Clinical Evaluation of Weakness

Diagnosis of Weakness

determine whether the "weakness" is due to a "myopathy’ or neurogenic cause

Causes:

Myopathy: group of diseases characterized by progressive weakness and wasting of muscle

Primary: involvement of the muscle fiber itself without involvement of peripheral or central nervous system causing muscle weakness

Clinical Course: Symmetrical Weakness that affects large proximal muscles of the pelvic and shoulder girdle. Muscle test: response is all or none with a "giving away" response

Neuropathies (neurogenic atrophy): weakness secondary to abnormality along the course of the peripheral nerve from the anterior horn cell to the neuromuscular junction. "Pathological fatigue" – repetitive use and cranial nerve dysfunction

Clinical Course: Distal weakness that may be asymmetrical. May have sensory loss, spontaneous muscle twitching (fasciculations), tiredness, fatigue

Clinical manifestations of muscle disease

weakness, lack of endurance, muscle pain (myalgia)

Clinical History: weakness

Initial site: proximal>distal involvement Þ myopathy; initial distal involvement Þ neurogenic

Age of onset: give indication as to type of muscular dystrophy (Duchenne <5yrs)

Rate of progression: polymyositis – rapid loss; facioscapulohumeral dystrophy (FSH) – very slow

Periodicity: waxing and waning – metabolic muscle disease; shift of electrolytes (periodic paralysis); myoneural junction block (myasthenia gravis)

primary myopathy Þ progressive process; no spontaneous reversal or fluctuation in weakness

Heredity: genetic counseling

Pain: Not present in dystrophic or congenital myopathy. Seen in metabolic muscle diseases and some polymyositis.

Physical Examination

Weakness: Myopathies. Always Proximal (except myotonic) and Symmetrical (except FSH)

Atrophy: decrease in size or bulk of muscle. Chronic muscle disease

Hypertrophy: Pseudohypertrophy – enlargement due to fat or fibrous tissue replacement of muscle parenchyma.

Tenderness: Polymyositis. May occur with ETOH myopathy, myoglobinuria, phosphorylase deficiency disease

Myotonia: inability to relax a stimulated muscle Þ Myotonic Dystrophy. Can demonstrate through EMG and reflex hammer of thenar eminence of the hand.

Contracture: limitation of range of motion of an extremity. Not due to joint disease or neural impulse. Due to imbalance of agonist and antagonist muscle strength.

Fasciculations: spontaneous firing of motor units resulting in twitching of resting muscles. Disease of Anterior horn cell or peripheral nerve cells. NOT seen in primary myopathy

Muscle stretch reflexes: Deep tendon reflex Þ primary muscle disease, depressed or absent in neuropathic disease.

Sensory involvement: NOT seen in primary myopathy. Peripheral or central neuropathic etiology

Clinical Tests

(1) Manual Muscle Test: evaluation of strength of individual muscles. Essential to select muscle with appropriate among of weakness

(2) Functional Testing: evaluation of patient’s performance of normal functional activities of daily living – indicative of patient’s physical ability

Test includes: walking a fixed distance; rising to an erect position form a chair, climbing stairs
Practical measure in follow-up examination of patients with neuromuscular disease.

Laboratory tests

(1) Serum Creatine Kinase: enzyme that catalyzes the transfer of phosphate from phosphocreatine to ADP forming ATP and creatine. Important in muscle contraction.

Primary myopathy: Ý Ý Ý Elevated. Due to escape of enzyme from fiber in active muscle degeneration and/or Ý in muscle membrane permeability. ALWAYS elevated in Duchenne Muscular Dystrophy

NOT always elevated in myopathic disease – Chronic or congenital myopathies. Can be elevated with local muscle trauma

Neurogenic: Normal or slightly elevated. Muscle fibers are more intact

(2) Urinary Creatine and Creatinine Excretion: Creatine is synthesized from glycocyamine (made in the kidney) by methylation with methionine in the Liver. Creatine is NOT synthesized in the muscle but it’s chief storage depot is the muscle. Found mainly as phosphocreatine in muscle. Excreted in urine as creatinine.

measured as % 24hr creatinuria = (Creatine/ (creatine + creatinine (mgm/24hrs)))

Creatinuria (Ý creatine in urine) – indicative of muscle dysfunction

NON-SPECIFIC INDICATOR of muscle dysfunction – CANNOT differentiate neurogenic from myopathic weakness

Can be a good indicator of amount of functioning muscle - Ý creatinine excretion indicates more normal muscle

(3) Electromyography(EMG): provides information on the state of the "motor unit" – anterior horn cell, motor axons, neuromuscular junction, and muscle fiber innervated.

distinguishes neurogenic from primary myopathy, determine presence of myotonic dystrophy, identify suitable muscle for biopsy

Primary Myopathy: ß amplitude, short duration, polyphasic. Motor unit has less fibers to fire Þ decreased EMG electric potentials (motor unit action potential)

Neurogenic Atrophy: Ý amplitude. Because of denervation – there are more muscles per nerve. Can be indicative of degenerative neuropathy (lower motor unit disease)

Muscle Biopsy – most important! The Cadillac of diagnostic tests

Biopsy is the definitive mode of confirmation for presence, severity, and type of muscle disease present
NEVER take a biopsy from the EMG site – due to local trauma that may be present because of EMG (take from contralateral muscle)
Pathological Classification: Myopathic vs. Neurogenic Process

	Myopathic	Neurogenic
Fiber size	Wide variation	Bimodal groups
Fiber shape	Round	Angulated
Nuclei	Centralization	Apparent increase
Necrosis	Yes	no
Regeneration	Yes	No
Fibrosis	Yes	No
Inflammation	Yes	No

Bimodal Groups: loss of motor unit Þ Neurogenic atrophy. Stimulation of remaining typed fibers (recall that motor unit determines the muscle type). A bimodal distribution of muscle fiber types results. Contrast to muscle disease which is erratic