Simon M. Petersen-Jones, DVetMed, PhD, DVOphthal, DECVO, MRCVS
Associate Professor, Department of Small Animal Clinical Sciences, College of Veterinary Medicine, D-208 Veterinary Medical Center, Michigan State University
East Lansing, MI, USA
The term fundus describes the structures that can be seen ophthalmoscopically at the posterior of the eye. Examination and assessment of this part of the eye is a skill that many veterinarians find difficult. However, by following a few basic rules and with a little practice fundus examination can be easy and once it is routinely performed an appreciation of the range of normal appearances can rapidly be built up.
Facilities and Equipment
1. A Room That Can Be Darkened
Examining eyes in a darkened room makes the task much easier and enables a bright clear image of the fundus to be obtained.
2. An Ophthalmoscope
a) A Direct Ophthalmoscope
The majority of practices will have a direct ophthalmoscope only. Check that the optics are clean and that it has fresh batteries (or in the case of rechargeable models is fully charged).
b) Indirect Ophthalmoscope
Indirect ophthalmoscopy offers many advantages over direct ophthalmoscopy, but unfortunately commercial indirect ophthalmoscopes are expensive and therefore not widely used. A simple form of indirect ophthalmoscopy can be performed using a magnifying lens and a pen light or transilluminator. This simple monocular form of indirect ophthalmoscopy is cheap and with practice can provide the viewer with a clear wide-field view of the fundus. The disadvantages are that both hands are occupied so an assistant may be needed to hold the animal's head in the correct position and hold the eyelids open. The Volk 2.2 Pan Retinal lens is an excellent lens for indirect ophthalmoscopy.
Dilating the pupil using a mydriatic drug such as tropicamide makes any form of ophthalmoscopy easier and allows the majority of the fundus to be examined.
It may be useful to start off by performing distant direct ophthalmoscopy and then to move in to examine the fundus (close direct ophthalmoscopy).
The ophthalmoscope is set to zero (for a person with normal eyesight), looking through the viewing aperture the animal's eye is observed from arm's length. A fundus reflection is obtained and any opacities in front of the fundus show up as dark shadows on the reflected light which is colored by the tapetal coloration. Once the tapetal reflection is in view the examiner moves closer until the ophthalmoscope is within a few cm of the eye. The fundus should now be in focus.
This technique gives a magnified view of a relatively small area of the fundus, the examiner needs to move around to build up a mental montage of as much of the fundus as possible.
Simple indirect ophthalmoscopy may be performed using a pen light and condensing lens (between 14 and 30 diopters).
Position yourself at arm's length from the patient so that the light is shining along the animal's visual axis. A good tapetal reflection should be seen.
Interpose the lens at right angles to the examining beam of light, about 5cm from the cornea. An aerial image of the fundus should now be seen. Move the lens closer or further from the cornea until the best image is achieved. I find it easiest to hold the edge of the lens between finger and thumb and rest my little finger on the animal's head. The hand holding the lens then moves with any head movements.
To view other parts of the fundus the examiner must be prepared to either move him or herself or to alter the position of the animal's head. To view the dorsal fundus, for example, the examiner needs to look up into the eye or lift the animal's nose upwards, and to view the medial fundus the examiner needs to view from the lateral side of the eye. Therefore quite a lot of moving about may be required to view the entire fundus. When changing the direction of viewing it is important to alter the lens so that it remains at 90° to the beam of the examining light.
Normal Appearance of the Fundus
The fundus consists of:
Optic nerve head
Retina (including retinal pigment epithelium)
Superficial retinal vasculature
Choroid underlying the retina--including in the tapetal area the tapetum
In some instances the underlying sclera may be visible
The appearance of the normal fundus varies considerably between individual animals, particularly so in dogs. This wide range of normality must be appreciated before abnormality can be detected.
Tapetal variation. The color and size/extent of tapetum can vary considerably. In some dogs and cats the tapetum may be absent. Color ranges in dogs from green, yellow, reddish, tan and blue. In cats there tends to be less variation in coloration. In dogs in general terms small breed dogs tend to have a less extensive tapetal area than large breed dogs. Color-dilute breeds may have a lack of tapetum.
Non-tapetal fundus variation. The coloration of the non-tapetal fundus is dependent on the degree of ocular pigmentation. With color-dilute animals the choroidal vessels may show through the pigment, and in some there may be a total lack of pigmentation allowing the choroidal vessels and sclera to be visualized.
Retinal vasculature. The pattern and number of vessels varies. Some small breed dogs may have less extensive vasculature than large breed dogs. This should not be confused with blood vessel attenuation. In cats the major venules and arterioles run together. There is usually less variation in feline vasculature than canine.
Optic nerve head. There is quite a lot of variation in the appearance of the canine optic nerve head. The optic nerve head consists of the axons of ganglion cells that converge on the lamina cribrosa to penetrate through the sclera to form the optic nerve. As the axons converge they gain a myelin sheath. In the dog this occurs prior to the lamina cribrosa and causes the optic nerve head to be slightly raised above the retinal surface. It also confers the shape of the optic nerve head that can vary from round to triangular. Earlier myelination of the axons can be seen adding to the extent of the optic nerve head and making its shape irregular. In the cat myelination occurs at the lamina cribrosa so the optic nerve head is slightly depressed from the retinal surface, grayish in color and circular. Blood vessels converging on the optic nerve head dip over the edge of the depression in the cat and are not seen on the surface of the optic nerve head. In dogs there is a complete or partial venous circle visible on the optic nerve head and the vessels rise above the surface of the retina to pass onto the elevated surface of the optic nerve head.
Abnormalities of the Fundus
The wide range of normal appearance of the fundus must be appreciated prior to diagnosing abnormalities. There are a limited number of changes in appearance that can develop in the fundus. The following are a few such changes:
Retinal thinning over the tapetal fundus results in tapetal hyperreflectivity. This is indicative of retinal degeneration. Causes:
Focal degeneration, may be a post-inflammatory change, be associated with geographic retinal dysplasia, etc.
Generalized thinning can result from diseases such as progressive retinal atrophy, secondary to retinal damage from conditions such as glaucoma, result from sudden acquired retinal degeneration (takes several weeks to develop), etc.
Change in Tapetal Color
In addition to the wide range of normal tapetal colors, alterations in coloration may result from inflammatory processes (chorioretinitis) or from changes in thickness of the overlying retina, or from proliferation and pigmentation of the overlying retinal pigment epithelium obscuring view of the underlying tapetum. Severe inflammatory changes may result in destruction of the tapetum.
Thickening of the retina results in reduced reflection from the tapetum in the tapetal area. The lesions may appear gray in color. Lesions in the nontapetal area appear as whitish or grayish lesions. Causes include: retinal edema and retinitis.
Rough or long coated dogs usually have an irregular tapetal/nontapetal border. This can give the appearance of darkly pigmented areas amongst islands of tapetal tissue. This is normal.
Inflammatory lesions can result in changes in pigmentation. In the nontapetal area depigmentation may develop (allowing visualization of the underlying sclera) with patches of pigment hypertrophy. Inflammatory lesions in the tapetal area may induce retinal pigment epithelium proliferation and pigmentation. The condition retinal pigment epithelial dystrophy results in accumulation of light brown pigment clumps (lipofuscin) across the tapetal fundus.
Hemorrhage at various layers from vitreous to choroid gives different appearances. Possible causes include: trauma, coagulopathies, hypertension, inflammation
Changes in the superficial retinal blood vessels can include:
Narrower or less obvious blood vessels, e.g., anemia, retinal degeneration, post-inflammatory change
Wider blood vessels e.g., hyperviscosity syndrome, hypertension
Segmental dilation and constriction of blood vessels, e.g., hypertension, inflammatory disease
Creamy colored--very high circulating lipid levels
Underlying choroidal vessels may be exposed--normal lack of pigmentation, depigmentation following inflammation, Collie eye anomaly.
This may range from small flat detachments (affected area just appears fuzzy) to total bullous detachments where a gray membrane with blood vessels on its surface is seen bulging into the vitreous cavity. If retinal tearing occurs the torn retina may fall ventrally to appear as a veil which obscures the optic disc. Causes: congenital abnormalities (retinal dysplasia, Collie Eye Anomaly), inflammatory, hypertension, associated with hypermature cataract, neoplastic, trauma, idiopathic.
Optic Disc Changes
Excess myelination at the optic disc can give the appearance of "pseudopapilledema". Swelling of the disc may result from true papilledema or papillitis. Depression of the surface of the disc may be congenital (e.g., coloboma) or acquired (e.g., glaucomatous cupping). The feline optic disc is slightly depressed compared to the canine.