Heart Disease in Sphynx Cats

HCM is probably the first thing that comes to mind when heart disease in Sphynx cats is mentioned and, understandably, this is also what breeders tend to concentrate on when they talk about heart screening. But HCM is not the only heart condition that has actually been reported in the breed and an echocardiogram checks considerably more than whether the ventricular wall is thickened.

There is a fairly old but still very useful Sphynx study by Chetboul et al. from 2012. They examined 114 Sphynx cats, from six months to ten years old. Of those, 75 had normal cardiovascular examinations and abnormalities were found in 39.

Twenty-three had HCM, which is probably the part of this paper most people know about. What tends to get much less attention is what was found in the other 16 cats. They had congenital heart disease and 15 of the 16 had mitral valve dysplasia, either by itself or together with another defect.

That is quite difficult to dismiss as an incidental finding. In this particular population MVD was found in almost every cat with congenital heart disease, and the authors themselves concluded that both HCM and mitral valve dysplasia appeared to be relatively common in the Sphynx cats they examined.

So when we talk about an annual “HCM scan”, the name is actually a little misleading. The cardiologist is not there just to measure the left ventricular wall and write HCM negative or positive on a certificate. The valves, chambers, blood flow and function of the heart are being examined as well. A Sphynx can have no evidence of HCM and still have an abnormal heart.

HCM

HCM itself is hypertrophy of the heart muscle, predominantly affecting the left ventricle. In practice this means that areas of the ventricular wall become thicker than they should be. How much this matters to an individual cat depends on considerably more than the thickness alone.

Some cats with HCM remain clinically well for years. In others the disease progresses, the left atrium enlarges and eventually complications such as congestive heart failure or arterial thromboembolism can occur. Sudden death is possible as well, although it is not the inevitable outcome of an HCM diagnosis.

Age is particularly important in the Sphynx because this is not a disease that can be excluded for life by scanning a young cat once.

Chetboul et al. already found an association between increasing age and HCM in their Sphynx population. Silverman et al. later reported familial clustering of the disease, adding evidence for a hereditary component.

The New Zealand study by Seo et al. is useful here for another reason. They followed 55 Sphynx cats that were apparently healthy when enrolled. Twenty were eventually diagnosed with HCM, at a median age of 5.8 years.

I would be very careful with turning that into a breed prevalence figure. Twenty out of 55 is roughly 36%, but it does not follow that 36% of Sphynx cats everywhere will develop HCM. It tells us what happened in that cohort.

For screening purposes, something else from that study is probably more important: disease appeared over time.

And that changes the meaning of a negative scan. If a Sphynx is normal at twelve months, we know that no detectable HCM was present at twelve months. We do not know that the cat will still be normal at six years old.

Mitral valve dysplasia

MVD deserves its own section because the Chetboul numbers are difficult to ignore, yet it receives nowhere near the attention HCM does in discussions about Sphynx screening.

The mitral valve separates the left atrium from the left ventricle. In mitral valve dysplasia the valve, or the structures associated with it, have developed abnormally. Unlike HCM appearing later in life, this is a congenital defect, so the abnormal anatomy is there from birth.

There is a wide spectrum. A mild abnormality may have little clinical consequence. With more significant dysplasia the valve may not close properly, allowing blood to regurgitate back into the left atrium. What happens to the heart then depends on the severity of the defect and its haemodynamic effect.

For me, the relevant part in relation to Sphynx screening is much simpler: 15 of 16 cats with congenital heart disease in the Chetboul study had MVD.

This is why I don’t particularly like describing echocardiography simply as an “HCM test”. If the report only tells an owner that their cat is “HCM clear”, quite a lot of information about what was actually examined has disappeared in that description.

Other cardiomyopathies

HCM is obviously the big one in Sphynx, but it is worth remembering that HCM and cardiomyopathy are not interchangeable words. HCM is one type of cardiomyopathy. Cats can also have restrictive cardiomyopathy, dilated cardiomyopathy and a few patterns that do not sit very neatly in either group.

RCM is quite different from HCM. You do not necessarily get these obviously thickened ventricular walls. The problem is more that the ventricle becomes stiff and does not relax properly, and the atria can become very large as a result. So you can have a cat with serious heart muscle disease without having the classic picture people associate with HCM.

DCM goes in almost the opposite direction. The ventricle becomes enlarged and the muscle does not contract properly.

Most people probably associate feline DCM with taurine deficiency, and for good reason. It used to be seen far more often before the taurine connection was understood and commercial diets were changed. But DCM did not simply stop existing when that happened. Cats can still develop it for other reasons.

What I would not do is take either RCM or DCM and add them to a list of “Sphynx heart diseases” as though we have the same evidence for them that we have for HCM. We don’t, at least not at the moment.

That does not mean a Sphynx cannot get RCM or DCM. Of course it can. It means I have not found good evidence showing that the breed itself is particularly predisposed to either one.

And then there are findings that are not really separate cardiomyopathies

This is another reason I think “HCM scan” is a slightly awkward name for what the cardiologist is actually doing.

An echo can pick up congenital valve defects, abnormal blood flow, changes in chamber size, problems affecting the right side of the heart and things that occur as part of HCM itself.

SAM is one of them.

It means systolic anterior motion of the mitral valve. Basically, during contraction part of the mitral valve moves into the outflow tract of the left ventricle. If it moves far enough it can partly obstruct the blood leaving the heart and you get turbulent flow.

This is something the cardiologist can actually watch happening on the echo. It is not something you are going to understand from seeing “5.4 mm” written on a certificate.

The same goes for the left atrium.

If the ventricle becomes stiff and filling pressures rise, the left atrium can enlarge. And once that happens, I am much more interested in the whole picture than I am in arguing about a fraction of a millimetre of ventricular wall.

Two cats can have similar wall measurements and still not have similar disease.

One may have a completely normal left atrium. The other may already have significant enlargement.

Those are not equivalent scans.

Left atrial enlargement

The left atrium is something I would pay quite a lot of attention to on the report, not only the ventricular measurements.

With HCM the ventricle can become stiff and filling becomes more difficult. Pressure then starts to affect the atrium and it may enlarge. How large it is can tell us quite a lot about what is going on with the heart, and two cats with similar wall measurements can look very different once you start looking at the atrium as well.

This becomes more important as the disease progresses because a large left atrium is also associated with complications we really do not want to see.

One is congestive heart failure. Fluid can accumulate in or around the lungs and sometimes the first thing noticed at home is simply a change in breathing. The cat may breathe faster at rest or start putting more effort into breathing. At the other end of the scale you can have a cat becoming seriously unwell very quickly.

Blood clots are another problem. They can form in cats with heart disease, particularly where there is significant left atrial enlargement and poor atrial function. If a clot leaves the heart and enters the arterial circulation it can lodge elsewhere, most famously where the blood supply divides towards the hind legs. That is the classic feline arterial thromboembolism people often call a saddle thrombus.

There are large feline HCM studies looking at this, so it is not some theoretical complication added to a list. (PubMed)

Arrhythmias belong somewhere in this discussion too, although there is not much point making them sound like one single thing. An abnormal rhythm can be relatively insignificant or very serious depending on what it actually is and what else is happening with the heart. In severe disease arrhythmias can affect how well the heart pumps and can be associated with collapse or sudden death.

About the 6 mm cut-off

I have a bit of a problem with the way 6 mm is sometimes discussed, because it can sound as though feline HCM works around one absolute number. Measure the wall, check which side of 6 mm it falls on and there is your answer.

That is not really how an echo is interpreted.

The number itself is useful, but body size is part of the problem with treating it as a universal cut-off. Chetboul actually looked at this in the healthy Sphynx cats in the 2012 study and found that weight affected several cardiac measurements, including ventricular wall thickness. Their conclusion was quite specific on this point: body weight should be taken into account when myocardial wall thickness is interpreted in Sphynx cats. (PubMed)

And I think this is one of those things that makes much more sense when you stop looking at the reference number for a moment and look at the actual cats. A tiny 2.5 kg female and a 6 kg male can both be perfectly normal adults, but there is a very substantial difference in body size between them.

This was later looked at in cats more generally. A study of 150 healthy cats found enough of an effect from body weight that the authors recommended weight-based reference ranges, particularly for cats at either end of the size range. (PubMed) There is also the much bigger dataset of almost 20,000 cats, again showing a clinically relevant relationship between body weight and cardiac dimensions. (PubMed)

I am not saying that a measurement of 6 mm should be ignored in a large cat. That would be going too far in the other direction. I just don’t think a measurement should be separated from the animal it came from.

There is also the question of where the measurement is taken because HCM does not have to thicken the ventricle uniformly.

A recent case report in a Sphynx described apical HCM. Most of the abnormal thickening was towards the apex of the left ventricle, and the cat later progressed to left atrial enlargement and congestive heart failure.

Obviously one cat cannot tell us how often this happens in Sphynx. I would not try to make that claim from a case report. What the case illustrates rather well is that a normal-looking measurement in one part of the ventricle does not guarantee that every other part looks the same.

And a normal scan?

It means the heart looked normal at that examination. I don’t think it needs to mean anything more complicated than that.

Where I would be cautious is turning it into a permanent status attached to the cat.

We know HCM can appear later in Sphynx. Chetboul found more HCM with increasing age and the Seo cohort is particularly useful because those cats were actually followed over time. Some developed HCM later despite entering the study apparently healthy.

This is also why, when I look at a breeding line, I am much more interested in the ages attached to the scans than simply counting how many relatives have a normal result. A normal one-year-old is good to see. A cat that is still normal at eight after repeated screening is giving us a different piece of information.

Neither result predicts the future with certainty, but the second cat has simply had many more years in which HCM could have shown itself.

And that is probably the easiest way to explain why I don’t see heart screening as a one-off test. The information becomes more useful as the cat gets older and as there are previous scans to compare with.

What I actually want to know from the scan

When I receive a heart report, I am not really looking for a number below 6 mm and then stopping there.

I want to know what the rest of the heart looks like.

Are the different areas of the ventricular wall normal for this cat? What does the left atrium look like? Are the valves normal? Is the blood flow normal? Is there SAM? Is there anything congenital there? Has anything changed since the previous examination?

That last question becomes particularly useful once a cat has several scans behind it.

And this brings me back to the slightly misleading name we use for these examinations. We call them HCM scans because HCM is the main reason we screen Sphynx cats in the first place. But the cardiologist is not scanning HCM.

They are scanning the heart.

References

1. Chetboul et al. 2012 — Sphynx-specific echo, HCM, MVD, age, pedigree

2. Seo et al. 2024 — longitudinal Sphynx HCM

3. Luis Fuentes et al. 2020 — ACVIM cardiomyopathy classification

4. Häggström et al. 2016 — 19,866 cats / body weight

5. Schober et al. 2017 — 150 healthy cats / weight-based ranges

6. Fox et al. 2000 — ARVC in cats