Feline hypertrophic cardiomyopathy diagnosis, staging and management

Doses on this pageEvery dose below is quoted from the cited source. Verify against the current label/formulary and the patient's full clinical picture.

Definition and who is affected

The ACVIM consensus defines the HCM phenotype as "Diffuse or regional increased LV wall thickness with a nondilated LV chamber" [1]. The panel classifies feline cardiomyopathy by phenotype. A cat is described as having a "hypertrophic cardiomyopathy phenotype" until a cause is sought, and as having HCM only if none is found [1]. A hyperthyroid cat with a thick wall is therefore described as having an HCM phenotype alongside hyperthyroidism [1].

Most affected cats are mixed breed [1]. The ACVIM panel lists the following breeds as believed to carry higher risk [1]:

• Maine Coon
• Ragdoll
• British Shorthair
• Persian
• Bengal
• Sphynx
• Norwegian Forest cat
• Birman

Two causative myosin binding protein C variants are known: A31P in Maine Coons and R820W in Ragdolls. Cats homozygous for either face higher risk [1][2]. Kittleson and Cote, writing in the Journal of Feline Medicine and Surgery (JFMS), describe a Sphynx variant in the ALMS1 gene but note it has not been proven to cause disease [2].

The sources differ on sex. The JFMS review reports male-to-female ratios near 3:1 in retrospective case series and lists possible explanations, including that males may simply develop more severe disease [2]. The Merck Veterinary Manual states that both sexes are equally predisposed but males tend toward more severe disease at a younger age [3].

Clinical presentation

Most cats with HCM are subclinical [1]. In cats with subclinical HCM, the ACVIM panel reports [1]:

• a systolic murmur in up to 80%, against 30% to 45% of healthy cats
• gallop sounds in 2.6% to 19%

Merck notes that at least one-third of cats with HCM have no murmur [3].

When signs appear, congestive heart failure (CHF) is the most common cause, followed by ATE. A minority of cats die suddenly with no warning [1]. Labored breathing is the most frequent presenting sign, though some cats only hide or stop eating [1].

Merck describes ATE as typically sudden hind limb weakness or paralysis with pain and weak or absent femoral pulses. It also notes that cough is uncommon in cats with heart failure [3].

In one first-opinion study cited by the ACVIM panel, dyspneic cats with any of the following were more likely to have CHF than another cause [1]:

• respiratory rates above 80 breaths/min
• gallop sounds
• rectal temperatures below 37.5 C
• heart rates above 200 bpm

Diagnosis and testing pitfalls

Echocardiography

The ACVIM panel calls echocardiography by a cardiology specialist the diagnostic test of choice [1]. Because thickening is often regional, the panel notes that M-mode alone can miss focal hypertrophy. It recommends measuring the thickest septum and free wall in at least two right parasternal views and averaging at least three cardiac cycles [1]. Where wall thickness falls between 5 and 6 mm and doubt remains, the panel recommends classifying the cat as equivocal and rechecking later [1].

Kittleson and Cote add several cautions [2]:

• Wall thickness measured by skilled operators can vary by up to approximately 20% on repeat exams, so they suggest rounding to the nearest 0.5 mm.
• End-systolic cavity obliteration is common in HCM but also appears in some normal cats.
• Furosemide shrinks the left atrium in cats. In one study they cite, median LA:Ao (left atrial to aortic root ratio) was 2.3 in untreated cats in heart failure versus 1.8 in cats given furosemide within the prior 12 hours.

Ruling out other causes

The ACVIM panel recommends that blood pressure measurement be considered for any cat with increased wall thickness. It also recommends serum thyroxine for cats 6 years or older with abnormal auscultation [1].

The JFMS review lists dehydration, hypertension, hyperthyroidism, acromegaly and a condition it abbreviates as TMT as "Impostors for HCM" [2]. Those authors also state that hypertension and hyperthyroidism do not produce severe hypertrophy, which they define arbitrarily as 7 mm or more. In a cat with that degree of thickening plus one of these disorders, they say the systemic disease is generally not the sole cause and likely exacerbates underlying HCM [2].

Biomarkers, radiographs and ECG

The ACVIM panel does not recommend the quantitative NT-proBNP assay for separating normal cats from cats with mild to moderate HCM. It recommends that any positive result be followed by echocardiography [1]. In one general practice study cited in the JFMS review, about half the cats with subclinical HCM had normal NT-proBNP [2].

The JFMS review also reports a single study in which a high-sensitivity troponin I assay was 88% sensitive and 95% specific for subclinical HCM. However, the authors advise against using such tests to rule HCM in or out in the general population [2].

Radiography is insensitive for mild or moderate change, and the ACVIM panel states the "valentine-shaped" heart is not specific for HCM [1]. The panel does not recommend ECG for screening because of low sensitivity [1]. Genetic testing for A31P and R820W is recommended for breeding Maine Coons and Ragdolls but not for other breeds [1].

Staging

The ACVIM panel adapts the AHA and ACVIM heart disease staging systems [1].

Left atrial size can be used to divide B1 from B2, though the panel says other factors should also be considered [1]. The JFMS review gives these thresholds for a right parasternal short-axis view [2]:

• Normal LA:Ao is under 1.6.
• LA:Ao above 1.8 to 2.0, or an LA diameter above 18 to 19 mm, places a cat in stage B2.

The ACVIM panel lists further risk markers [1]:

• a gallop or arrhythmia
• reduced left atrial fractional shortening
• extreme hypertrophy
• reduced systolic function
• spontaneous echo contrast or thrombus
• a restrictive filling pattern

Management by stage

Stage B1

For stage B1, the ACVIM panel generally does not recommend treatment and advises annual monitoring for progression [1]. The panel says atenolol may be considered in B1 cats with severe dynamic outflow tract obstruction if it can be given consistently [1]. The JFMS authors define severe systolic anterior motion of the mitral valve as a peak outflow velocity above 4.5 to 5.0 m/s. They suggest atenolol at "6.25-12.5 mg/cat PO q12h" when it can be given without stress [2].

Two placebo-controlled trials found no effect of ramipril or spironolactone on left ventricular mass, per the ACVIM panel. Those studies were small and limited to one breed [1].

Stage B2

For stage B2, the ACVIM panel recommends clopidogrel in cats at risk of ATE. It says other antithrombotics may be added for cats at very high risk [1]. It also recommends that owners monitor resting or sleeping respiratory rate [1].

Acute CHF

For acute CHF, the ACVIM panel recommends [1]:

• oxygen
• IV furosemide "as multiple boluses of 1 to 2 mg/kg or a constant rate infusion"
• thoracocentesis for pleural effusion

The panel calls IV fluids contraindicated in congested cats [1]. The JFMS review cites a higher initial dose of 3-6 mg/kg for severe cases, so the two sources do not give identical dosing [2].

Chronic CHF

For chronic CHF, the ACVIM panel recommends clopidogrel "18.75 mg/cat PO q24h, with food" in cats with a CHF history and moderate to severe atrial enlargement [1]. The panel says pimobendan can be considered when clinically relevant obstruction is absent [1]. The JFMS authors note a prospective study found no benefit from pimobendan and say they reserve it for stage D [2]. Benazepril did not delay treatment failure in a randomized placebo-controlled trial [1].

Refractory disease and ATE

For refractory disease, the ACVIM panel says torsemide may be considered in place of furosemide in cats with persistent CHF despite furosemide doses above 6 mg/kg/day PO. It also says calorie intake should take priority over sodium restriction [1].

In acute ATE, the panel makes analgesia with a mu opioid agonist the priority in the first 24 hours, recommends anticoagulant treatment started as soon as possible, and does not recommend thrombolytics [1].

Prognosis and client communication

The ACVIM panel reports a 5-year cumulative incidence of cardiac death of about 23% in cats with HCM, regardless of age at diagnosis [1]. In one study described in the JFMS review [2]:

• about 7% of subclinical cats developed heart failure within a year, and 20% within 5 years
• roughly half of those in heart failure died within 2 months

Merck gives a median survival of 3 months for cats in CHF [3]. Cats whose CHF followed stress, IV fluids, anesthesia or long-acting corticosteroids may live longer [1][2]. Unlike in people, outflow tract obstruction does not appear to worsen prognosis in cats [1].

For owner conversations, the sources support explaining sleeping respiratory rate monitoring. The JFMS authors suggest it mainly for cats with moderate to severe atrial enlargement to avoid overvigilance [2]. They also suggest owners keep a log and call a veterinarian when the sleeping rate increases [2].

Both long sources flag adherence. The JFMS review notes clopidogrel is bitter and suggests placing it in a small gelatin capsule or pill pocket [2]. The ACVIM panel notes that salivation, retching or vomiting with clopidogrel can be minimized by giving it in an empty gelatin capsule followed by water [1]. The JFMS review advises against breeding affected intact cats [2].

Before using any dose aboveVerify against the current label/formulary and the patient's full clinical picture.