PLL

Primary Lens Luxation

Testing method

Ophthalmoscopic and slit-lamp biomicroscopic examination for early signs of lens instability, including iridodonesis (trembling of the iris), phacodonesis (trembling of the lens), and the appearance of an aphakic crescent (a visible gap between the lens edge and the pupil margin). Increased intraocular pressure may also be present. A DNA test is available for most affected breeds and is highly recommended alongside annual clinical examination within the at-risk age range.

What is primary lens luxation?

The lens of the eye is held in position by a series of fine fibres called zonules. In primary lens luxation (PLL), these zonules break down as a result of an inherited genetic defect, causing the lens to become unstable and eventually dislocate from its normal position. A lens that has moved partially out of position is said to be subluxated; a lens that has moved completely is luxated.

A luxated lens is a serious ocular emergency. Depending on the direction of displacement, it can block the drainage of fluid from the eye, causing a rapid and painful rise in intraocular pressure (secondary glaucoma), or it can move forward into the anterior chamber where it may contact and damage the cornea. Without prompt treatment, the elevated pressure causes permanent damage to the optic nerve and retina, leading to irreversible blindness.

The condition affects both eyes, though the two eyes do not usually luxate at the same time. There is typically a lag of weeks to months between the first and second eye. This means that once PLL is identified in one eye, close monitoring of the fellow eye is essential.

Breeds affected

PLL is certified under the BVA scheme in the Border Collie, Miniature Bull Terrier, Smooth Fox Terrier, Wire Fox Terrier, Lancashire Heeler, Parson Russell Terrier, Sealyham Terrier, and Tibetan Terrier. The condition has also been described in several other terrier breeds not currently on the BVA schedule.

DNA testing

A DNA test targeting the ADAMTS17 splice site mutation is available and is widely used in the affected terrier breeds and the Lancashire Heeler. The test identifies dogs as clear, carrier, or affected. Because approximately 5% of carrier dogs can develop clinical PLL, carrier status is not without risk and carrier dogs should still be monitored clinically within the at-risk age range.

The DNA test is not applicable to all breeds — the Shar Pei and Brittany Spaniel, for example, do not carry the ADAMTS17 mutation despite being susceptible to lens luxation. For these breeds, clinical examination remains the primary screening method.

Early signs to watch for

Between clinical examinations, owners of at-risk breeds should be aware of early warning signs that may indicate lens instability or luxation: a visible trembling or shimmering of the iris when the dog moves its head (iridodonesis), sudden onset of eye pain or squinting, redness of the eye, cloudiness of the cornea, or a visible change in the appearance of the pupil. Any of these signs warrant urgent veterinary attention - do not wait for a scheduled screening appointment.

What the examination involves

The BVA examination for PLL involves slit-lamp biomicroscopy and ophthalmoscopy to look for early signs of zonular breakdown. The examination requires pupil dilation to allow full assessment of the lens position and the zonules. Annual examination from 3 years of age is recommended for all at-risk breeds, continuing until at least 7 years of age.

At a glance

Inheritance: Autosomal recessive in most affected breeds. In the majority of terrier breeds and the Lancashire Heeler, the genetic defect is a splice site mutation in the ADAMTS17 gene, for which a DNA test is available. Approximately 5% of heterozygous carriers can develop PLL, so carrier status is not entirely without risk. The Border Collie has a possibly autosomal recessive pattern but may involve a different mechanism. Note that some breeds reported to be affected by PLL — notably the Shar Pei and Brittany Spaniel — do not carry the ADAMTS17 mutation, so the DNA test is not applicable to those breeds.
Age of onset: Clinical signs are not usually observed before 3 years of age or after 7 years, with a mean age of onset around 4 years. The Border Collie tends to present similarly, between 4 and 7 years of age.
Prognosis: Primary lens luxation is a serious condition requiring prompt veterinary attention. A luxated lens can obstruct aqueous drainage, causing acute glaucoma, or can damage the cornea. The condition is bilateral, though there is usually a lag between the first and second eye becoming affected. Early detection through regular screening allows affected dogs to be monitored so that treatment can be initiated promptly when signs develop.

Book an eye test

Select a clinic location and available date online.