Low blood potassium and hypokalaemic polymyopathy
Hypokalaemia is the term used to describe low levels of potassium in the blood. Most of the potassium in the body is contained within cells, but a smaller amount is in the blood and in the fluid (tissue fluid) around the cells. The correct balance between the amount of potassium inside cells and in the blood and fluid around cells is critical for their normal function, especially cells such as muscle fibres.
Blood potassium levels may decline for a number of different reasons, but if the levels drop too low then this can cause multiple adverse effects. One of the most obvious signs can be generalised muscle weakness which, depending on the severity of the hypokalaemia can be anything from mild to very severe, sometimes with an inability to even stand up.
What causes low blood potassium (hypokalaemia)?
In healthy cats, blood potassium concentrations are controlled very tightly to ensure the body functions normally. Some potassium is lost in the urine, but this is controlled by a hormone (aldosterone) produced by the adrenal glands. Usually the potassium in the diet is sufficient to replace the amount lost in the urine each day.
Hypokalaemia can be caused by a variety of conditions:
- Inappetence or anorexia – prolonged severe inappetence or anorexia may cause a mild drop in blood potassium, but this is not usually marked
- Chronic kidney disease (CKD) – this is probably the single most common cause of significant hypokalaemia in cats. For unknown reasons, some cats with CKD can lose excessive amounts of potassium in their urine and suffer hypokalaemia as a result
- Severe vomiting or diarrhoea – prolonged or severe vomiting and diarrhoea can result in excessive loss of potassium (in the vomitus or in the diarrhoea)
- Primary aldosteronism – this is a disorder where there is abnormal production of the hormone aldosterone, usually as a result of an adrenal tumour. With excessive aldosterone production the kidneys will fail to conserve potassium and large amounts will be lost in the urine
- Periodic hypokalaemia in Burmese and related cats – hypokalaemia may develop as an inherited disease in some Burmese and related cats. In these cats, there is intermittent hypokalaemia, although the exact cause of the low potassium is uncertain.
- Other disease – rarely, hypokalaemia can result from other diseases (for example, hyperthyroidism, acid-base disturbances, etc), or sometimes as a result of treatment of other diseases (for example, some forms of diuretic therapy, during treatment for diabetes etc).
What are the signs of hypokalaemia?
In mild cases, there will be no obvious signs from hypokalaemia and unless a blood test is done there would be no outward indication that potassium levels are low. However, the low blood potassium can still cause unwanted effects – low blood potassium can contribute to damage to the kidneys and to a poor appetite, for example.
In more severe cases, the most noticeable sign of hypokalaemia is the development of muscle weakness. Usually the first and most obvious sign of this is an inability for the cat to hold its head up in a normal position – the neck remained drooped, so-called ‘ventroflexion’ of the neck. While other causes of muscle weakness can also cause ventroflexion of the neck, low blood potassium is the most common cause. See video below.
With lower blood potassium levels it will be obvious that the muscle weakness will be affecting other areas of the body too – the limbs will become weak, the cat may be reluctant to walk and move, the cat may appear stiff and/or wobbly. In extreme cases the cat will be unable to stand, and even the respiratory muscles may be affected making it difficult to breath. Muscles may also become painful.
How is hypokalaemia diagnosed?
Hypokalaemia is easy to diagnose as the potassium concentration in the blood can readily be measured (and shown to be low) with a simple blood test. Usually, with muscle weakness, there will also be indicators in a blood sample that the muscles have been damaged (enzymes such as creatine kinase – CK, and aspartate aminotransferase – AST, that are usually present inside muscle cells may leak out into the blood when the muscle is damaged by hypokalaemia).
If hypokalaemia is confirmed, further investigations may be required to determine the underlying cause – these may include further blood and urine tests, X-rays and/or ultrasound examinations. Sometimes more specialist tests (such as measurement of the hormones aldosterone and renin) may be required.
How can you diagnose Periodic Hypokalaemia in Burmese and related cats?
Periodic hypokalaemia of Burmese and related cats is usually first seen in young kittens from 2-6 months of age, although occasionally signs may not be evident until the cat is around a year of age. The defect causes a transient increase hypokalaemia (possibly through excessive urinary loss of potassium) and so typically causes intermittent signs of muscle weakness (including ventroflexion of the neck). The severity of signs will vary according to how severe the hypokalaemia is, but affected cats often have moderate to severe signs for several days and then may have periods of apparent normality in between that can last a few weeks. Clinical signs may be induced by exercise, stress or cold weather. Severely affected cats may collapse and even die of respiratory paralysis.
This condition affects Burmese and related cats such as Asian, Australia Mist, Bombay, Burmilla, Cornish Rex, Devon Rex, Singapura, Sphynx, Tiffanie and Tonkinese cats. The defect is inherited as a simple autosomal recessive trait – that means to be clinically affected, the pair of genes that determine the presence or absence of disease must both be defective. A cat with only one defective gene will be a ‘carrier’ and can pass this on to offspring, but will not themselves be affected. The gene controlling the disease is termed WNK-4 and a simple gene test is now available (using a buccal mouth swab or a blood sample) to determine the genetic status of a cat. All potential breeding cats from the Burmese and related breeds should ideally be tested before being used for breeding in an attempt to eradicate this defect from the breed.
Treatment of hypokalaemia
Treatment of hypokalaemia in cats will depend on the underlying cause, but usually potassium supplementation is needed to correct the immediate problem. Depending on the severity of the hypokalaemia, this will usually initially be through potassium-supplemented intravenous fluids, but oral supplementation (ideally with potassium gluconate in liquid, powder or tablet form) for longer term supplementation may be needed. Cats with periodic hypokalaemic polymyopathy usually need continued potassium supplementation to avoid recurrent episodes, but some will appear to spontaneously improve by the time they reach 1-2 years of age so may not need life-long supplementation.
Your vet will need to do repeat blood tests in most cases to ensure the correct dose of potassium is being used, as this will vary from one cat to another.
Negative gene register
International Cat Care has established a register for breeders with cats that have been tested for the WNK-4 gene defect.
Terms of the register:
- International Cat Care will only accept test results if the cat has been microchipped.
- International Cat Care will need to have a copy of a signed certificate from the genetic testing laboratory, showing that the microchip number had been verified by a vet and counter-signed by the vet in question.
- International Cat Care need to make a charge of £5 per cat for creating and maintaining the register.
- International Cat Care will record results as either ‘normal’, ‘carrier’, or ‘affected’ as per the results from the laboratory in order to identify and distinguish carriers from non-carriers to allow appropriate breeding selections.
If you would like to have your cat entered on the register please send your certificate(s) as outlined in the terms above, together with a cheque made payable to ‘International Cat Care’ to:
International Cat Care, Place Farm, Tisbury, Wiltshire SP3 6LW, UK