Neuromuscular disease implies a disorder of the peripheral nerve, neuromuscular junction or muscle.

Clinical signs

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.

Causes

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.

 Diagnosis  

Blood samples – haematology and biochemistry

Many diseases can be eliminated/suggested by routine tests for example electrolyte disorders (hypokalaemia, hypocalcaemia,) and endocrinopathies (Cushing’s syndrome, hypothyroidism, hyperthyroidism, diabetes mellitus). A significantly elevated creatine kinase (>500u/l) indicates muscle damage and is suggestive of a dystrophy, inflammation or metabolic imbalance such as hypokalaemia. However creatine kinase can also increase after prolonged recumbency or because the animal is breaking down muscle for energy. In protozoal diseases such as toxoplasmosis or Neosporosis, liver parameters may also be elevated.

Other blood tests

If serum biochemistry is suggestive then specific tests for endocrine disorders may be recommended. Blood samples may also be recommended to test for myasthenia gravis (acetylcholine receptor antibody); protozoal problems (Toxoplasma gondii and Neospora caninum) and / or mitochondrial myopathies(blood pH and abnormal levels of organic acids). It pet’s problem suggests a specific inherited problem then a specific genetic test may be recommend e.g. Centronuclear Myopathy (Labrador retriever myopathy) or exercise induced collapse in Labrador Retrievers.

Electrophysiology

electrophysiology (electromyography (EMG) with nerve function tests) is used for diagnosing neuromuscular disease and specifically localising the problem to the nerve, neuromuscular junction or a neuropathy. It can also provide information as to the aetiology of the problem; for example it is possible to ascertain if the disease is more likely to involve the myelin or the axon.

An EMG recording from a muscle. Abnormal spontaneous activity can indicate a neuropathy or myopathy

Trace of a sciatic motor nerve conduction velocity.

The amplitude of the compound muscle action potential (numbered wave form) is reduced and so is the conduction velocity (53.1m/s). This dog had a neuropathy.

 

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. Comparative Neuromuscular Laboratory, Davis University, California http://neurolab.vet.ed.ac.uk

 

Other imaging

Other tests e.g.radiographs or ultrasound may be recommended to look for associated or contributing diseases e.g. myasthenia gravis can be induced by thymoma. Although extravagantly expensive MRI can be useful to identify disease muscle to increase accuracy of biopsy

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.

 

Management megaoesophagus

  • Feed from a height
    • Stairs if unable to stand
    • Maintain raised position 10mins after feeding
  • Food
    • Small meals 4-6 per day
    • High calorie (e.g. Hills p/d or n/d)
    • Experiment food types (e.g. sticky food in small balls)
  • Low dose erythromycin
    • gastrointestinal pro-kinetic (stimulates motilin receptors)
    • 5-1mg/kg PO q8hours (paediatric suspension)
  • Treatment myasthenia gravis
    • Pyridostigmine bromide
    • Immunosuppression (steroids and other agents)

Appendix

Differentials of myopathies – common conditions highlighted in bold

 

Examples
D Degenerative E.g. Hereditary myopathies such as  muscular dystrophy
A Anomalous Myotonic myopathy, Myositis ossificans (progressive calcification of connective tissue in striated muscle)
M Metabolic Hypokalemia, Hypernatraemia,  Hypocalcaemia, Hyperkalemic periodic paralysis, Hypokalemic periodic paralysis, Hyperthyroidism, Hypothyroidism, Hyperadrenocorticism, Hypoadrenocorticism, Primary hyperparathyroidism, Hypoparathyroidism, mitochondrial myopathy, lipid storage myopathy
N (para) Neoplastic Thymoma, Bronchogenic carcinoma, Myeloid leukaemia, Tonsillar carcinoma
  Nutritional Vitamin E / Selenium deficiency.

Muscles hot, swollen and painful. Poor hair coat.

I Inflammatory Polymyositis
  Infectious Neosporosis, Toxoplasmosis, Trichinella,
T Traumatic Myositis ossificans traumatica
  Toxic Ionophores (Monensin)
V Vascular Ischaemic myopathy

 

Differentials of neuromuscular transmission disorders – common conditions highlighted in bold

  Examples
A Anomalous Congenital myasthenia gravis
N (para) Neoplastic Thymoma (myasthenia gravis – especially Abyssinian)

Lambert-Eaton syndrome – antibodies against Ca 2+channels (not reported veterinary medicine)

I Inflammatory Acquired myasthenia gravis
T Toxico-infectious Botulism – unlikely in cats

Tick paralysis (dogs in appropriate areas only)

T Toxic Organophosphates

Aminoglycosides, Tetracyclines, lincomycin, clindamycin

 

 

Differentials of peripheral neuropathies – common conditions highlighted in bold

  Examples
D Degenerative Idiopathic (in many cases a degenerative neuropathy may be diagnosed without determining a cause; some may be associated with an age-related degeneration)

Breed related disorders (see additional table)

Storage diseases, primary hyperoxaluria, peripheral and central axonopathy

M Metabolic Diabetes mellitus, Hyperthyroidism, Cushing’s syndrome, Hypothyroidism, Insulinoma
N (Para)neoplastic Bronchogenic carcinoma, Thymoma, Mammary adenocarcinoma, Malignant melanoma, Osteosarcoma, Thyroid adenocarcinoma, Mast cell tumour, Lymphosarcoma, leiomyosarcoma, haemangiosarcoma, synovial cell sarcomas, adrenal adenocarcinoma
I Inflammatory Acute polyradiculoneuritis , Post vaccinal acute polyradiculoneuritis
  Infectious Rabies, Toxoplasmosis, Neosporosis,
  Idiopathic Idiopathic Polyradiculoneuritis, facial neuropathy, laryngeal neuropathy, distal denervating disease
T Traumatic Brachial plexus avulsion, Sacrocaudal fractures
  Toxic Lead, Mercury, Thallium, Acrylamide, Hexacarbon, Ionophores, Salinomycin, Organophosphate (delayed), Coyotillo (Buckthorn), Dysautonomia
V Vascular Aortic thromboembolism

 

Table 2 Inherited peripheral nervous system or muscular disease for which there is a DNA test and / or breed screening program

Disease Breed Test Other info / Laboratory
Centronuclear Myopathy (Hereditary Myopathy) Labrador retrievers DNA (mouth swabs) Animal Health Trust

Laboklin, Alfont University

Hypokalemic myopathy Burmese cat DNA Langford veterinary Services

 

Leonberger Polyneuropathy

 

Leonberger DNA (blood, mouth swabs) University of Minnesota,

University of Bern

Malignant Hyperthermia All breeds DNA (blood, mouth swabs) Laboklin
Muscular Dystrophy Golden Retriever DNA (blood, mouth swabs) Laboklin
Myostatin Deficiency (“Bully” Whippet)/ Double Muscling Whippet DNA (blood, mouth swabs) Laboklin
Myotonia congenita Schnauzer DNA (blood, mouth swabs) Laboklin
Pyruvate Dehydrogenase Phosphate 1 Deficiency (PDP1) Sussex Spaniels
Clumber Spaniels
DNA (mouth swabs) Animal Health Trust,

Laboklin

Polyneuropathy, neuropathy Greyhound DNA (blood, mouth swabs) Optigen,

Laboklin

Spinal Muscular Atrophy Maine Coon DNA (blood, mouth swabs) Antagene

# testing recommended but not required for the Kennel Club Assured Breeder Scheme

Animal Health Trust http://www.aht.org.uk/cms-display/genetics_tests.html

Antagene http://www.antagene.com/index.motsclefs.Vets-.rubrique_id.185.coderub1.3.coderub2.0.html.coderub2.0.html

Laboklin http://www.laboklin.co.uk/laboklin/showGeneticTest.jsp?testID=8227D

Langford Veterinary Services  http://www.langfordvets.co.uk/lab_pcr_tests.htm#gen

Optigen http://www.optigen.com/opt9_test.html

University of Missouri / Orthopaedic Foundation for Animals http://www.offa.org/dnatesting/dm.html

University of Minnesota http://www.vdl.umn.edu/ourservices/canineneuromuscular/canineneuro/home.html

UC Davis http://www.vgl.ucdavis.edu/services/PDE.php

Universiteitskliniek voor Gezelschapsdieren, DNA diagnostiek, Q. Stassen, PO box 80.154, NL-3508 Utrecht euroadmin@boerboelinternational.eu