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Fundoscopy: normal findings

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Examination techniques

  • Examination of the fundus is achieved by ophthalmoscopy.
  • Direct ophthalmoscopy Ophthalmoscopy: direct provides a small field of view of a magnified image, and is useful when examining an area in greater detail.
  • Indirect ophthalmoscopy Ophthalmoscopy: indirect provides a wide field of view, is the best way of scanning the entire fundus and is possible even with partial opacity of the ocular media. 

Examination preparation

  • The patient needs to be examined in a darkened room.
  • Topical 0.5-1% tropicamide Tropicamide results in pupil dilation after 20 mins, and this affords easier fundus examination. However, this should be avoided in cases where glaucoma Glaucoma or lens luxation Eye: lens luxation are suspected.

Normal fundus components

  • Before being able to recognize abnormal fundi, it is essential to be familiar with the normal fundus and all its many variants in the species being examined. The feline fundus has much less variation than that of the dog.

1. The optic disk

  • The optic disk is the optic nerve head, and it is also referred to as the optic papilla.
  • It consists of the axons of the retinal ganglion cells which course radially from the retina and turn at a right angle to exit the eye through a sieve-like perforation in the sclera, called the lamina cribosa.
  • Myelination of the nerve fibers does not occur at the disk, unlike in the dog, and thus the disk usually appears pale white or grey . Myelination does occur posterior to the disk but very occasionally can be seen ophthalmoscopically radiating from the disk.
  • The normal optic disk is almost circular in shape. There is much less variation than in the dog.
  • It is located within the tapetum (when a tapetum is present).
  • The optic disk is normally slightly cupped, with the vessels dipping inwards at the edge .
  • The peripapillary area can have a narrow zone of tapetal hyperreflectivity, termed conus, which is normal. There may be a rim of pigment around the disk, which may or may not be complete. Both are visible in .

2. The tapetal fundus

  • The tapetal fundus is the roughly triangular-shaped area with a granular appearance.
  • The tapetum lucidum is a region within the choroid located between the choriocapillaris and the medium vessel layer of the choroid.
  • Specialized tapetal cells contain reflective rods, which are made up of a riboflavin-zinc complex. The tapetum is thicker than in the dog, and the cells are more precisely oriented.
  • The function is to reflect light not absorbed by the retina back to the retina for a second chance of stimulation. Therefore, in low lighting environments, animals with tapeta have increased vision.
  • The tapetum is highly reflective in cats.
  • The extent of the tapetum varies, but most commonly it is present. Variations include absence (most often in color-dilute eyes) or incomplete development with the presence in small islands (uncommon). Sometimes it is thin and some choroidal vessels can be seen through it .
  • It varies in color and may appear yellow , yellow-green , blue-green  or blue.
  • The junction between the tapetal and nontapetal fundus is termed the tapetal-nontapetal border and it is usually clearly defined. However, it is possible to have tapetal islands making it more irregular in outline.

3. The nontapetal fundus

  • The nontapetal fundus encompasses all regions of the fundus apart from the tapetal fundus and the optic disk.
  • It is heavily pigmented due to concentration of melanin granules in the Retinal Pigment Epithelium. It is usually dark grey-brown to black.
  • Subalbinotic animals have no pigment or only light pigment in this region. Because of this, it is possible to view the underlying choroidal blood vessels and sometimes the white sclera.

4. Retinal vasculature

  • Cats have a holangiotic pattern to their retinal blood vessels . Thus, most of the retina has a direct blood supply.
  • Three major primary veins emerge from near the edge of the optic disk extend peripherally.
  • One vessel typically extends upwards, and the remaining two to three vessels typically extend laterally and downwards.
  • The vessels are usually not present on the central area of the optic disk.
  • Three slightly narrower cilioretinal arterioles also radiate from the perimeter of the disk.
  • The vessels arc around the area centralis, which is the area of highest cone density, and is located lateral and slightly dorsal to the optic disk .

Normal variations with age

  • The tapetum is absent at birth and the fundus appears brown-grey in the young kitten.
  • The tapetum subsequently develops with a tapetal area starting to become apparent by 2 weeks of age. This zone starts to appear lilac as the tapetum develops.
  • By 6 weeks of age the tapetum is typically lilac-blue in color.
  • The tapetum reaches maturity and develops its future color by around 3 months of age.

Normal variations with subalbinism

  • Certain breeds sometimes lack ocular pigment, such as the Himalayan or Siamese Siamese, and these have a subalbinotic fundus.
  • Subalbinotic fundi differ because of a reduction in pigment in the retinal pigment epithelium. The tapetum mat be present, sparse or absent.
  • The degree of subalbinism varies from complete albinism, with no tapetum and no pigmentation through to segments or regions of subalbinism at varying locations within the fundus .
  • The choroidal vessels can be seen because of sparse pigmentation within the choroid and retinal pigment epithelium.
  • The nontapetal fundus is described as tigroid, because of the appearance of the choroidal vessels .
  • Sometimes the white sclera can also be seen .

Further Reading

Publications

Refereed papers

Other sources of information

  • Mitchell N & Oliver J (2015) The vitreous and fundus. In:  Feline Ophthalmology - The Manual. Editorial Servet, Grupo Asis. pp 173-191 ISBN 978-84-16315-11-6.
  • McLellan G J & Narfström K (2014) The Fundus. In: BSAVA Manual of Canine and Feline Ophthalmology, D Gould & G McLellan (eds), 3rd edn. Chapter 18. pp 322-356.
  • Barnett K C & Crispin S M (1998) Fundus. In: Feline Ophthalmology. An Atlas & Text. KC Barnett & SM Crispin. Saunders. Chapter 14, pp 155-180.