Definition

Originally known as “Ectopia Lentis”, the term may still be found in literature, but lens luxation/subluxation is the most current usage.

Ectopia: Greek for “out of position”
Lentis: Greek word for “lens”

Therefore, ectopia lentis means displacement or malposition of the eye’s lens from its normal location. It is no longer centered. This condition usually applies to bilateral lens displacement. When the lens is off center or partially dislocated, but still held within the lens space, it is termed subluxated or subluxation. When the lens is completely dislocated out of its normal position altogether, and no longer behind the iris, it is termed luxated or luxation.

Clinical Signs

May observe any of the following:

The occurrence of/or sudden change in the appearance of the eye. In the event both eyes are involved the appearance of luxation or subluxation may seem to occur more gradually in one eye than the other. This is more likely to be seen in cases where the condition was due to genetics.

Anterior Luxation

• Pain
• Tearing (excessive Porphyrin [rust-colored tears] in the rat)
• Floating lens
• Prolapse of lens
• Abnormal pupil
• If corneal involvement cloudiness in the eye may be seen

Posterior Luxation

• May not be apparent initially
• Pupillary abnormality (pupillary abnormality, a generic term, meaning an abnormality due to either the iris sphincter or iris dilator muscles not functioning properly, or any change in size or shape of the pupil)

Subluxation

• Trembling (vibration) of iris or lens
• Mild redness of conjunctiva
• Eye may appear cloudy/ white
• Change in pupillary shape or contour (Distortion of pupil shape and/or size with lens movement)
• Aphakic (“a” meaning absent, “phak” meaning lens) crescent (pupillary aperture appears shaped like a crescent where the lens is no longer in place)

*Note: for additional information on recognizing various signs of pain or discomfort refer to: Signs of Pain In Rats.

Etiology

The lens, a focusing device of the eye located behind the iris, is held in place by zonular fibers that extend from the ciliary body to the lens equator, but is not within the ciliary body. It can be visualized through the pupil.
The vitreous body (a gelatinous substance) is behind the lens (between the lens and the retina), and gives the eye its shape. The suspensory ligaments composed of straight fibers called zonules keep the lens centered in its normal position.

A displacement of the lens can be due to either a primary or secondary condition. When the fibers (zonules) that suspend the lens and hold it in place are weak due to a congenital defect, the condition is said to be primary. When the fibers (zonules) become stretched, weakened, or break due to disease or conditions such as inflammation, cataracts, glaucoma, cancer, or as a result of trauma (e.g., injuries to the cornea or iris), the condition is said to be of a secondary nature.

Note: while lens dislocation can occur in only one eye, it is more common for primary lens luxations to be bilateral, although they may not necessarily develop at the same time.

When luxation of the lens occurs, the lens floats freely in the eye. This can result in the lens either drifting or being pushed into the anterior chamber of the eye (anterior luxation), or to fall toward the back of the eye (posterior luxation).

With anterior lens luxation, conditions such as glaucoma and uveitis, as well as corneal damage can develop. As the luxated lens rubs on the iris, poking through the pupil opening, it can apply pressure to the cells that line the inner surface of the cornea, thereby damaging the cornea. With uveitis, and inflammatory eye conditions, it can cause the pupil of the eye to constrict. This results in the lens of the eye becoming trapped in the anterior chamber of the eye which then leads to the obstruction of flow of aqueous humour and results in the development of glaucoma (increased intraocular pressure).

When posterior lens luxation occurs, there is less of an issue. The lens falls backward into the vitreous humour lying flat so there is a lesser chance of the development of inflammation and glaucoma, although glaucoma can occur.

In subluxation of the lens, there is only a partial displacement initially, just shifting the lens slightly from its normal location. This can eventually cause prolapse of the vitreous humour into the anterior chamber of the eye leading to glaucoma.

Lens luxation or subluxation can also be associated with other developmental eye anomalies such as microphakia (an abnormally small lens), spherophakia (abnormally round lens), and colobomas (equatorial lens defects). Because of the complications that can arise from anterior or posterior luxation or subluxation it is important to have a veterinarian assess these conditions for appropriate treatment.

*Note to breeders: primary lens luxation involving only one eye is thought to be characterized by an autosomal dominant condition. However, primary lens luxation that involves both eyes (even though presentation may be delayed in opposing eye) is believed to be an autosomal recessive heritable condition meaning that each parent rat must at least be a carrier of the disorder. Should either of these types of conditions present it is recommended that breeders not breed from the affected litter.

Figures

Case History and Photos

• Fig. 1: Anterior lens luxation in female rat (Gemma)

Diagnostics

External examination of the eye.

Use of ophthalmoscope to view eye surface, anterior chamber, vitreous, and retina

Use of slit lamp to visualize

Measurement of intraocular pressure.

Treatment

Lens surgery is challenging at best, even in larger animals. In very small animals, such as rats, surgical intervention of the type needed to do a repair may not be considered prudent or in the rat’s best interest. In humans as well, the condition depending upon severity, may just be closely observed, and medically rather than surgically managed. Rats can do well with observation and medical treatment for any secondary involvement arising from complete or partial lens displacement.

The treatment of lens luxation varies depending on the location of the lens, the presence of secondary eye conditions, and any associated pain. The main goals of treatment include reducing intraocular pressure (IOP) if possible, treating underlying causes, and surgical enucleation where pain cannot be controlled.

Treatment, as directed by a veterinarian may include the following:

• Treating, and/or control of glaucoma by reducing intraocular pressure (IOP) with an osmotic agent, topical miotic, topical or oral antiglaucoma medication, or a topical anti-inflammatory agent, or may include a combination of medications.
• Treating, and/or control of anterior uveitis. Treatment may include the use of topical ophthalmic anti-inflammatory agents, or an oral anti-inflammatory agent along with a topical ophthalmic or oral antibiotic.
• Treating corneal abrasions with a topical ophthalmic antibiotic.
• Enucleation of the eye may be necessary when pain is not able to be controlled.

For information pertaining to medications refer to the Rat Medication Guide.

Nursing Care

• Treat for any secondary eye conditions as directed by the veterinarian.
• Monitor lens position if the lens is loose, but still in place.
• If a primary lens luxation is diagnosed in one eye, the other eye must be closely monitored for degeneration of the zonules and loosening of the lens.
• Notify veterinarian if any of the following occur: redness, eye swelling, pain or squinting.

Outcome

• Pain relieved
• Intraocular pressure controlled
• Any secondary complications treated

Prevention

• Early veterinary attention and treatment of eye conditions, infections, and injuries can help to prevent or reduce the incidence of lens luxation or subluxation.
• Supervise children when they are playing with pet rats.
• Discontinue breeding lines with known ocular genetic disorders.

Consultation

• Steven R. Hollingsworth, DVM, DACVO
  Chief, Ophthalmology Service, School of Veterinary Medicine
  University of California, Davis

References

1. Armstrong, R., & Monie, I. (1966). Congenital eye defects on rats following maternal folic-acid deficiency during pregnancy. J Embryol Exp Morphol, 16(3), 531-42. Retrieved March 28, 2011, from http://dev.biologists.org/content/16/3/531.full.pdf
2. Hirani, R. (2006). Adelaide Research and Scholarship: Molecular interactions of latent transforming growth Factor-&# 946; binding Protein-2 (LTBP-2) with fibrillins and other extracellular matrix macromolecules [electronic resource]: LTBP-2 competes with LTBP-1 for binding to Fibrillin-1 suggesting that LTBP-2 may modulate latent TGF-&# 946; storage. Adelaide Research and Scholarship. Retrieved March 28, 2011, from http://hdl.handle.net/2440/37860
3. Tanaka, K., Inagaki, S., Matsumoto, H., Ikemoto, F., & Doi, K. (1996). Posterior lens rupture in rats. Toxicol Pathol, 24(5), 639-41. Retrieved March 28, 2011, from http://tpx.sagepub.com/content/24/5/639.full.pdf
4. 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.