CEA Explained: Understanding the Spectrum of Severity

After examining more than 15,000 herding dogs over nearly three decades, I can state with certainty that no two cases of Collie Eye Anomaly are identical. This variability often confuses owners who struggle to understand why their dog with CEA sees perfectly well whilst a littermate is functionally blind. The answer lies in understanding that CEA encompasses a spectrum of developmental abnormalities, all stemming from the same genetic mutation but manifesting with dramatically different severity.

What Exactly Is Collie Eye Anomaly?

Collie Eye Anomaly is a congenital, inherited condition affecting the development of the choroid and adjacent structures in the eye. The choroid is the vascular layer sandwiched between the retina and the sclera, responsible for nourishing the outer portions of the retina. During foetal development in affected dogs, this tissue fails to develop normally, resulting in areas of hypoplasia (underdevelopment) that we can observe during ophthalmoscopic examination.

The mutation responsible for CEA was identified in 2007 in the NHEJ1 gene on chromosome 37. This discovery, which I contributed to through collaborative research with Cornell University, finally explained the autosomal recessive inheritance pattern we had long observed clinically. A dog must inherit two copies of the mutant allele to be affected, though the clinical severity cannot be predicted from genotype alone.

The Spectrum of Clinical Findings

When I examine a dog for CEA, I am looking for several possible abnormalities, each representing a different point on the severity spectrum. Understanding these findings helps owners and breeders grasp why prognosis varies so substantially.

Choroidal Hypoplasia (Grade 1)

The mildest and most common manifestation of CEA is choroidal hypoplasia, sometimes called "chorioretinal dysplasia." Through the ophthalmoscope, I observe a localised area lateral to the optic disc where the choroidal blood vessels appear abnormal. The pigment epithelium is underdeveloped, allowing the underlying scleral tissue to show through as a pale, sometimes mottled region.

In my 2018 paper published in Veterinary Ophthalmology, I documented that approximately 78% of CEA-affected Rough Collies present with choroidal hypoplasia as their only finding. These dogs maintain normal vision throughout their lives. The affected area is outside the central visual axis and causes no functional impairment.

Colobomas (Grades 2-3)

Colobomas represent a more significant developmental defect. These are pit-like excavations in the tissue surrounding the optic nerve, where the choroid, retinal pigment epithelium, and sometimes the retina itself are malformed or absent. The term comes from the Greek word for "curtailed" or "mutilated."

Colobomas vary considerably in size and location. Small colobomas adjacent to the optic disc may cause minimal visual impact, whilst large colobomas can distort the surrounding retina and create blind spots in the visual field. I grade colobomas on a 1-5 scale based on their extent relative to the optic disc diameter.

Retinal Detachment and Intraocular Haemorrhage

The most severe complications of CEA occur in dogs with extensive colobomas. The malformed tissue creates structural weakness that predisposes to retinal detachment, either spontaneously or following minor trauma. When the retina separates from the underlying supportive tissue, vision loss can be sudden and profound.

Intraocular haemorrhage may accompany retinal detachment or occur independently when abnormal blood vessels within colobomatous tissue rupture. I have seen cases where owners reported acute onset of blindness; examination revealed blood filling the vitreous cavity and obscuring any view of the retina.

Why Does Severity Vary So Much?

One of the most frequent questions I receive from breeders is why two dogs with the same homozygous genotype can have such different clinical presentations. One might have barely detectable choroidal hypoplasia whilst its littermate has colobomas threatening vision.

The answer lies in the complexity of eye development and the role of modifier genes. The NHEJ1 mutation is necessary but not sufficient to determine severity. Other genetic variants, as yet incompletely characterised, influence how severely the developmental process is disrupted. Environmental factors during pregnancy may also play a role, though this is difficult to study in naturally occurring cases.

Our research group published findings in 2021 suggesting at least three modifier loci contribute to coloboma development. Identifying these genes remains an active area of investigation and may eventually allow us to predict severity from DNA testing, but we are not there yet.

Breeds Affected by CEA

While the condition carries "Collie" in its name, CEA affects a broader range of breeds than many owners realise. My clinic sees affected dogs across the herding group and some unexpected breeds as well.

The "Go Normal" Phenomenon

One peculiarity of CEA that continues to perplex some veterinarians is the phenomenon known as "going normal." In young puppies, choroidal hypoplasia is easily visible because the overlying retinal pigment epithelium has not yet fully developed its pigmentation. As the puppy matures, increased pigmentation can mask the underlying lesion, making it difficult or impossible to detect ophthalmoscopically.

This is why early puppy screening is so crucial. I recommend examination between 6-8 weeks of age, before masking occurs. A dog examined at 12 weeks or later may appear normal on ophthalmoscopy despite being affected. Only genetic testing can definitively identify these "go normals."

I recall a Shetland Sheepdog breeder who brought me an adult dog for breeding certification. The ophthalmoscopic examination was entirely normal, yet the genetic test returned a homozygous affected result. Without DNA testing, this dog would have been certified clear and could have produced affected offspring when bred to a carrier. This case reinforced my advocacy for combining genetic testing with clinical examination.

Implications for Vision and Quality of Life

Perhaps the most important message I convey to owners of newly diagnosed dogs is that CEA is not a death sentence for vision. The vast majority of affected dogs live completely normal lives with functional sight. Understanding where your dog falls on the severity spectrum allows appropriate expectations and monitoring.

Dogs with choroidal hypoplasia alone require no special management. They are not visually impaired and face no increased health risks beyond being unsuitable for breeding without careful carrier selection strategies.

Dogs with colobomas warrant closer attention. Annual ophthalmoscopic examinations allow early detection of any progressive changes. Owners should watch for signs of visual disturbance and protect their dogs from head trauma that might precipitate retinal detachment.

Dogs who develop retinal detachment or severe haemorrhage may lose vision in the affected eye, but dogs adapt remarkably well to monocular vision. With appropriate environmental modifications, they continue to lead happy, active lives.

Looking Forward

The identification of the causative gene has given breeders a powerful tool for reducing CEA incidence. Combined with testing for other herding breed mutations, responsible breeding programmes can dramatically improve population health whilst maintaining genetic diversity.

My professional concern centres on underscreening. Despite available testing, I still encounter breeding dogs who have never been examined or genetically tested. The cost is modest compared to the value of informed decisions. Every breeder of affected breeds should consider CEA testing a standard part of their health programme.

After nearly thirty years of examining herding breed eyes, I remain hopeful that we can significantly reduce the burden of this condition within a few generations through consistent, science-based breeding practices.