, field of view 68°) and a 90D lens (image magnification 0.76¥, field of view 74°) are valuable for examination of small eyes and pupils, but their use may be regarded as specialist because they are best used in conjunction with slit lamp biomicroscopy.

•Commercially-manufactured monocular indirect ophthalmoscopes (lens and light source in one instrument) are also available and have the advantage that they are simple to use and the image viewed is upright (Figure 1.4(b)). Models of this type are useful for the widely varying conditions under which ophthalmoscopy is performed in large animal ophthalmology.

Figure 1.4 Indirect ophthalmoscopy. The technique is performed in the dark and is most rewarding when the pupil is dilated. (a) Monocular indirect ophthalmoscopy using a condensing lens and penlight. (b) Monocular indirect ophthalmoscopy using an American Optical