Persistent Pupillary Membrane (PPM) in Veterinary Medicine

1. What Is PPM?

Embryology Recap: During fetal development, the lens and anterior chamber are covered by a vascular membrane called the tunica vasculosa lentis (TVL), which includes the pupillary membrane anteriorly. This membrane supplies the developing lens with nutrients. In most species, this membrane completely regresses before or shortly after birth. When regression is incomplete, the remnants = PPM.

2. Species Affected

3. Classification of PPM

Type 1 – Iris-to-Iris (Iris-Iris)

• Strands run from one part of the iris to another part of the iris
• Most common and mildest form
• Usually do NOT affect vision
• No lens or corneal involvement
• Appear as thin, thread-like bands crossing the pupil

Type 2 – Iris-to-Lens (Iris-Lenticular)

• Strands run from the iris to the anterior lens capsule
• Can cause focal anterior lens opacity (cataract) at attachment point
• The area where PPM attaches to the lens may develop into a "rosette" or "pie-crust" cataract
• Potentially vision-threatening if large or centrally located
• Clinically significant

Type 3 – Iris-to-Cornea (Iris-Corneal)

• Strands run from the iris to the posterior corneal surface
• Can cause focal corneal opacity (leukoma) at the attachment site
• May impair vision if in the visual axis
• Most clinically significant type

Type 4 – Sheets / Plaques

• Large, sheet-like membranes rather than individual strands
• Can significantly obstruct the visual axis
• Rare but severe

4. Clinical Signs & Presentation

Visual Appearance

• Fine, brownish or pigmented filaments or strands crossing the pupil
• May look like a spider web across the pupil
• Originate from the iris collarette (not the pupillary margin — this differentiates them from synechiae)
• May be single or multiple strands
• Present in one or both eyes (bilateral is common, especially in dogs)

Symptoms Depending on Type

5. Diagnosis

A. Ophthalmic Examination

• Pupil dilation (mydriasis) using:
  • Tropicamide 1% (drug of choice) — dilates pupil to reveal full extent of PPM strands
  • Allows complete visualization of strand insertion points
• Slit-lamp biomicroscopy — most accurate tool to trace strand origin and insertion
• Retroillumination technique — backlight through the lens, PPM strands appear as dark silhouettes against the tapetal reflex

B. Differentiation from Other Conditions

C. Electroretinogram (ERG)

• Used if vision impairment is suspected to evaluate retinal function behind any opacity

6. Genetics & Breeding Implications

• In Basenjis, PPM is autosomal dominant with incomplete penetrance — extensively studied by OFA (Orthopedic Foundation for Animals) and CERF (Canine Eye Registry Foundation, now part of OFA-CAER)
• Affected dogs should NOT be used for breeding in lines where PPM severity is high
• Iris-to-iris forms in Basenjis are graded on a severity scale (Grade 1–5):
  • Grade 1–2: Minor, non-vision-threatening
  • Grade 3–5: Significant, breeding discouraged

7. Treatment

A. No Treatment Required (Most Cases)

• Iris-to-iris PPM without vision impact = monitor only
• Many animals live normal lives with no intervention
• Re-examine annually (especially young animals, as strands sometimes regress in the first few months of life)

B. Medical Management

C. Surgical Management

1. Surgical membrane dissection / membranectomy
   • PPM strands are carefully cut and removed under general anesthesia
   • Performed with fine iris scissors or vitrectomy instruments
   • Post-op image (from database) shows the result after bilateral PPM resection — clear visual axis achieved:

1. Phacoemulsification (cataract surgery)
   • If iris-to-lens PPM has caused a significant cataract affecting vision
   • Combined with intraocular lens (IOL) implantation
   • Performed by a veterinary ophthalmologist (DACVO)
2. Nd:YAG laser treatment
   • Laser photodisruption of PPM strands — used in human medicine, experimental in veterinary patients
   • Limited availability; requires specialized equipment

8. Prognosis

9. Examples to Illustrate

Example 1 — Basenji Dog, 8 Weeks Old

Example 2 — Domestic Shorthair Cat, 4 Months Old

10. What to Tell the Owner (Owner Communication Guide)

"During development before birth, the eye has a temporary 'covering' over the pupil that supplies blood to the growing lens. Normally, this covering disappears before or shortly after birth. In your pet, small strands of this covering remained behind — these are called Persistent Pupillary Membranes."

"In most cases, no. The most common form — where the strands connect only to the iris itself — does not affect vision at all and requires no treatment. It is simply something we monitor once a year." "However, if the strands are attached to the clear outer surface of the eye (cornea) or to the lens, they can sometimes cause cloudiness. This is less common but may need treatment or referral to a specialist."

"For the vast majority of animals with PPM, the answer is no. Most live completely normal, full-quality lives with no vision problems. Only the rare, severe forms involving large membranes or significant cataracts could potentially affect vision, and even those can often be managed or surgically treated."

"PPM has a hereditary component in certain breeds, especially Basenjis. If you plan to breed your pet, I would recommend having a formal eye certification done through OFA-CAER to document the findings and inform breeding decisions."

"Watch for any cloudiness or white spots developing in the eye, squinting, excessive tearing, or if your pet seems to be bumping into things. If you notice any of these, contact us promptly."

"I recommend an eye check every 6–12 months to make sure things remain stable. No special care is needed at home for now."

Summary Table