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Group of genetically transmitted muscle disorders
characterized by progressive weakness of variable age onset depending on the
clinical type.
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Cytoplasmic Body Myopathy; Distal Myopathy with Rimmed
Vacuoles (DMRV); Hereditary Inclusion Body Myopathy; Quadriceps-Sparing
h-IBM; Nonaka Myopathy.
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Several patterns: autosomal dominant,
autosomal recessive (chromosome 9, band p1-q1), and sporadic.
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Cytoplasmic inclusions predominantly in type 1
fibers in the skeletal muscle. Smooth and cardiac muscles may be affected.
Muscle biopsy reveals red-rimmed vacuoles, cytoplasmic or intranuclear
filaments, and occasionally intracellular amyloid deposition. Progressive
muscular weakness and atrophy occur as a consequence of the degenerative
changes.
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Clinical course (symptoms appear from early to late
life), tendon reflexes generally depressed, normal or elevated creatine
phosphokinase, electromyogram showing predominant myopathic changes. Muscle
biopsy reveals the characteristic vacuolar myopathy described. Sialuria is a
characteristic feature of the quadriceps-sparing autosomal recessive
myopathy.
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A “malignant course” has been described in
adolescents who showed a delayed onset of walking in childhood with easy
fatigability. By age 14 to 15 years, they presented with scoliosis and weakness
in the face, sternocleidomastoid, proximal limbs, and respiratory, spinal,
and cardiac muscles. Most patients succumb from cardiorespiratory failure.
Earlyand late-onset adult inclusion body myopathies are characterized by
distal muscle weakness of the upper and lower limbs that may progress to the
girdles and result in various degrees of incapacitation. The variability of
the clinical manifestations makes classification of these myopathies
challenging: a quadriceps-sparing autosomal recessive myopathy of adult
onset has been recognized as a distinct entity; it is caused by a defective
mutation in the UDP-N-acetylglucosamine (UDP-GlcNAc)
2-epimerase/N-acetylmannosamine (ManNAc) kinase (or GNE) gene and was first
described in Iranian Jews but it is not limited to this ethnic group.
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Obtain a thorough evaluation of
motor function and exercise tolerance. Inquire about previous complications
(cardiac, respiratory, other), especially those following surgeries. Seek
any familial history of muscle weakness. Evaluate pulmonary function test
(forced vital capacity [FVC], peak expiratory flow rate [PEFR], forced
expiratory volume in 1 second [FEV1], FEV1/FVC, arterial blood gas
analysis, chest radiographs) and cardiac function (ECG, echocardiography,
and, if necessary, dobutamine stress echocardiography or radionuclide
imaging). Plasma levels of creatine phosphokinase, serum
glutamic-oxaloacetic transferase, lactate dehydrogenase, Na, and K. Avoid
elective surgery if cardiac and/or respiratory function is significantly
compromised (ventricular ejection fraction <0.5, FVC <25%, PEFR <30%).
Inquire about corticosteroids or other immunosuppressive agents; some
patients may be treated with these agents even though they are unsuccessful
in controlling the disease.
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Complications related to the various
degrees of muscle weakness with the possibility of respiratory insufficiency
and cardiomyopathy. If severe, spinal deformity may compress the upper
respiratory tract, especially in the prone position. Careful dosing and
monitoring of nondepolarizing muscle relaxants are required. Mechanical
ventilatory support may be required postoperatively.
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Use of succinylcholine may trigger
hyperkaliemia, arrhythmias, rhabdomyolysis, myoglobinuria, and cardiac
arrest. Because of the muscle weakness, careful titration of the
nondepolarizing muscle relaxants (under control of nerve stimulator) is
necessary. In a spontaneously breathing patient, opioids should be given
with caution, and, when possible, regional anesthesia ...