Anesthetics

People with neuromuscular disorders must take great care if they are to have a local or general anesthetic. Even someone with very mild, or non-existent symptoms, or someone who has a family history of a disorder, needs to let the anesthetist know well in advance so that tests can be carried out and proper care after the operation can be arranged.

Many people are afraid of having an anesthetic, mainly through ignorance, but when we look at the rate of complications and even deaths arising from anesthesia we see that it is in fact very safe. This safety is the result of a thorough understanding of the patient's medical condition with a careful assessment before the operation, marked technical improvements in monitoring facilities such as High Dependency Units (HDU) and Intensive Care Units (ICU).

Patients with neuromuscular disorders (NMDs) deserve special attention when it comes to anesthesia because many of the agents used (gases and chemicals) have effects on both muscle and nervous tissue. The main areas of concern are how the anesthetic agents will affect the muscle and how they will affect the heart which is itself a muscle. A skeletal deformity such as scoliosis, or curvature of the spine, can also affect the way the patient responds to anesthesia so it is important to consider that too.

Anesthetics and the heart

An article printed in the Winter 1995 No. 20 edition of The Search showed how people with NMDs can sometimes have associated heart disease. This can occur as a cardiomyopathy, when the heart muscle doesn't work effectively, or as a defect in the way the electrical activity of the heart is transmitted, a conduction defect. The anesthetic vapors, the smelly agents such as ether and halothane which are inhaled, can reduce the effectiveness of the heart's muscle contractions and also aggravate any conduction defect. The vapors are all slightly different from each other, some having more effect on the heart than others. So it is important that the anesthetist makes a good assessment of the heart's condition before the operation which would include the level of physical activity that the patient can manage, and an ECG. Occasionally a more extensive assessment is needed.

Anesthetics and breathing

Doctors need to measure how weak the patient's muscles are, usually by assessing the amount of physical activity that the patient can perform, and by taking a blood test to measure levels of a muscle enzyme, creatine kinase (CK). Any anesthetic agent which affects the muscles will also affect the muscles we use to breathe. Strong analgesic or sedative agents will affect these muscles indirectly, and muscle relaxants will have a direct effect on them. As breathing (or respiration) may already be difficult for patients with NMDs, these drugs should be used cautiously, and monitoring of breathing after the operation is absolutely essential. As a result, the patient is usually best cared for in a High Dependency Unit or Intensive Care Unit immediately after the operation. The muscles used for swallowing can also be affected which is another reason why good post-operative care is important.

Muscle Relaxants

Muscle relaxant drugs should only be used if essential because they tend to have a more profound and prolonged effect in NMD patients compared to other patients. One type of muscle relaxant, called suxamethonium, should usually be avoided. It causes the release of potassium ions (K+) from the muscle tissue into the blood. In normal patients this is usually of little practical significance. In patients with NMD the muscle may normally leak K+ so that a further increase in the levels of K+ in the blood may cause abnormal heart rhythms. A preoperative blood test to check K+ levels is therefore important.

Local anesthetics

A local anesthetic works by preventing the normal electrical activity in the nerve around which the anesthetic agents are placed. For minor procedures, such as stitches for cuts, they are probably the first choice for patients with NMD because they have few if any side-effects. However for major local anesthetic techniques, e.g. spinal or epidiural, careful assessment of the patient is needed and the type of NMD considered well before the operation.

Changes in body temperature and preoperative ‘starvation'

Patients with NMD do not tolerate changes in body temperature or the starvation often associated with anesthesia or surgery as well as normal patients, so steps need to be taken to minimize these problems by keeping the patient warm and well hydrated using drips.

Malignant hyperthermia (MH) and Central Core Disease

Malignant hyperthermia (MH) is an inherited disorder which causes an unexpected, sometimes fatal, reaction in the patient to certain anesthetic drugs. Because some patients with NMD have sometimes experienced similar problems during anesthesia there have been claims that patients with NMD may also have MH. However, it is generally accepted that the only neuromuscular condition truly related to MH is Central Core Disease (CCD), although this is not always the case. Patients with CCD should be considered potentially susceptible to MH unless proved otherwise by a special type of muscle biopsy which screens for MH.

by Dr. P.J. Halsall and Professor F.R. Ellis