Myopathies produce generalized, symmetrical weakness
muscle tone is usually diminished, but sensation is preserved.
Metabolic disorders affecting muscle strength are painless in nature
Reflexes are present but diminished
DISEASE HISTORY STRENGTH DEEP TENDON REFLEX SENSATION WASTING Myelopathy Trauma, infection, cancer Normal to decreased Increased Normal to decreased No Motor neuron disease (ALS) Progressive difficulty swallowing, speaking, walking Decreased Increased Normal Yes Neuropathy Recent infection Ascending weakness Normal or decreased Distal > proximal Decreased Decreased Yes Neuromuscular junction disease Food (canned goods) Tick exposure Easy fatigability Normal to fatigue Normal Normal No Myopathy Thyroid disease Previous similar episodes Decreased Proximal > distal Normal Normal Yes
Ramsay-Hunt syndrome refers to herpes zoster involving the tympanic membrane, ear canal, and other areas in the distribution of the sensory branches of the facial nerve.
Herpetic zoster may result in motor abnormalities in up to 25% of cases.
Weakness or diplopia only on exertion is a common complaint in early cases of multiple sclerosis. Another early presenting sign is retrobulbar neuritis; in fact, 50–75% of cases occur in patients who develop multiple sclerosis.
Steroids may transiently exacerbate weakness in patients with polymyositis and should not be started on patients who will be discharged from the ED.
Acute periodic paralysis appears to involve abnormalities in cellular function, possibly related to potassium transport. The disease is most common in young men. No specific physical findings may be found and it is often misdiagnosed as hysterical in origin
2. A 34-year-old woman with known myasthenia gravis presents in respiratory distress. She is unable to move without assistance. Her vital signs are: temperature 36 ◦ C (96.8 ◦ F), heart rate 50/min, blood pressure 100/60 mm Hg, respiratory rate 35/min and shallow. She is drooling and has upper airway rhonchi and bilateral wheezing. Her respiratory rate appears to be decreasing. You immediately:
a. Administer 2–4 mg of intravenous edrophonium.
b. Perform endotracheal intubation.
c. Administer 1 mg of atropine; if there is an improvement in her wheezing, administer pralidoxime.
A 34-year-old woman with myasthenia gravis presents with flank pain and fever of 103.4 ◦ F. She is allergic to penicillin, and despite boluses of intravenous fluid and antibiotic therapy, she becomes hypotensive. A medicine that you can safely use in her management is:
Aminoglycoside antibiotics have some curare-type effects on the motor endplate; if they are used in the myasthenic patient, the physician should be prepared to treat paralysis and respiratory arrest. Obviously, these patients are more susceptible to muscle-paralyzing agents as well. Phenytoin, quinidine, procainamide, and lithium can also adversely affect patients with myasthenia gravis.
An extract from the bark and stems is the source of a potent isoquinoline alkaloid used in the deadly poison curare. Amazonian Indians use the gummy extract to coat the poison darts of their blowguns. The alkaloid D-tubocurarine blocks acetylcholine receptor sites at neuromuscular junctions, causing relaxation and paralysis of muscles, including respiratory organs and the heart.
In fact, D-tubocurarine has been used to relax the heart muscle during open heart surgery.
It has also been used to treat the spastic paralysis of tetanus toxin from the bacterium Clostridium botulinum . Tetanus toxin causes uncontrollable muscle contraction throughout the body.
results from an autoimmune attack directed against the voltage-gated calcium channels (VGCCs) on the presynaptic motor nerve terminal.
This results in a loss of functional VGCCs at the motor nerve terminals. The number of quanta released by a nerve impulse is diminished.
However, because presynaptic stores of ACh and the postsynaptic response to ACh remain intact, rapid repetitive stimulation or voluntary activation that aids in the release of quanta will raise the endplate potential above threshold and permit generation of muscle action potential.
As neuromuscular transmission is completed at additional neuromuscular junctions, a transient increase will occur in the strength of the muscle.
Parasympathetic, sympathetic, and enteric neurons are all affected.
Clinically, this phenomenon is noted by the appearance of previously absent tendon reflexes following a short period of strong muscle contraction by the patient.
Autoantibodies (immunoglobulin G [IgG]) develop against ACh nicotinic postsynaptic receptors for unknown reasons, although certain genotypes are more susceptible. 2 Cholinergic nerve conduction to striated muscle is impaired by a mechanical blockage of the binding site by antibodies and, ultimately, by destruction of the postsynaptic receptor. Patients become symptomatic once the number of ACh receptors is reduced to approximately 30% of normal. The cholinergic receptors of smooth and cardiac muscle have a different antigenicity than skeletal muscle and are not affected by the disease. The role of the thymus in the pathogenesis of myasthenia gravis is not entirely clear, but 75% of patients with myasthenia gravis have some degree of thymus abnormality (eg, hyperplasia in 85% of cases, thymoma in 15% of cases). Given the immunologic function of the thymus and the improvement in the clinical condition of patients following thymectomy, the thymus is suspected to be the site of autoantibody formation. However, the stimulus that initiates the autoimmune process has not been identified