Primary lens luxation (PLL) is thought to be heritable in most breeds in which it is seen, although clinical signs are generally not seen until the dog is an adult.

Secondary lens luxation is not heritable, and occurs secondary to another disease process within the eye.

In the terrier breeds (such as the Jack Russell terrier) PLL is associated with an inherited degeneration of the zonules, or the thin ligaments that suspend the lens in place behind the iris (the coloured part of the eye) and in front of the vitreous (a clear, gel-like substance).

The genetic mutation has been characterised in a number of breeds, and a genetic test is available.

Lens luxation refers to the lens being in an abnormal position inside the eye.

Clinical signs in the fox terrier are usually not seen until the dog is in middle age, and include a sudden onset of pain (squinting, tearing etc), redness, and cloudiness of the cornea.

The lens may partially or fully luxate into the front chamber of the eye, causing acute glaucoma (increased pressure within the eye).

Sometimes the lens may fall backwards into the posterior (back) chamber of the eye, which may displace the vitreous forwards.

This may then also lead to a blockage of drainage of fluid from the eye and a secondary glaucoma.

Glaucoma (increased fluid pressure within the eye) is a common consequence of lens luxation, and can rapidly lead to blindness.

Lens luxation is a veterinary emergency, and if you notice the signs of PLL in your dog’s eye you should see your vet immediately.

Diagnosis is by examination of the inside of the eye by a veterinarian, and possibly an ultrasound of the eye.

Treatment of PLL is aimed at reducing the fluid pressure within the eye and preserving vision in acute cases, then removing the lens surgically.

Blind eyes may be removed to treat pain.

Genetic testing is available for the screening of breeding animals, so that two carriers (or any affected animals) are not bred.

Although the disease is treated as a recessive one, carrier animals will also occasionally develop lens luxation.