HCM in Cats: Testing Hypertrophic Cardiomyopathy
· By Dr. Tang
TL;DR — Hypertrophic cardiomyopathy (HCM) is the most common heart disease in cats, affecting roughly 15% of random-bred cats, and it is the leading cause of feline sudden death and saddle-thrombus paralysis. The reason it slips through care is that many HCM cats have no murmur — so the dog-style approach (wait for a loud murmur) fails. The correct screen is NT-proBNP first: above 265 pmol/L it separates cardiac from respiratory dyspnea with 90% sensitivity / 88% specificity. And here is the trap most owners miss: a negative genetic test does not clear a cat — only two MYBPC3 mutations are known, so at-risk breeds still need annual echo, not a one-time gene swab.
In Plain Terms
HCM makes the heart’s pumping chamber wall thicker, which sounds like it should pump better — but the opposite happens. A thick wall is stiff, so the chamber cannot relax and fill properly between beats. The cat compensates for a long time, then fails in one of two ways: fluid backs up into the lungs (heart failure), or a clot forms in the stretched chamber and shoots down to block the blood supply to the hind legs.
Neither event gives much warning. That is the whole problem.
How Common It Really Is
| Population | Prevalence |
|---|---|
| Random-bred cats | ~15% |
| Bengal | ~16.7% |
| Maine Coon | ~10.1% |
| Ragdoll | ~2.9% (higher in other cohorts) |
For context, HCM is at least as common in cats as in humans, where the figure is 1 in 500. A ~15% prevalence makes it one of the most common serious diseases a feline practitioner will ever see — yet it is under-diagnosed precisely because the early signs are invisible.
Why Auscultation Fails Cats
In dogs, the murmur is a reliable early flag for mitral valve disease. In cats, it is not. A large fraction of HCM cats have no audible murmur, or only a soft, transient one that disappears when the cat is stressed or panting in the exam room. Even a gallop rhythm — a better feline marker — is easy to miss in a purring cat.
The practical consequence: a cat without a murmur can still have significant HCM. Leading with NT-proBNP is how you catch the silent cases; treating auscultation as a clearance is how you miss them.
The Two Testing Tools
NT-proBNP — the screen and the triage
When the stiff, thickened heart struggles to fill, it stretches and releases NT-proBNP. The value of the test is two-fold:
- Screening at-risk breeds before symptoms.
- Triaging the dyspneic cat — heart failure or asthma/effusion? A value above 265 pmol/L distinguishes cardiac from respiratory causes with 90% sensitivity and 88% specificity, and a multicenter study of 167 dyspneic cats put the positive predictive value at 92% and negative predictive value at 85%.
Two caveats that keep it honest: (1) NT-proBNP is cleared by the kidneys, so a cat with concurrent CKD can read falsely high — interpret it with creatinine/SDMA; (2) a normal value does not rule out early HCM, because a mildly thickened heart may not yet be under enough load to raise the marker.
Echocardiography — the gold standard
Echo measures the left-ventricular wall thickness and left-atrial size, which is the key predictor of thromboembolism risk. It is the only test that confirms and stages HCM. The workflow that works: NT-proBNP screens and triages, echo confirms and characterizes, NT-proBNP then tracks progression between annual echoes.
The Genetics Trap
Two causative mutations are known, both in the cardiac myosin-binding protein C (MYBPC3) gene:
| Breed | Mutation | Discovered |
|---|---|---|
| Maine Coon | A31P | 2005 |
| Ragdoll | R820W (C820T) | 2007 |
A positive test is genuinely useful — it confirms high risk, and the A31P mutation exceeds 30% incidence in screened Maine Coons. But a negative test is not a clean bill of health. The reasons are concrete: there are almost certainly undiscovered mutations in other breeds, and sporadic (non-genetic) HCM exists in every population. In other words, the gene test tells you a cat is at risk; it cannot tell you a cat is safe.
The correct protocol for a Maine Coon or Ragdoll breeder: gene test to identify carriers, and start echocardiographic screening young (from 1–2 years) and repeat annually regardless of the gene result.
What HCM Actually Does to a Cat
Two catastrophic endpoints define the disease:
- Congestive heart failure — fluid accumulates in the lungs, presenting as fast, labored breathing (a resting respiratory rate above ~30–40 breaths/min in a sleeping cat is a red flag).
- Arterial thromboembolism (saddle thrombus) — a clot forms in the enlarged left atrium and lodges at the aortic bifurcation, causing sudden, painful hind-limb paralysis. This is frequently the first sign of HCM — the cat goes from “normal” to “dragging its back legs” in minutes.
Both are why screening before symptoms matters: once a cat presents in failure or with a clot, the disease has already declared itself, and the prognosis is far worse than if it had been caught on a wellness screen.
A Screening Protocol That Works
- Breed risk (Maine Coon, Ragdoll, Bengal, Sphynx): annual NT-proBNP + periodic echo from 1–2 years of age.
- Any cat with a murmur, gallop, or arrhythmia: echo to characterize.
- The dyspneic cat: NT-proBNP immediately — a value above 265 pmol/L points to the heart, below it to the lungs.
- Pre-anesthesia in older or at-risk cats: a screening NT-proBNP is cheap insurance.
- Known HCM: serial NT-proBNP to monitor progression and guide recheck intervals.
How the Test Runs
- Draw blood (serum/plasma).
- Load the feline NT-proBNP cartridge into a quantitative immunofluorescence analyzer.
- The analyzer quantifies the signal against a calibration curve.
- Read a numeric pmol/L result in 10–20 minutes — storable and comparable to the cat’s previous values.
Related products: Feline NT-proBNP Test · FIA680 Analyzer · FIA880 Analyzer
FAQ
What is HCM in cats?
1 thickened wall, 2 endpoints — HCM thickens the left ventricular wall, reducing filling (diastolic dysfunction). It is the most common feline heart disease and the leading cause of feline sudden death and saddle-thrombus paralysis.
How common is HCM?
~15% of random-bred cats — with breed peaks of 10.1% (Maine Coon), 16.7% (Bengal), and a published 2.9% (Ragdoll). It is at least as common as human HCM (1 in 500).
Can a genetic test diagnose HCM?
Partially — 2 mutations, 2 breeds (A31P in Maine Coons, R820W in Ragdolls). A positive confirms risk (A31P >30% incidence), but a negative does not rule out HCM — undiscovered mutations and sporadic cases exist. Annual echo is still required.
Why is HCM hard to catch early?
Because many HCM cats have no murmur — so auscultation, which works in dogs, fails in cats. NT-proBNP is the better screen: >265 pmol/L is 90% sensitive / 88% specific for cardiac vs respiratory dyspnea.
What is the danger of HCM?
2 events, no warning — congestive heart failure (fluid in the lungs) and arterial thromboembolism (sudden hind-limb paralysis from a saddle thrombus). Both can be the first sign of the disease.
How is HCM monitored?
Echo is the gold standard (wall thickness + left-atrial size predict clot risk); NT-proBNP screens, triages, and tracks progression. High-risk breeds screen from 1–2 years and repeat annually.
Key Takeaways
- The murmur is a trap — up to half of HCM cats have no murmur, so waiting for one misses the disease. In cats, lead with NT-proBNP (cut-off 265 pmol/L), not auscultation.
- The screen number is 265 — a feline NT-proBNP above 265 pmol/L separates cardiac from respiratory dyspnea at 90% sensitivity / 88% specificity, with a 92% PPV / 85% NPV in a 167-cat study.
- Genetics screen, they don’t clear — A31P (Maine Coon, >30% incidence) and R820W (Ragdoll) confirm risk, but a negative test is not a clean bill of health. At-risk cats still need annual echo.
- 2 silent killers — HCM ends in heart failure or a saddle thrombus, and the clot is often the first sign. The prognosis flips once a cat presents that way, so screen before symptoms.
- Read NT-proBNP with the kidneys — azotemic cats read falsely high, so pair it with creatinine/SDMA (SDMA upper limit 14 µg/dL) or you’ll chase a cardiac diagnosis in a CKD cat.
References
- Payne JR, et al. Population characteristics and survival in cats with hypertrophic cardiomyopathy. J Vet Intern Med. 2010. PMID 20840126
- Fox PR, et al. NT-proBNP in feline respiratory distress. J Vet Intern Med. 2011;25(5):1010–1016. PMID 21848947
- Meurs KM, et al. A substitution mutation in the myosin binding protein C gene in Ragdoll HCM. Genomics. 2007;90(2):261–264. PMID 17553619
- Meurs KM, et al. A31P MYBPC3 mutation in Maine Coon HCM. Hum Mol Genet. 2005. PMID 15829503
- Myosin-binding protein C DNA variants in domestic cats and their association with HCM. PMC3602388
This content is for educational and product-selection purposes only. It is not a substitute for veterinary diagnosis — any cat with suspected heart disease should be evaluated by a veterinarian. Reference ranges are platform-dependent; always use the intervals validated for your specific analyzer. Product specifications are as published by Migibio (Guangzhou Magic Biotech Co., Ltd.) and may change.