MUSCULAR DYSTROPHIES
PATRICK ROSS, MD
overview
■ Definition: A group of diseases of primary muscular atrophy of unknown cause. There is degeneration of muscle fibers & an increase in content of fat & fibrous tissue. The group includes Duchenne’s, Becker’s, facioscapulohumeral, limb-girdle, congenital, others.
■ Duchenne’s: 1 in 3,300 male births. Most common & severe. X-linked recessive but female “carriers” w/ severe disease have been reported. Onset of weakness 2–6 y, starting in pelvic girdle & then proximal muscle groups. The earlier the onset, the more rapid the downhill course. Disease leads to wheelchair dependence in 3–10 y & to death after 5–20 y, usually from respiratory illness.
■ Becker’s dystrophy: 1 in 18,000–33,000 male births. X-linked reces- sive. Onset mean age 12 y. Much more benign course than Duchenne’s. Death occurs at an average age of 42 y, usually from pneumonia.
■ Facioscapulohumeral (FSH) dystrophy: 1 in 20,000. Autosomal dom- inant (boys= girls). Weak pectoral & facial muscles. Present in teens w/ weak shoulders & winging of the scapula. May have slight decrease in vital capacity & some cardiac involvement. Life span is reduced minimally.
■ Limb-girdle: “wastebasket” classification of 5 predominantly auto- somal recessive diseases. Severity is midway between Duchenne’s & FSH.
■ Congenital: “wastebasket” used to classify onset of the disease at birth.
■ Usual Rx: None, although some pts may be receiving steroids.
preop
Issues/Evaluation
■ Respiratory: seen by reductions in inspiratory & expiratory effort as well as decreased vital capacity & total lung capacity. Obtain baseline room air saturation & consider PFTs. Pts who appear to be doing well may be severely limited after anesthesia. Pts have decreased cough & are more prone to pneumonia.
174 Muscular Dystrophies
■ Kyphoscoliosis occurs as result of muscular weakness & compounds decreased respiratory compromise.
■ Cardiac abnormalities: severity of cardiac & skeletal disease does not correlate. May range from EKG abnormalities, arrhythmias, mitral valve prolapse, & cardiomyopathies. Atrial arrest or sudden death reported w/ some disease states. Obtain baseline EKG & rhythm strip. Consider echo. CK-MB fraction is present in skeletal mus- cle myopathies & thus cannot be used as a marker of cardiac injury.
■ Malignant hyperthermia has been associated w/ this group of dis- eases. Speculated that inherent membrane defect renders the muscle more susceptible to injury induced by anesthesia.
■ Gastric hypomotility: a feature for some pts
What To Do
■ Consider regional anesthesia where appropriate.
■ Avoid succinylcholine as it increases potassium & can trigger MH.
■ Avoid premedication when possible as pts at increased risk for aspiration.
■ Treat as if a full stomach.
■ Teach pulmonary toilet & prepare pt for postop CPAP or mechanical ventilation.
intraop
■ Dose anesthesia to pt response rather than age/weight, trying to “minimize” anesthesia.
■ Pts may be more “sensitive” to nondepolarizing neuromuscular blockers. Complete reversal seen by peripheral neuromuscular mon- itoring does not predict adequate respiratory muscle recovery.
■ Anticipate & be prepared to treat arrhythmias.
■ Place NG or OG tube to evacuate stomach contents for gastroparesis.
Management
■ Avoid succinylcholine as above.
■ Consider modified rapid-sequence induction for gastroparesis.
■ Anticipate arrhythmias.
postop
■ Provide close cardiorespiratory monitoring.
■ Anticipate need for further respiratory support, including adequate pulmonary toilet, CPAP or mechanical ventilation.
Myasthenia Gravis 175
MYASTHENIA GRAVIS
EMILY REINYS, MD WILLIAM A. SHAPIRO, MD
overview
■ Definition: chronic autoimmune disease characterized by weak- ness/fatigue of voluntary skeletal muscles w/ progressive use
■ Disease results from antibody-mediated impairment of neuromus- cular transmission at nicotinic acetylcholine receptors
■ Usual Rx
➣
Anticholinesterase agents (typically, pyridostigmine), which inhibit the enzyme responsible for the hydrolysis of acetylcholine➣
Thymectomy for pts refractory to anticholinesterase therapy➣
Occasionally, immunosuppressants and/or corticosteroids➣
Less commonly, plasmapheresispreop
Issues/Evaluation
■ Great variability in severity of disease, ranging from limited extraoc- ular muscle involvement to generalized weakness w/ respiratory impairment
■ Determine whether pt has been taking anticholinesterase meds in the periop period. These agents will markedly decrease sensitivity to nondepolarizing neuromuscular blockers & markedly prolong dura- tion of succinylcholine.
■ Consider type/location of surgery & whether neuromuscular block- ers will be required.
■ Other medications such as local anesthetics (esters) & aminoglyco- sides can exacerbate muscular weakness.
What To Do
■ Obtain prior anesthetic records (if available) to determine previ- ous response to general anesthesia, neuromuscular blockers & any postop complications.
■ Perform preop testing w/ peripheral nerve stimulator to assess a baseline train-of-four; this helps predict pts w/ increased sensitivity to nondepolarizing neuromuscular blockers (T4/T1<0.9 will have increased sensitivity).
■ Inform pt of possible need for postop ventilatory support.
176 Myasthenia Gravis Myocardial Infarction
■ Administer very little, if any, sedative premed to avoid respiratory depression.
intraop
■ Dose of neuromuscular blocker varies widely among pts & should be titrated individually.
■ Inherent muscle relaxant effects of halogenated volatile anesthetics may eliminate or decrease need for neuromuscular blockers.
■ Prolonged effects of opioids, especially in causing respiratory depression, detract from the use of these meds for maintenance of anesthesia.
Management
■ Use peripheral nerve stimulator to determine dose & dosing interval of neuromuscular blockers (if needed). Document peripheral nerve stimulator response before using any neuromuscular blocking agent.
■ If neuromuscular blockers are required for intubation, administer 1/3–1/2 usual dose of nondepolarizing agent & assess response w/ peripheral nerve stimulator. If depolarizing agent is used for intuba- tion, dose required may be up to twice the usual dose.
■ Neuromuscular blockade reversal w/ anticholinesterase agents (neostigmine, edrophonium) is recommended at end of case; may not be effective if pt has been taking pyridostigmine in periop period.
■ Prior to extubation, ensure adequate muscle strength: check for sus- tained head lift>5 sec, regular respiratory pattern, adequate tidal volumes, return of 4/4 train-of-four ratio.
postop
■ Close observation in PACU/ICU for respiratory distress.
■ Pts too weak to be extubated may require prolonged ventilatory sup- port and/or pyridostigmine infusion.