Episodic weakness in cats
(Hypokalemic myopathy)
Breeds affected
Mainly Burmese
Clinical symptoms
Episodic weakness is a lethal recessive genetic disease which manifests between 4 - 10 months of age (average 7.4 months). The cat appears normal until an attack is triggered by factors such as excitement or mild stress. During an episode, the head is held close to the chest when walking or resting. When walking, the head nods up and down. When walking, the forelegs are stiff, straight and high stepping while the hind legs flex normally, but are more widely splayed. The pupils are dilated and the claws extended. Afflicted eats can only walk a short distance and cannot land from a jump. and, if forced to jump, cannot land properly. Serum creatine kinase levels are markedly increased.
Weakness is defined as a loss of muscular strength resulting in an animal becoming completely or partially recumbent, or ataxic. Generalised muscle weakness is referred to as asthenia. Weakness can be continuous or episodic. Many conditions produce a continuing weakness due to advanced disease, and the diagnosis is often more apparent (e.g. kidney or liver disease, haemorrhage). Heart failure also produces weakness (forward failure). On exertion or exercise the animal becomes weak and may become recumbent (appear to collapse).Causes of episodic weakness can be divided into three broad categories:
| (1) | Pre-syncopal episodes. |
| (2) | Neurological or neuromuscular diseases, e.g. myasthenia gravis, polymyopathy or polymyositis, cauda equina syndrome, wobbler syndrome. |
| (3) | Endocrinopathies and metabolic diseases, e.g. hyperkalaemia, hypothyroidism, hypoparathyroidism. |
- There are quite a range of possible neurological causes of collapse including central CNS disorders (e.g. hepatic encephalopathy, narcolepsy, cataplexy, Scottie cramp, episodic falling over in Cavalier King Charles Spaniels), peripheral neuropathies, disorders of neuromuscular transmission (e.g. myasthenia gravis) and myopathies (e.g. polymyositis, Labrador and golden retriever myopathy, hypokalaemic polymyopathy in cats). The reader is referred to References and Further Reading for a detailed discussion on these causes.
- Peripheral or central vestibular disease often causes ataxia and collapsing. In the veterinary profession vestibular syndrome is sometimes referred to as a 'stroke'. Within the medical profession, this term refers to a cerebrovascular accident (CVA) due to thromboembolism, and such events are very rare in dogs and cats.
There are a number of case reports of muscle-related diseases of cats. Descriptions include: nutritional myopathy secondary to vitamin E deficiency myositis secondary to Clostridium chauvoei and Clostridium septicum infections fibrotic myopathy of the semitendinosus muscle and quadriceps contracture secondary to trauma. Episodic weakness and signs of depression have been noted in young domestic short-hair cats (less than 1 year of age) with hypernatremia secondary to hypodipsia. The most common clinical sign of hypernatremic myopathy is ventral flexion of the neck. Causes of hypodipsia include lesions of the hypothalamus, and mechanical inability to swallow--a potentially serious complication of hypertrophic feline muscular dystrophy. The association between myositis and malignant neoplasia (paraneoplastic myopathy) is likely to be reported in the future. Myopathies in cats may occur in association with FeLV or FIV infections (e.g., FeLV-associated immunosuppression may enable encystment of Sarcocystis spp. in muscle).