Myasthenia Gravis

Dear Dr. Jody:  Our six year old German shepherd, “Harley”, has recently started regurgitating and choking on his food and water immediately after swallowing it.  What can we do to give him some relief from this aggravating condition?

The symptom you are describing is suggestive of a condition known as megaesophagus which often stems from yet another condition known as myasthenia gravis (MG).  MG is a frustrating condition in which nerve impulses that are meant to stimulate muscle contraction are not strong enough to cause or maintain a muscle contraction.

In a healthy animal, the brain sends messages to muscles via nerve cells, or neurons.  A neuron is made up of three parts:  its “head” is located in the spinal cord; its “body” or axon is a long strand that runs from the spinal cord to the muscle, and its “foot” is a projection inside the muscle at a site called the neuromuscular junction.  When an impulse travels from the spinal cord through the neuron’s head down through its axon and into the neuromuscular junction, it stimulates contraction of the muscle by releasing a chemical transmitter called acetylcholine from the foot process.  Once the chemical message has been transferred to the muscle and the muscle has contracted, an enzyme comes along and “mops up” all of the leftover acetylcholine and recycles it.  If this enzyme did not remove the leftover acetylcholine, the muscle would continue to contract indefinitely.  This entire process is a very effective and efficient mechanism, when it works.

In animals and humans with myasthenia gravis, the body develops antibodies against the neuromuscular junctions and destroys them.  The result?  When a nerve sends a signal to the neuromuscular junction to secrete acetylcholine and thereby cause a muscle contraction, very little acetylcholine makes it across the badly damaged junction, and the muscle cannot contract.

The most common symptom of MG in dogs is a secondary condition called megaesophagus (ME).  In dogs with ME, the esophagus loses its ability to contract during the swallowing of food or water.  If the dog tries to swallow and the esophagus isn’t contracting properly, food or water can pool inside the baggy saggy esophagus before reaching the stomach.  This often leads to choking or regurgitation of the food and water.  If the dog chokes on this material and inhales any of it, potentially fatal aspiration pneumonia can develop.

Unfortunately we cannot repair damaged neuromuscular junctions.  The most common treatment for MG is a medication called pyridostigmine which prevents the little “janitor enzyme” from mopping up the leftover acetylcholine.  By keeping this chemical transmitter at the neuromuscular junction longer, it may be possible to slightly improve muscle contraction.  A second medication that may be helpful called bethanechol.  Bethanechol causes more acetylcholine to be released from the neuron at the level of the neuromuscular junction.  The hope is that the more acetylcholine that is released, the more will make it across the damaged neuromuscular junction and stimulate a muscle contraction.  Thirdly, medications such as steroids may be used to combat the immune system’s destruction of the neuromuscular junction in the first place. 

It is important for dogs suffering from MG and the resultant megaesophagus to be on medications to prevent erosion of the esophagus with stomach acid.  Because the esophagus lacks tone, it is not uncommon for stomach acid to reflux up into the esophagus, causing heartburn and painful ulcers.

Affected dogs may have an improved ability to swallow food and water if they are fed a liquid diet with their body held in a vertical position so that gravity can help bring the food and water down into the stomach.  Devices can be built that assist the dog to maintain this otherwise unnatural posture during and after feeding. 

Myasthenia gravis is a relatively uncommon yet highly frustrating disease.  With the correct diagnosis and treatment, some dogs can experience relief of their symptoms.