Neuromuscular disorders

What are the clinical signs of neuromuscular disease?

Weakness is the predominant sign of neuromuscular disease. Mild cases may only have exercise intolerance whereas severe cases may be unable to stand. The gait is often characteristic - initially relatively normal then becoming progressively more stilted and weak. Cats will often “bunny-hop”. Poor “hopping” responses can often demonstrate the weakness. Conscious proprioception and the time taken to correct a knuckled over paw is usually normal. If spinal and/or cranial nerve reflexes are depressed then a neuropathy is more likely. Myasthenic animals often have an absent or fatigable palpebral reflex and poor eye movement.

“Hopping” ability (one of the tests in a neurological examination) is often poor when there is neuromuscular disease.

Spinal reflexes may be reduced

Palpebral responses may be weak or fatigable.

What is the cause of neuromuscular disease?
The list of causes of neuromuscular disease is huge and, as with most of neurology, it is easiest to subcategorise then with the DAMNIT(V) format. There are many neuromuscular diseases that have not been classified.

This Vizsla has lost muscle, particularly over the head as a consequence of polymyositis. He was presented because of difficulty swallowing. This condition is increasingly recognised in young adult Hungarian Vizsla dogs. (for more information visit http://vizslamyositis.blogspot.com/ and http://vetneuromuscular.ucsd.edu/cases/2007/Sept07.html) see the two links in the right sidebar.

Burmese Cat with periodic hypokalemic paralysis.

He is weak and displaying neck ventriflexion - a posture characteristic for this disease. Periodic hypokalemic paralysis is characterised by low serum (blood) potassium and an extremely high creatine kinase. It is hypothesised to be a sodium channelopathy and is thought to be an autosomal recessive problem in the Burmese. Cases typically present when 4- 6 months old and 50% of affected cats “grow out of” the problem.

Click on pictures right to get a bigger view.

Differentials of myopathies, (first image on the right)

Differentials of neuromuscular transmission disorders (second picture on the right)

Storage diseases, primary hyperoxaluria, peripheral and central axonopathy (third picture on the right)

Muscle and nerve biopsy
Although relatively easy to perform it is important that biopsies are handled correctly and submitted to an appropriate laboratory. A dedicated neuromuscular laboratory should be able to provide the full range of histologic and histochemical stains and electron microscopy e.g. Neuromuscular Laboratory, Edinburgh University and Comparative Neuromuscular Laboratory, Davis University, California

Nerve biopsy from a 1 year old female domestic shorthair diagnosed with a neuropathy. The male sibling was more severely affected. The nerve biopsy revealed lipid droplets within macrophages endoneurium & subperineurally (arrows) suggestive of a lysosomal storage disease. Histopathology sections were prepared, analysed and photographed by Dr Caroline Hahn Neuromuscular Laboratory, University of Edinburgh

Nerve biopsy from a 1 year old female domestic shorthair diagnosed with a neuropathy. The male sibling was more severely affected. The nerve biopsy also revealed axonal degeneration with scattered myelin ovoids and no evidence of nerve regeneration. Histopathology sections were prepared, analysed and photographed by Dr Caroline Hahn Neuromuscular Laboratory, University of Edinburgh.

Other imaging
Other tests e.g. radiographs (x-rays) or ultrasound may be recommended to look for associated or contributing diseases e.g. myasthenia gravis can be induced by a tumour in the chest called thymoma

How is neuromuscular disease treated?
Therapy obviously depends as to the precise nature of the problem. Some neuromuscular diseases have a specific therapy for example endocrine disorders (hyperthyroidism, diabetes mellitus); Myasthenia gravis (Pyridostigmine and immunosuppressive therapy); and antiprotozoal agents for Toxoplasmosis and Neosporosis. For many degenerative neuropathies there is no treatment however some improve when given dietary supplements such as the mitochondrial co-factor L-Carnitine 50mg/kg twice daily and anti-oxidants such as gamma linoleic acid and omgea-3-fatty acids. Many severely tetraparetic animals will require considerable supportive care as well. For example, a pet which is unable to swallow may require feeding through a (PEG) tube into the stomach

This 16 year old cat has diabetes mellitus and associated neuropathy particularly of the sciatic nerve. The cat is weak and walking on her hocks (plantigrade stance). She improved after the diabetes mellitus was stabilised and her diet was supplemented with L-Carnitine and omega 3 and 6 fatty acids.

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Diagnosis and treatment