Inclusion Body Myopathy

Group of genetically transmitted muscle disorders characterized by progressive weakness of variable age onset depending on the clinical type.

Cytoplasmic Body Myopathy; Distal Myopathy with Rimmed Vacuoles (DMRV); Hereditary Inclusion Body Myopathy; Quadriceps-Sparing h-IBM; Nonaka Myopathy.

Several patterns: autosomal dominant, autosomal recessive (chromosome 9, band p1-q1), and sporadic.

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.

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.

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.

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.

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.

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 is highly suitable.

Sporadic Inclusion Body Myositis (IBM): Acquired myopathic process of unknown origin presenting as weakness or impairment of muscle function in affected areas (predominance on knee extensor and wrist/finger flexor joints).