A neuromuscular disorder in a pet can become very serious very quickly. A seemingly healthy animal may collapse during exercise or become paralysed over the course of a few hours without apparent cause. Obtaining an accurate diagnosis is crucial since, with appropriate early treatment, many conditions can be treated in a manner that facilitates the animal’s recovery.
What is the neuromuscular system?
The nervous system is divided into two parts: the central nervous system (brain and spinal cord) and the peripheral nervous system (nerves and muscles). The neuromuscular system is composed of nerves (so-called peripheral nerves) that control the muscles of the limbs. There is a junction between the peripheral nerves and the muscles that is referred to as the neuromuscular junction.
What does the neuromuscular system do?
For any activity involving voluntary movement of the body (standing up, walking, running, chewing or swallowing, etc.), the neuromuscular system is essential. Throughout the body, each muscle is controlled by its own peripheral nerve. When a leg is moved, the brain sends a command signal down the spinal cord, which is then relayed to the leg’s muscles via a peripheral nerve.
How do nerves transmit messages?
The nerve sends a signal in the form of an electrical current. This is referred to as an electrical impulse. Upon reaching the end of the nerve, the electrical impulse must be transmitted to the muscle to cause it to contract. The neuromuscular junction is the gap between the nerve and the muscle. It is only through the use of a chemical messenger that the electrical signal can bridge this gap. Acetylcholine is the chemical messenger which bridges this gap. Upon release from the end of the nerve, the messenger flows across the gap and attaches to a receptor on the muscle, causing the muscle to contract. The term neuromuscular disorder refers to a problem with signal transmission from a nerve to a muscle. There are many causes of the problem, including a failure of the nerve to transmit electrical nerve impulses (disease of the nerve or neuropathy), the failure of the chemical messenger to bridge the gap between the nerve and the muscle (disease of the neuromuscular junction or junctionopathy), or the failure of the muscle to contract (disease of the muscle or myopathy).
What are the signs of a neuromuscular disorder?
The symptoms of neuromuscular disorders in dogs and cats include muscle weakness and loss of muscle mass. It is possible that the animal will be unable to stand or exercise normally if this muscle weakness affects the limbs. It is possible that the weakness of the muscle will only be intermittent and may be triggered by exercise in a mild form. There is a possibility that this weakness will affect all four legs or only the back legs. The condition often manifests itself as short strides, a stiff gait, and muscle tremors when an animal starts to exercise, which progresses to weakness or collapse if the animal continues to exercise. Neuromuscular disorders may, in their most severe form, completely paralyse an animal, preventing it from supporting its own weight, holding its head upright, and even having difficulties breathing due to involvement of the muscles of the chest wall and diaphragm.
Other muscles in the body may also be affected by these disorders. Some types of neuromuscular disorders may also affect the muscles of the throat or oesophagus (the tube that transports food from the mouth to the stomach). Animals with weak swallowing muscles may have difficulty swallowing and may often bring back food after eating.
What diseases affect the neuromuscular system?
It is possible for the neuromuscular system to be affected by a variety of diseases. Neuromuscular diseases can be divided into those that affect the various components of the neuromuscular system directly (peripheral nerves, neuromuscular junction, muscles) and those that affect the neuromuscular system indirectly.
A number of hormonal disorders (such as hypothyroidism or diabetes), some cancers, kidney disease, heart disease, and lung disease can indirectly affect the neuromuscular system.
Diseases that directly affect the various components of the neuromuscular system include infectious diseases (particularly toxoplasmosis, neosporosis, botulism or tetanus), immune-diseases causing inflammation of the muscle (myositis), inflammation of the nerve or destruction of the muscle receptors (myasthenia gravis), toxic diseases, inherited diseases and degenerative diseases.
How can neuromuscular diseases be investigated?
In the event that you suspect a patient may suffer from a neuromuscular disease, you will first want to eliminate diseases from other parts of the body. It may include taking a complete blood profile, taking chest and abdominal X-rays, and performing an abdominal ultrasound.
Electromyograms (EMGs) are specialised tests that can be performed to determine whether the problem arises from the neuromuscular system. An EMG machine records small electrical signals in individual muscles within an anaesthetised animal. Using an EMG machine a vet can evaluate how well the muscles are working. It is also possible to measure the electrical activity of the nerves. It may be necessary to perform a blood test to examine for antibodies to the acetylcholine receptor if a disorder affecting the junction between nerves and muscles such as myasthenia gravis is suspected. Myasthenia gravis can also be diagnosed by administering a relatively short-acting drug called an anticholinesterase inhibitor. The drug acts as a short-acting antidote for myasthenia gravis (see factsheet on myasthenia gravis). If given the correct dose, affected animals can experience a dramatic increase in muscle strength (though the effects wear off after a few hours). In this stage of the investigation, a spinal tap may be considered in order to check for inflammation of the nerve roots (idiopathic polyradiculoneuritis).
In the event that the preliminary tests fail to reveal the cause of the problem, it may be necessary to take a biopsy of the nerve and muscle to determine whether the problem is caused by an inflammation or degeneration. A degenerative process may be transient and of unknown origin (such as distal denervating disease or chronic relapsing demyelinating neuropathy), or it may be permanent and slowly progressive (such as an inherited neuropathy or myopathy e.g., muscular dystrophy).
Can neuromuscular disorders be treated?
In order to treat and prognosis neuromuscular disorders, it is important to determine their primary cause. Neuromuscular disorders caused by cancer, inherited neuropathies, or inherited myopathies carry a poor prognosis. The prognosis for other conditions (myositis, myasthenia gravis, distal denervating disease, underactive thyroid gland, and most forms of polyradiculoneuritis) is generally good with adequate treatment. There is no such thing as a quick fix in treating neuromuscular disorders, and in some cases it can take weeks before any improvement is noticeable (even in animals that go on to recover completely).