When I hand a breeder an examination report describing their dog's choroidal hypoplasia as "Grade 2, bilateral, temporal to the optic disc," I can see the questions forming before they are asked. What does Grade 2 mean? Is it worse than Grade 1? Will it progress? Should this dog be removed from the breeding programme? These are reasonable questions, and the answers matter enormously for breeding decisions. Yet grading systems for choroidal hypoplasia remain poorly explained in most resources available to breeders. This article aims to change that.
Why We Grade Choroidal Hypoplasia
Choroidal hypoplasia is the hallmark finding of Collie Eye Anomaly and by far its most common manifestation. As I have discussed in my overview of the CEA severity spectrum, approximately 78% of affected Rough Collies present with choroidal hypoplasia as their sole finding. But not all choroidal hypoplasia is identical. The underdeveloped choroidal tissue varies in extent, location, and the degree to which normal structures are disrupted.
Grading provides a standardised language for describing these variations. Without it, examination reports would rely on subjective descriptions that vary between examiners and over time. A grading system allows breeders to compare findings between dogs, track changes within a dog across examinations, and make informed decisions about which pairings are likely to produce milder or more severe offspring.
I must emphasise upfront that no single universally adopted grading system exists for choroidal hypoplasia. Several classification schemes are used internationally, and the terminology varies between the American College of Veterinary Ophthalmologists (ACVO), European College of Veterinary Ophthalmologists (ECVO), and individual clinical practices. What I present here synthesises the most widely used approaches and reflects the system I employ in my own practice.
The Grading Scale for Choroidal Hypoplasia
I classify choroidal hypoplasia on a four-point scale that considers three factors: the extent of the lesion, the degree of choroidal thinning, and the visibility of underlying scleral tissue.
Grade 1: Focal, Mild
A small area of choroidal pallor, typically less than 2 optic disc diameters in extent, located temporal to the optic disc. The choroidal vasculature is visible but appears slightly attenuated or irregular. The underlying sclera is not prominently visible. This is the mildest detectable form of choroidal hypoplasia.
Grade 2: Moderate Extent, Mild to Moderate Thinning
A larger area of choroidal pallor, spanning 2-4 optic disc diameters. The choroidal vessels are clearly abnormal, with reduced density or calibre. The sclera may be faintly visible through the thinned choroid. The retinal pigment epithelium shows reduced pigmentation over the affected area.
Grade 3: Extensive, Moderate Thinning
A broad area of choroidal hypoplasia exceeding 4 optic disc diameters. The choroidal vasculature is markedly reduced, and the white scleral tissue is clearly visible through the affected region. The retinal pigment epithelium is significantly depigmented. The lesion may extend in multiple directions from the optic disc.
Grade 4: Extensive, Severe Thinning
The most severe choroidal hypoplasia without coloboma formation. Very large area of nearly absent choroidal tissue with prominent scleral show. The choroidal vasculature is sparse or absent over the affected region. Often approaches the appearance of a coloboma at its margins, and careful examination is required to distinguish Grade 4 choroidal hypoplasia from early coloboma formation.
What Each Grade Means Clinically
Let me translate these clinical descriptions into practical meaning for owners and breeders.
Grades 1 and 2: The Majority of Cases
In my experience, approximately 85% of dogs with choroidal hypoplasia fall into Grades 1 or 2. These dogs have functionally normal vision and will maintain it throughout their lives. The affected area lies outside the central visual axis and causes no detectable visual impairment. Most owners of Grade 1 or 2 dogs would never know anything was amiss without examination.
These dogs require no special management, no activity restrictions, and no ongoing treatment. Annual ophthalmoscopic examinations are prudent but primarily for monitoring purposes rather than because progression is expected. I have followed hundreds of Grade 1 and 2 dogs over their lifetimes, and the choroidal hypoplasia remains stable in the vast majority.
Case Example: Grade 1 Bilateral
A five-year-old Rough Collie I have examined annually since puppyhood illustrates the stability of mild choroidal hypoplasia. At his initial 7-week examination, I documented bilateral Grade 1 choroidal hypoplasia, each lesion approximately 1.5 disc diameters in size, positioned temporal to the optic disc. Over five years and five annual examinations, the lesions have not changed in size, appearance, or location. His vision is clinically normal by every measure. His owner competes with him in agility, where he navigates courses with precision that would be impossible with significant visual compromise.
Grade 3: Warranting Closer Attention
Grade 3 choroidal hypoplasia does not typically impair central vision, but the extent of choroidal thinning places these dogs in a category that merits closer monitoring. The larger affected area means more retinal tissue is supported by compromised choroid, and whilst complications remain uncommon, the statistical risk is higher than for Grades 1 or 2.
I recommend annual ophthalmoscopic examination for Grade 3 dogs and advise owners to watch for any behavioural signs of visual change. Practically speaking, most Grade 3 dogs live entirely normal lives, but informed monitoring provides early detection should complications develop.
Grade 4: The Borderline Cases
Grade 4 choroidal hypoplasia represents the severe end of the spectrum without reaching coloboma territory. These cases require careful differentiation from early colobomas, and I occasionally request a second opinion from a colleague when the distinction is genuinely ambiguous. Dogs with Grade 4 findings warrant more frequent monitoring, perhaps every 6-12 months, particularly during the first two years of life when complications are most likely to emerge if they are going to.
From a breeding perspective, Grade 4 findings in a dog raise the question of modifier gene load. Whilst we cannot yet test for modifier genes directly, a dog with severe choroidal hypoplasia may carry modifier variants that predispose to more severe expression in offspring. This does not mean such a dog should never be bred, but it is a factor I discuss with breeders when reviewing examination results.
Grading in the Context of Colobomas
Choroidal hypoplasia and colobomas are distinct but related findings within the CEA spectrum. Choroidal hypoplasia refers to underdevelopment of the choroidal tissue. Colobomas are focal excavations or pits in the tissue surrounding the optic nerve, representing a more severe developmental failure.
A dog may have choroidal hypoplasia alone, colobomas alone (though this is rare without some accompanying choroidal hypoplasia), or both findings simultaneously. When I examine a dog and find both choroidal hypoplasia and colobomas, I grade each finding separately. A report might read: "Grade 2 choroidal hypoplasia bilaterally, with a small coloboma (less than 1 disc diameter) adjacent to the optic disc in the left eye."
The presence of colobomas changes the clinical picture substantially. While choroidal hypoplasia of any grade is essentially a cosmetic finding with no functional impact in most dogs, colobomas carry structural risk including potential retinal detachment. Breeders should understand that the choroidal hypoplasia grade and the coloboma status are separate pieces of information, both important but for different reasons.
How to Use Grading Data in Your Breeding Programme
The genetic test for CEA returns a binary result for breeding purposes: clear, carrier, or affected. It does not distinguish between an affected dog with Grade 1 choroidal hypoplasia and one with Grade 4. This is where clinical grading provides additional value for breeding decisions.
Principle 1: Grade Correlates Loosely with Modifier Gene Load
Research into CEA modifier genes is ongoing, but clinical observation over decades has shown me that severity tends to run in families. Lines producing predominantly Grade 1 findings tend to continue producing mild cases. Lines with Grade 3 or 4 findings are more likely to produce puppies with colobomas. I discuss the [current state of modifier gene research and its implications for breeding](/articles/modifier-genes-cea-severity-current-research/) in a dedicated article. This pattern is not absolute, but it is a consistent enough finding to inform breeding decisions.
Principle 2: Favour Milder Grades When Using Affected Dogs
When a breeder must use an affected dog (because of its other outstanding qualities), preferring a dog with Grade 1 or 2 findings over one with Grade 3 or 4 is a reasonable selection criterion. Both dogs carry the same NHEJ1 genotype, but the milder phenotype suggests a more favourable modifier gene profile.
Principle 3: Track Grades Across Generations
Maintain records not just of genetic status but of clinical grades across your breeding programme. If you notice that a particular pairing consistently produces puppies with higher grades than either parent, this may indicate unfavourable modifier gene combinations. Conversely, pairings that produce lower grades suggest beneficial modifier combinations worth repeating.
| Breeding Scenario | Genetic Outcome | Grading Consideration |
|---|---|---|
| Clear x Clear | All puppies clear | No grading relevant; no affected offspring |
| Clear x Carrier | 50% clear, 50% carrier | No affected offspring to grade; carrier status determined by DNA only |
| Clear x Affected (Grade 1-2) | All puppies carriers | Favourable; mild parental phenotype suggests lower modifier burden |
| Clear x Affected (Grade 3-4) | All puppies carriers | Acceptable but note severity; monitor offspring if bred in next generation |
| Carrier x Carrier | 25% clear, 50% carrier, 25% affected | Grade affected puppies; severity data informs future decisions |
Principle 4: Do Not Overweight Grading
Grading provides useful supplementary information, but it should not override the fundamental breeding strategies based on genetic status. A Grade 1 affected dog is still affected and will pass the mutation to every offspring. A clear dog with no grading data at all is still the preferred mate from a CEA reduction standpoint. Grading helps refine decisions within the framework of genetic testing; it does not replace it.
Variability Between Examiners
I must address a practical reality that breeders encounter: grades assigned by different examiners may not always agree. Grading choroidal hypoplasia involves subjective judgement, particularly at the boundaries between grades. What I call Grade 2, another experienced ophthalmologist might classify as a high Grade 1 or a low Grade 3.
This inter-examiner variability is real but should not undermine confidence in the grading system. The broad categories are reliable. No competent examiner will confuse Grade 1 with Grade 4. Disagreements occur at boundaries and are typically clinically insignificant. If you seek a second opinion and receive a grade that differs by one step from the original, this does not indicate error; it reflects the inherent subjectivity of grading continuous biological variation.
For consistency within your breeding programme, I recommend using the same examiner for all dogs when possible. This ensures that grading comparisons between your dogs reflect true differences rather than examiner variation.
The ECVO and ACVO Examination Frameworks
The European College of Veterinary Ophthalmologists and the American College of Veterinary Ophthalmologists provide standardised examination protocols for hereditary eye disease screening. These frameworks describe findings in consistent language and facilitate record-keeping across the profession.
Under the ACVO examination scheme used in North America, the examiner records whether CEA-related findings are present and describes their nature and severity. The standardised eye examination provides a framework that ensures consistent documentation. Under the ECVO scheme used in Europe, similar standardised recording exists. Both systems document choroidal hypoplasia, colobomas, and any complications, though the precise format and terminology differ.
When receiving examination certificates from either system, breeders should look for the specific clinical description rather than relying solely on the pass/fail classification. A dog that "fails" due to mild Grade 1 choroidal hypoplasia has a very different clinical picture from one that fails due to Grade 4 findings with colobomas. The narrative description in the examiner's notes carries more breeding-relevant information than the categorical result alone.
Longitudinal Grading: Does Choroidal Hypoplasia Change Over Time?
One question I am frequently asked is whether choroidal hypoplasia grades change as a dog ages. The short answer is that the underlying lesion does not truly change, but its clinical appearance can shift in both directions.
In puppies, choroidal hypoplasia may appear more prominent because the overlying retinal pigment epithelium has not yet fully developed. As pigmentation increases with age, the choroidal hypoplasia can become harder to detect or may appear smaller. This is the "go normal" phenomenon, and it can make an initially Grade 2 lesion appear Grade 1 or even undetectable at subsequent examinations. This masking effect is especially pronounced in [Shetland Sheepdogs, where fundus pigmentation develops more rapidly](/articles/cea-in-non-collie-breeds-australian-shepherds-shetland-sheepdogs/) than in Collies.
Conversely, I have occasionally observed apparent enlargement of choroidal hypoplasia areas in older dogs, likely reflecting age-related thinning of the retinal pigment epithelium that unmasks previously hidden lesion margins. This does not represent progression of the disease but rather unmasking of the existing lesion.
For breeding purposes, the initial puppy examination at 6-8 weeks provides the most accurate clinical grading because it precedes the pigmentation changes that complicate later assessment. This is yet another reason I advocate strongly for early examination.
Practical Takeaways
Understanding choroidal hypoplasia grading empowers breeders to make nuanced decisions rather than treating all affected dogs identically. Here is what I want you to take from this article:
- Grades 1 and 2 are functionally insignificant. Dogs with these findings have normal vision and normal lives.
- Grade 3 warrants monitoring but rarely causes problems. Annual examinations are prudent.
- Grade 4 sits at the boundary of concern and merits careful evaluation for concurrent colobomas.
- Grading provides supplementary data for breeding decisions but does not change the fundamental importance of genetic testing.
- Severity tends to run in families, so tracking grades across generations informs your selection strategy. The [international programmes that have achieved the greatest CEA reductions](/articles/international-cea-prevalence-data-breeding-program-success-stories/) combine grading data with genetic testing in open databases.
- Use the same examiner consistently for meaningful comparisons within your programme.
- Early puppy examination at 6-7 weeks provides the most accurate grading before pigmentation masking occurs.
The grading system is a tool, not a verdict. It helps you understand where your dogs sit on the severity spectrum and guides the incremental improvements that, over generations, reduce both the frequency and severity of CEA in your lines. Combined with comprehensive hereditary eye disease screening, clinical grading ensures that breeding decisions rest on the most complete information available.