MUSCLE DISEASES

Weakness is the predominant feature of muscle disease. In contrast to weak-ness secondary to a neuropathy, reflexes and sensation are normal in muscle disease and wasting of muscle occurs late. Muscle disease is acquired or congenital (Table 17.23). Investigation of suspected muscle disease is dis-cussed on page 738.

Table 17.23 Muscle disease: classification

Acquired

Genetic

Inflammatory
Polymyositis/Dermatomyositis
Inclusion body myositis
Viral, bacterial, parasitic infection
Sarcoidosis

Dystrophic
Duchenne
Facioscapulohumeral
Limb girdle and others

Endocrine*
Corticosteroids/Cushing’s syndrome
Thyroid disease
Hypocalcaemia
Osteomalacia
Hypokalaemia
Alcohol excess
Drugs (diamorphine, amphetamine, statin)

Myotonic
Myotonic dystrophy
Myotonia congenital
Channelopathies
Hypokalaemic periodic paralysis
Hyperkalaemic periodic paralysis
Normokalaemic periodic paralysis

Myasthenic
Myasthenia gravis
Lambert–Eaton myasthenic
myopathic syndrome (LEMS)

Metabolic (all rare)
Myophosphorylase deficiency
(McArdle’s syndrome)
Other defects of glycogen and
fatty acid metabolism
Mitochondrial disease
Malignant hyperpyrexia (p. 661)

Acquired muscle disease

Polymyositis and dermatomyositis are discussed on page 305. Inclusion body myositis is a rare idiopathic inflammatory myopathy occurring in men over 50 years. There is pharyngeal muscle weakness and slowly progressive weakness of the distal muscles. There is no specific treatment. Muscle disease associated with drugs, toxins and endocrine disease usually produces weakness of the limb girdles (proximal myopathy). Typically the patient is unable to rise from a seated position without the use of their arms.

Myasthenia gravis (MG)

MG is characterized by weakness and fatiguability of ocular, bulbar and proximal limb muscles. The heart is not affected. It can present at any age but peaks occur in the 3rd and 6th decade. It is twice as common in women as in men.

Aetiology

In most cases, autoantobodies to acetylcholine receptors (anti-AChR anti-bodies) at the postsynaptic membrane of the neuro-muscular junction cause receptor loss. Myasthenia gravis is associated with thymic hyperplasia in about 70% of patients under 40 years of age, and in about 10% a thymic tumour is found.

Clinical features

Presentation is usually with fatiguability of muscle on sustained or repeated activity that improves after rest. The ocular muscles are the first to be involved in about 65% of patients resulting in ptosis that is often partial or unilateral and improves after sleep or with the application of an ice pack on the lid (specific to MG). There is extraocular progression of weakness usually from top down leading to difficulty in talking, chewing, swallowing and res-piratory difficulties. Fatigue can be demonstrated by ptosis on sustained upward gaze or asking the patient to sit with the arms outstretched and looking for a slow downward drift.

Investigations

■ Anti-AChR antibodies are specific for MG and are found in the serum in 90% of cases of generalized MG but in less than 30% of pure ocular disease. Antibodies against a muscle-specific receptor tyrosine kinase (anti-MuSK antibodies) are found in Anti-AChR negative disease.

■ Nerve stimulation tests show a characteristic decrement in evoked poten-tial following stimulation of the motor nerve.

■ Mediastinal imaging with CT or MRI to look for a thymoma.

■ The Tensilon test (positive if intravenous injection of edrophonium, an anticholinesterase, results in rapid temporary improvement in weakness) is seldom required.

Differential diagnosis

Thyroid opthalmopathy, myotonic dystrophy and brainstem cranial nerve lesions present with ocular and/or bulbar symptoms. Botulism causes rapid onset of muscle weakness, often with pupillary paralysis and there is a history of ingestion of contaminated food. Generalized muscle weakness occurs in motor neurone disease (differs in involvement of upper and lower motor neurones), Lambert-Eaton myasthenic myopathic syndrome is due to a presynpatic defect on the neuromuscular junction in patients with small cell lung cancer. Limb muscle weakness is the usual presenting symptom, exercise improves symptoms and reflexes are decreased or absent.

Management

Anticholinesterases (e.g. oral pyridostigmine: starting dose 60 mg three times a day and then titrated according to the response) increase availability of acetylcholine at the receptor and are the mainstay of treatment. Excessive treatment may cause a ‘cholinergic crisis' with worsening weakness, hyper-salivation, abdominal pain and diarrhoea. However, weakness is more often due to myasthenia unless high doses of pyridostigmine are used (>360 mg per day).

Immunosuppressant drugs are used in patients who do not respond to pyridostigmine or who relapse on treatment. Corticosteroids produce an improvement in 70% of cases. Azathioprine, mycophenolate and other immunosuppressants are also used.

Plasmapheresis and intravenous immunoglobulin have rapid onset and transient benefit and are used occasionally in myasthenic crisis (life-threatening condition with weakness of respiratory and bulbar muscles and may be drug induced, e.g. aminoglycosides, β-Blockers) or as a bridge to other therapies (thymectomy or immunosuppressants).

Thymectomy is performed in patients with a thymoma (because of the potential for malignancy) and in selected patients without a thymoma who are most likely to benefit (<50 years, + ve anti-AChR antibodies) and enter a remission without medication.

Muscular dystrophies

This is an inherited group of progressive myopathic disorders resulting from defects in a number of genes needed for normal muscle function. The Duchenne and Becker muscular dystrophies are inherited as X-linked reces-sive traits caused by a mutation in the dystrophin gene on chromosome 21. Patients with Duchenne muscular dystrophy present in early childhood with weakness in the proximal muscles of the leg. There is progression to other muscle groups with severe disability and death in the late teens. There is no curative treatment. Patients with Becker muscle dystrophy present later and the degree of clinical involvement is milder. Other dystrophies present later in life and are summarized in Table 17.24.

Ebook Essentials of Kumar and Clark's Clinical Medicine, 5e

1. Ethics and communication

Ethics and communication

2. Infectious diseases

Infectious diseases

3. Gastroenterology and nutrition

Gastroenterology and nutrition

4. Liver, biliary tract and pancreatic disease

Liver, biliary tract and pancreatic disease
LIVER BIOCHEMISTRY AND LIVER FUNCTION TESTS
SYMPTOMS AND SIGNS OF LIVER DISEASE
JAUNDICE
HEPATITIS
NON - ALCOHOLIC FATTY LIVER DISEASE (NAFLD)
CIRRHOSIS
COMPLICATIONS AND EFFECTS OF CIRRHOSIS
LIVER TRANSPLANTATION
TYPES OF CHRONIC LIVER DISEASE AND CIRRHOSIS
PRIMARY SCLEROSING CHOLANGITIS
BUDD - CHIARI SYNDROME
LIVER ABSCESS
LIVER DISEASE IN PREGNANCY
LIVER TUMOURS
GALLSTONES
THE PANCREAS
CARCINOMA OF THE PANCREAS
NEUROENDOCRINE TUMOURS OF THE PANCREAS

5. Haematological disease

Haematological disease
ANAEMIA
Assessment and treatment of suspected neutropenic sepsis
HAEMOLYTIC ANAEMIA
INHERITED HAEMOLYTIC ANAEMIAS
ACQUIRED HAEMOLYTIC ANAEMIA
MYELOPROLIFERATIVE DISORDERS
THE SPLEEN
BLOOD TRANSFUSION
THE WHITE CELL
HAEMOSTASIS AND THROMBOSIS
THROMBOSIS
THERAPEUTICS

6. Malignant disease

Malignant disease
MYELOABLATIVE THERAPY AND HAEMOPOIETIC STEM CELL TRANSPLANTATION
THE LYMPHOMAS
THE PARAPROTEINAEMIAS
PALLIATIVE MEDICINE AND SYMPTOM CONTROL

7. Rheumatology

Rheumatology
COMMON INVESTIGATIONS IN MUSCULOSKELETAL DISEASE
COMMON REGIONAL MUSCULOSKELETAL PROBLEMS
BACK PAIN
OSTEOARTHRITIS
INFLAMMATORY ARTHRITIS
THE SERONEGATIVE SPONDYLOARTHROPATHIES
Clinical features, Investigations
INFECTION OF JOINTS AND BONES
AUTOIMMUNE RHEUMATIC DISEASES
SYSTEMIC INFLAMMATORY VASCULITIS
DISEASES OF BONE
THERAPEUTICS

8. Water, electrolytes and acid–base balance

WATER AND ELECTROLYTE REQUIREMENTS
BODY FLUID COMPARTMENTS
REGULATION OF BODY FLUID HOMEOSTASIS
PLASMA OSMOLALITY AND DISORDERS OF SODIUM REGULATION
DISORDERS OF POTASSIUM REGULATION
DISORDERS OF MAGNESIUM REGULATION
DISORDERS OF ACID - BASE BALANCE
THERAPEUTICS

9. Renal disease

Renal disease
INVESTIGATION OF RENAL DISEASE
GLOMERULAR DISEASES
NEPHROTIC SYNDROME
URINARY TRACT INFECTION
TUBULOINTERSTITIAL NEPHRITIS
HYPERTENSION AND THE KIDNEY
RENAL CALCULI AND NEPHROCALCINOSIS
URINARY TRACT OBSTRUCTION
ACUTE RENAL FAILURE/ACUTE KIDNEY INJURY
CHRONIC KIDNEY DISEASE
RENAL REPLACEMENT THERAPY
CYSTIC RENAL DISEASE
TUMOURS OF THE KIDNEY AND GENITOURINARY TRACT
DISEASES OF THE PROSTATE GLAND
TESTICULAR TUMOUR
URINARY INCONTINENCE

10. Cardiovascular disease

COMMON PRESENTING SYMPTOMS OF HEART DISEASE
INVESTIGATIONS IN CARDIAC DISEASE
CARDIAC ARRHYTHMIAS
HEART FAILURE
ISCHAEMIC HEART DISEASE
RHEUMATIC FEVER
VALVULAR HEART DISEASE
PULMONARY HEART DISEASE
MYOCARDIAL DISEASE
CARDIOMYOPATHY
PERICARDIAL DISEASE
SYSTEMIC HYPERTENSION
ARTERIAL AND VENOUS DISEASE
ELECTRICAL CARDIOVERSION
DRUGS FOR ARRHYTHMIAS
DRUGS FOR HEART FAILURE
DRUGS AFFECTING THE RENIN - ANGIOTENSIN SYSTEM
NITRATES, CALCIUM - CHANNEL BLOCKERS AND POTASSIUM - CHANNEL ACTIVATORS

11. Respiratory disease


Respiratory disease
TUBERCULOSISnd
DIFFUSE DISEASES OF THE LUNG PARENCHYMA
OCCUPATIONAL LUNG DISEASE
CARCINOMA OF THE LUNG
DISEASES OF THE CHEST WALL AND PLEURA
DISORDERS OF THE DIAPHRAGM

12. Intensive care medicine

Intensive care medicine

13. Drug therapy, poisoning, and alcohol misuse

Drug therapy, poisoning, and alcohol misuse

14. Endocrine disease

Endocrine disease
PITUITARY HYPERSECRETION SYNDROMES
THE THYROID AXIS
MALE REPRODUCTION AND SEX
FEMALE REPRODUCTION AND SEX
THE GLUCOCORTICOID AXIS
THE THIRST AXIS
DISORDERS OF CALCIUM METABOLISM
DISORDERS OF PHOSPHATE CONCENTRATION
ENDOCRINOLOGY OF BLOOD PRESSURE CONTROL
DISORDERS OF TEMPERATURE REGULATION
THERAPEUTICS

15. Diabetes mellitus and other disorders of metabolism

DIABETES MELLITUS
DIABETIC METABOLIC EMERGENCIES
COMPLICATIONS OF DIABETES
SPECIAL SITUATIONS
HYPOGLYCAEMIA IN THE NON - DIABETIC
DISORDERS OF LIPID METABOLISM
THE PORPHYRIAS

16. The special senses

THE EAR
THE NOSE AND NASAL CAVITY
THE THROAT
THE EYE

17. Neurology

COMMON NEUROLOGICAL SYMPTOMS
COORDINATION OF MOVEMENT
THE CRANIAL NERVES
COMMON INVESTIGATIONS IN NEUROLOGICAL DISEASE
UNCONSCIOUSNESS AND COMA
STROKE AND CEREBROVASCULAR DISEASE
EPILEPSY AND LOSS OF CONSCIOUSNESS
NERVOUS SYSTEM INFECTION AND INFLAMMATION
HYDROCEPHALUS
HEADACHE, MIGRAINE AND FACIAL PAIN
SPINAL CORD DISEASE
DEGENERATIVE NEURONAL DISEASES
DISEASES OF THE PERIPHERAL NERVES
MUSCLE DISEASES
MYOTONIAS
DELIRIUM
THERAPEUTICS

18. Dermatology

Dermatology

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