Endocrine myopathies

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A. Hypothyroid myopathy. The most commonly recognized endocrine myopathy occurs as a feature of hypothyroidism; it may antedate the diagnosis of hypothyroidism by several months. Symptoms and signs of this entity range from mild aches and pains, muscle cramps, and proximal weakness to apparent muscle hypertrophy and the mounding phenomenon, or myoedema (a transient focal ridging of muscle in response to percussing or pinching the muscle). In the usual form, proximal weakness may be observed, although atrophy is rare.

Hypertrophic muscles may exhibit myotonia-like contractions that are electrically silent on EMG.

1. Laboratory studies. Serum CK is often markedly elevated. A variety of EMG changes have been noticed, most of which indicate a myopathic process. Muscle biopsy may show focal necrosis, regeneration, and vacuolization of fibers, but findings are usually normal. Fiber size is quite variable. Sarcolemma nuclei are numerous, enlarged, and centrally placed. Mucoprotein deposits occur in one-third of patients. Histochemical staining has shown a decrease in type 2 fibers that is directly proportional to disease severity and serum CK levels. CK levels are not increased in hypopituitarism, and a myopathy has not been reported with hypopituitarism; nonetheless, in a patient with hypothyroidism, hypopituitarism should be considered.

2. Treatment of hypothyroidism is thyroid hormone replacement. With proper therapy, serum CK levels return to normal over several months.

B. Thyrotoxic myopathy. The manifestations of this disorder range from complaints of diffuse weakness, easy fatigability, and mild atrophy to severe proximal muscle weakness with pronounced atrophy. Laryngeal and pharyngeal muscles are not involved. Serum muscle enzymes are not elevated, even in the severe form; creatinuria, however, is present. EMG may show myopathic changes. Muscle biopsy may reveal only small fiber size or atrophy of fibers with replacement of fat and lymphocyte infiltrates. (Less common myopathies in association with hyperthyroidism are exophthalmic ophthalmoplegia, thyrotoxic periodic paralysis, and the association of Graves' disease in 5% of patients with myasthenia gravis).

C. Acromegalic myopathy. Proximal muscle weakness and easy fatigability occur in up to 50% of patients with acromegaly. Myalgia and cramps may occur in a few patients. Muscle mass, however, is increased. Serum CK and aldolase are usually normal but may be slightly elevated. EMG shows myopathic findings. Muscle biopsy shows no consistent pattern, but there is some evidence that type 2 fiber hypertrophy may occur. With treatment of the pituitary tumor, strength recovers gradually over months or years.

IV. Muscular dystrophy This group of primary muscle diseases is characterized by degeneration of muscle fibers, which occurs on a genetic basis. Although there is no specific treatment for these disorders, it is important to distinguish them from treatable forms of myopathy. Genetic counseling is suggested for patients and their families.

A. Duchenne's pseudohypertrophic muscular dystrophy. The onset of this sex-linked disease is usually in early childhood. Pelvic-girdle involvement, manifested by frequent falls, difficulty in climbing stairs, difficulty in rising from the floor, and a peculiar gait, is characteristic. The onset is insidious, and progression is slow. Gradually, the trunk and shoulder muscles become involved, causing the patient to require a wheelchair by age 12. Patients usually die in their twenties of either pulmonary infection or the cardiomyopathy associated with the disease.

1. Physical examination early in the disease reveals enlarged, firm (pseudohypertrophic), but weak calves and sometimes quadriceps and deltoids. Facial and distal muscles usually retain normal strength.

2. Laboratory studies. Serum muscle enzymes are increased. Creatinuria is present. Muscle biopsy shows fiber size variation and fat infiltration, depending on the stage of the disease. Dystrophin, a normal muscle protein bound to the sacrolemma, is absent. EMG reveals low-amplitude potentials of short duration. Electrocardiographic abnormalities, such as a prolonged PR interval, slurred QRS complex, ST-segment depression or elevation, and usually right bundle-branch block, are seen late in the disease. Female carriers may have mild abnormalities of serum CK, muscle biopsy, and EMG but are clinically asymptomatic.

B. Becker's muscular dystrophy is similar to the Duchenne form but milder, with later manifestations of disability. It is also an X-linked dystrophinopathy, with reduced synthesis of dystrophin.

C. Fascioscapulohumeral dystrophy is an autosomal dominant disease that affects female and male subjects equally. The usual age of onset is between 9 and 20 years, although adult onset has been recognized. The symptoms vary in severity; thus, diagnosis may be difficult. Shoulder-girdle weakness and winged scapulae are usually the first findings. Facial muscles are involved and may be the first muscles affected in some patients. The face is flattened, and the mouth moves asymmetrically, unable to pucker or whistle. Axial and pelvic muscles may become involved late in the disease; however, distal muscles are usually spared. Cardiac disease is rare, and patients usually live a normal life span. Serum muscle enzymes may be elevated slightly, and elevated urinary creatine is common.

D. Limb-girdle dystrophy is inherited in an autosomal recessive form and affects female and male subjects equally. Its onset occurs during the second or third decades. Shoulder- or pelvic-girdle muscles are involved first, with gradual progression to other muscle sites over years, although facial muscles are spared. Cardiac disease resulting from limb-girdle dystrophy is rare.

1. Laboratory studies. Serum muscle enzymes are slightly elevated, and creatinuria is present. Muscle biopsy shows fibrous and fatty replacement with necrosis of single fibers. Sarcolemmal nuclei are increased in number, forming chains centrally in the fibers. These changes may also be seen in Duchenne's dystrophy and fascioscapulohumeral dystrophy.

Some forms of this disorder are secondary to a genetic lack of production of dystrophin-associated glycoproteins.

E. Myotonic dystrophy. Myotonia is an inability to relax a muscle normally after contraction. It may be elicited by grasping with the hand or by directly percussing muscle groups, such as those of the forearm or tongue, or the thumb adductors.

Myotonic dystrophy is an inherited disease that begins early in adult life. It is manifested by distal muscle weakness and atrophy. Deep tendon reflexes are reduced. Ptosis may be present, and closure of the eyelids is also weak. Atrophy of the temporalis and sternocleidomastoid muscles is severe. Other clinical features include early frontal alopecia, cataracts, blepharitis, conjunctivitis, and testicular atrophy. Mental retardation may occur. Dystrophic cardiac disease occurs late. The disease can be quite variable, and some patients and affected family members may manifest only one or two features.

1. Laboratory studies. EMG reveals myopathic changes and characteristic afterpotentials of myotonia. Serum muscle enzymes are usually normal, and creatinuria is rare. Histopathologic features are similar to those of other dystrophies; however, there may be prominent rows of sarcolemmal nuclei, spirals of myofibrils, and areas of clear sarcoplasm, devoid of myofibrils. Type 1 fiber atrophy is present. Peripheral nerves and anterior horn cells are normal. This disease is associated with an expanded cytosine-thymine-guanine (CTG) repeating motif in the noncoding region of the myotonin protein kinase gene.

2. Treatment. Quinine in a dose of 300 to 600 mg PO q6h can relieve the myotonia; however, there is no known cure. Supportive therapy and physical therapy may prolong mobility and prevent contractures.

F. Congenital myopathies are rare inherited diseases that begin during infancy. Progression may be quite insidious. Diagnosis rests on muscle biopsy.

V. Inflammatory myopathies This group of myopathies is characterized by inflammation of unknown etiology within the muscles.

A. For full discussions of idiopathic polymyositis, dermatomyositis, myositis of other rheumatic diseases, and myositis associated with carcinoma, see C.h.a.p.te..r 2.Z..

B. Sarcoid myopathy. Muscle biopsies in small numbers of sarcoid patients without symptoms of muscle pain or weakness have revealed noncaseating granulomas typical of the disease. The occurrence of asymptomatic muscle involvement has clouded the issue of whether a true sarcoid myopathy exists. Nonetheless, there are sarcoid patients with symptomatic muscle involvement. Muscle pain and tenderness are most often seen in acute sarcoidosis with erythema nodosum. Symmetric proximal muscle weakness can be seen in chronic sarcoid (see Chapters.!).

1. Laboratory studies. EMG shows a nonspecific myopathic pattern. Muscle biopsy shows noncaseating granulomas, surrounding lymphocytic infiltrates, muscle fiber necrosis, and fiber regeneration.

2. Differential diagnosis. Similar granulomatous reactions have been seen in various malignancies, leprosy, syphilis, tuberculosis, Crohn's disease, drug reactions, and several fungal infections.

3. Treatment. Administration of 20 to 40 mg of prednisone daily has been effective in about half of patients.

VI. Infectious myositis and myopathy Several microorganisms have been implicated in the onset of myositis. Both diffuse and proximal myopathies have been reported. This group of myopathies is to be distinguished from the isolated muscle involvement seen in tropical pyomyositis, streptococcal myositis, and clostridial myonecrosis.

A. Trichinosis is caused by the nematode Trichinella spiralis. It is transmitted by ingestion of uncooked or poorly cooked pork or bear meat. Within 2 days of ingestion of the cysts, diarrhea, nausea, abdominal pain, and fever occur. By the end of the first week, patients may have fever, periorbital edema, conjunctivitis, muscle pain and tenderness, and an erythematous maculopapular rash. Muscle weakness may be mild but is often quite severe. Muscle invasion may last 6 weeks. Myocarditis and encephalitis may occur during this stage of illness. The most commonly invaded muscles are those of the diaphragm, eye, tongue, shoulder, and calf.

1. Laboratory studies. Biopsy of the deltoid or gastrocnemius muscle should be performed during the third or fourth week of illness. Pressing the tissue between glass slides will reveal the uncalcified larvae. Calcified cysts represent former infection. The muscle shows a severe myositis with neutrophilic, eosinophilic, and lymphocytic infiltrates. Fiber degeneration and necrosis are present. Serum muscle enzymes are characteristically elevated. By the end of the second week, a 15% to 50% eosinophilia is present. Serologic tests become positive by the end of the third week.

2. Treatment is with 25 mg of thiabendazole per kilogram twice daily for Z days. Patients with myocarditis, encephalitis, or severe hypersensitivity manifestations should be treated with 40 mg of prednisone daily during thiabendazole administration.

B. Toxoplasmosis. Toxoplasma gondii has been proved to cause myositis in an occasional patient. A recent case report of a patient with polymyositis and cerebellar ataxia is an example of this problem. The patient had severe muscle cramps, coarse fasciculations, and no weakness or muscle tenderness. Serum CK was markedly elevated. EMG indicated a chronic peripheral neuropathy. Muscle biopsy showed chronic interstitial myositis, fiber necrosis, and encysted T. gondii. Toxoplasma organisms were grown in mice injected with a suspension of the muscle biopsy.

Serologic evidence of Toxoplasma infection in patients with polymyositis consists of elevated complement-fixation titers, positive findings on Sabin-Feldman dye tests, and specific immunoglobulin M antibodies in a subgroup of patients. Most rheumatologists do not routinely order these tests. However, some patients with positive serologies defined early in the disease course did respond to antibiotic therapy for toxoplasmosis.

C. Viral myositis. A number of different viruses may cause an illness similar to polymyositis. Often, a prodromal illness is caused by the virus. It is unclear whether the myositis is a post-infectious immune phenomenon or a true infection of muscle. Virus-like particles have been found in muscle in some cases. Biopsy shows myositis with fiber necrosis and regeneration. Myoglobinuria may occur, especially with influenza and herpes group myositis. Viruses implicated are hepatitis B and C virus, echovirus, coxsackie virus, herpes simplex virus, and influenza virus. Late atrophy of muscles may occur. In addition, muscle biopsy findings at times may be normal. Human immunodeficiency virus (HIV) and medications such as zidovudine (AZT) can be responsible for severe myopathy as well as a syndrome indistinguishable from polymyositis.

D. Miscellaneous infections causing myopathy. It should be noted that numerous other microorganisms have been implicated as etiologic agents in myositis. Some have occurred only in immunocompromised patients. Agents found include Candida tropicalis, Mycoplasma pneumoniae, Trypanosoma cruzi, and Echinococcus alveolaris.

VII. Fibromyalgia presents as variable muscle pain and multiple tender points with normal muscle strength. The sedimentation rate and muscle enzymes are normal. It is a diagnosis of exclusion. (See Chapter 49 for a full discussion.)

VIII. Polymyalgia rheumaticc presents in patients over 50 with proximal soreness and stiffness and constitutional symptoms. It may be associated with temporal arteritis. The Westergren sedimentation rate is elevated to over 50 mm/h in most patients, and many are anemic. (See Chapter.26 for a full discussion.)

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