A Greek word meaning “curtailed.” It is used to describe conditions where normal tissue in or around the eye is missing from birth.

Clinical Signs

May observe any of the following:

  • Pigment changes in the eye
  • Keyhole or notch like appearance in the eye which may or may not appear to have jagged edges
  • Smaller than usual eye
  • Pupil may seem to be misaligned
  • Pupil may be non-reactive to light
  • Papillary shape change to the eye


A coloboma is a defect or absence of a part of the ocular (eye) tissue. They can be present in different parts of the eye depending on the part that is missing such as in the following areas:

  • choroids (part of the uvea)
  • ciliary body (part of the uvea)
  • eyelid
  • iris (part of the uvea)
  • lens
  • optic nerve
  • retina

Colobomas are usually congenital resulting from the failure of part of the eye to fully develop, it is suggested that inheritance may be autosomal dominant, autosomal recessive, or X-linked.

Normally, as the eye develops, a gap called the optic fissure, which brings nourishment to the eye in utero, begins to close near the center and continues rostrally and caudally. When it doesn’t close the coloboma occurs. Colobomas can occur in one or both eyes.

Colobomas seen in the iris and ciliary body (uveal colobomas) occur if there is a failure in closure of the anterior aspect of the optic fissure. Colobomas of the iris do not normally change the function of the eye, but it can produce changes to the shape of the pupil.

When the posterior portion of the optic fissure fails to close completely it can cause a keyhole-like, or a notch-like, appearing defect to occur in the optic disc. This type of defect can be associated with “posterior thinning and outpouching” of the sclera and can be a cause of large, multilocular, cysts to form caudal to the globe such as retrobulbar cysts5.

In some cases, a coloboma can be due to pathological reasons or as a result of surgical determination.

Colobomas may also be associated with other conditions of the eyes:

  • One eye a different color than the other known as heterochromia
  • A small eye referred to as microphthalmia.
  • A thicker than normal cornea.
  • Glaucoma; a condition where the pressure in the eye is increased.
  • A malformation of the retina known as retinal dysplasia.
  • Involuntary eye movements referred to as nystagmus.
  • Staphyloma indicating protrusion of the back of the eyeball.

A coloboma may occur alone, or may be associated with other body anomalies.

Sometimes the other anomalies can be minor or sometimes more serious, depending on the organ involved or where in the body it is located (e.g., heart or kidney defect). Where there are numerous anomalies, the condition may be referred to as a genetic syndrome.

Some researchers have proposed that certain environmental factors may contribute to developing coloboma, either in humans or in animals. These findings have been published over time in the research literature, but there has been no systematic analysis of possible links (Blain, 2009).

While colobomas do not present a painful condition for the rat, or interfere with the rat’s activity, vision may be altered, or other associated eye conditions may present that will require treatment.


Case History and Photos

  • Fig. 1: Coloboma of the iris of the right eye in female rat (Maya)
  • Fig. 2: Suspected coloboma of the iris (bilateral) in female rat (Jessie)


Ophthalmoscopy: May or may not reveal changes depending on severity. Changes may reveal a tiny pit in the optic disk, a notching of the optic disk, or an excavated disk.


No treatment is required.

Individual treatment should be addressed only as other disease processes of the eye(s) become apparent.

Nursing Care

  • Include a check of the appearance of the eye(s) when doing regular weekly home health checks on the pet rat.


  • Does not interfere with rat’s activities
  • No presence of pain


  • Because colobomas can be an inherited trait breeders should thoughtfully consider whether to continue breeding parents, or offspring with the defect.
  1. Blain, D. (2009). Facts About Uveal Coloboma. Retrieved January 12, 2011, from
  2. Canine Coloboma. (2007). Watford GSD Help & Information. Retrieved January 12, 2011, from
  3. Cazabon, S., & Bradbury, J. (2003). Ocular coloboma and radial aplasia: syndrome or association. Eye, 17(4), 532-4. Retrieved January 12, 2011, from
  4. Nicholls, J., & Tansley, K. (1938). Colobomata of the optic nerve sheath in rats. Br J Ophthalmol., 22(3), 165-8.
  5. Thomson, R. G., McGavin, M. D., Carlton, W., & Zachary, J. F. (2001). Coloboma. Thomson’s Special Veterinary Pathology (3rd ed., pp. 655-656, 679, 688). St. Louis: Mosby.


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