Primary Lens Luxation (PLL) in the Biewer Terrier*

Understanding an Inherited Eye Disorder

Educational Information for Biewer Terrier Owners

Introduction

Primary Lens Luxation (PLL) is an inherited eye disorder that can lead to severe pain and permanent blindness if not recognized and treated promptly. Although PLL has not been reported as a common clinical disease in the Biewer Terrier, the breed carries ancestry from the Yorkshire Terrier, a breed known to harbor the ADAMTS17 mutation associated with PLL. For this reason, the Biewer Terrier Club of America (BTCA) has been recommending DNA testing for Primary Lens Luxation prior to breeding as part of its CHIC health screening recommendations.

What is Primary Lens Luxation?

The lens is normally suspended behind the iris by thousands of tiny fibers called zonular ligaments. These fibers hold the lens in its proper position so that light can be focused accurately onto the retina.

In dogs affected with Primary Lens Luxation, these supporting fibers gradually weaken and break. As a result, the lens becomes unstable and eventually shifts from its normal position.

The lens may:

  • Partially displace (lens subluxation), or
  • Completely displace (lens luxation)

When the lens moves into the front of the eye, it can rapidly obstruct normal fluid drainage, causing acute glaucoma, one of the most painful emergencies in veterinary ophthalmology. Immediate treatment is essential to preserve vision.

Cause

The most common inherited form of PLL is associated with a mutation in the ADAMTS17 gene.

This gene is responsible for producing a protein involved in maintaining the strength and integrity of the microscopic fibers that suspend the lens.

When both copies of the gene carry the mutation, these fibers become progressively weaker with age, allowing the lens to dislocate.

Recent research has shown that while the ADAMTS17 mutation is the major genetic cause of PLL, additional genetic factors likely influence whether and when the disease develops. This helps explain why some dogs carrying the mutation never develop clinical disease while others do.

How is PLL Inherited?

Most laboratories report PLL as an autosomal recessive inherited condition, although more recent evidence suggests that dogs carrying one copy of the mutation may have a small increased lifetime risk of developing lens instability compared with genetically clear dogs.

Clinical Signs

PLL most commonly develops between 3 and 8 years of age, although it may occur earlier or later.

Owners must seek immediate veterinary attention if they observe:

  • Sudden redness of the eye
  • Excessive tearing
  • Squinting
  • Cloudy appearance of the eye
  • Enlarged pupil
  • Vision loss
  • Eye pain
  • Rubbing at the eye

These signs represent a true ophthalmic emergency. Delaying treatment may result in permanent blindness within hours to days.

Diagnosis

Diagnosis is made by a board-certified veterinary ophthalmologist (ACVO Diplomate) through a complete ophthalmic examination.

Diagnostic evaluation may include:

  • Slit-lamp examination
  • Measurement of intraocular pressure
  • Gonioscopy when indicated
  • Dilated retinal examination

Dogs showing lens instability should receive prompt evaluation before complete luxation develops.

Treatment

Treatment depends upon whether the lens is partially or completely displaced.

Management may include:

  • Emergency medications to reduce intraocular pressure
  • Anti-inflammatory medications
  • Surgical lens removal
  • Long-term glaucoma management

Recent veterinary ophthalmology research continues to evaluate surgical techniques for anterior lens luxation. Current evidence suggests that early intervention provides the best opportunity for preserving vision and minimizing complications.

DNA Testing

A DNA test is available through several veterinary genetic laboratories.

Testing identifies whether a dog is:

  • Clear
  • Carrier
  • Affected

DNA testing allows breeders to make informed breeding decisions that reduce the likelihood of producing affected puppies while preserving valuable genetic diversity.

Results may also be submitted to the Orthopedic Foundation for Animals (OFA) DNA database.

 BTCA Recommendation

The Biewer Terrier Club of America recommends Primary Lens Luxation (PLL) DNA testing before breeding as part of its current CHIC health screening recommendations.

Because inherited eye disease cannot always be identified by DNA testing alone, the BTCA also recommends routine ACVO (CAER) eye examinations throughout a breeding dog’s life, since some ocular disorders develop later in adulthood.

*This educational material is intended for informational purposes only and is not a substitute for professional veterinary diagnosis or treatment. Owners should consult their veterinarian or a board-certified veterinary ophthalmologist regarding any concerns about their dog’s eye health.

Educational Resources & Further Information