Myotonic dystrophy: Preop eval
Clinical - Neuromuscular Diseases and Disorders
It is characterized by muscle wasting (muscular dystrophy), cataracts, mental retardation, heart conduction defects, endocrine changes, and myotonia (delayed muscle relaxation following contraction). Pre-op evaluation of patient with this disease should be emphasized with:
1). Cardiac involvement. Both myocardium and the Hos-Purkinje system can be affected, which can lead to cardiomyopathy and AV block/arrhythmia respectively. Cardiac reserve and conduction abnormalities should be evaluated.
2). Pulmonary involvement. Muscular dystrophy may affect patient’s ventilation function and the ability to protect the airway. Pulmonary function usually shows restrictive lung disease.
3). GI involvement including gastric atony and pharyngeal muscle weakness, which predispose patients to gastric content aspiration.
4). Endocrine abnormalities include DM, thyroid/adrenal/gonadal dysfunctions.
MD Type 1 (Steinert’s disease) has an incidence is 1: 8000 (and represents 98% of all patients with MD). The principle defect is on Chromosome 19. It is characterized by muscle weakness and wasting (most prominent in the cranial and distal limb musculature), periodic myotonia, progressive myopathy, insulin resistance, defects in cardiac conduction, neuropsychiatric impairment, cataracts, testicular atrophy, and frontal balding in males. Mitral valve prolapse is found in 20% of patients. Of note, the typical cranial muscle weakness and wasting are manifested not only in the facial, temporalis, masseter, and sternocleidomastoid muscles, but also in the vocal cord apparatus.
MD Type 2 is quite rare and represents 2% of all patients with MD. The principle defect is on Chromosome 3. It is characterized by myotonia (90% of those affected), muscle dysfunction (82% weakness, pain, and stiffness), and less commonly, cardiac conduction defects, iridescent posterior subcapsular cataracts, insulin-insensitive type 2 diabetes mellitus, and testicular failure (in males, obviously).
- General considerations for MD are similar to those for other muscular dystrophies
- The majority of complications are pulmonary: smooth muscle atrophy, which leads to poor gastric motility, when coupled with a diminished cough reflex, promotes aspiration
- Succinylcholine will produce contractions lasting for several minutes, thus making intubation and ventilation a challenge; thus…
- Short-acting nondepolarizing muscle relaxants or avoidance of relaxation is therefore advised
- Triggering factors, such as hypothermia, shivering, and mechanical or electrical stimulation, may cause a myotonic reaction
- MD patients can be very sensitive to anesthetic agents, with hypersomnolence and CO2 retention sometimes being observed
- Pacing equipment should be readily available because a third of AV blocks may not respond to atropine