A 42-year-old homeless man comes to the emergency department due to painful muscle spasms. The patient has had frequent involuntary contractions of the jaw, neck, and trunk muscles, which are triggered by sensory stimuli. He has a history of injection drug use and has had several injection site infections. Physical examination shows neck stiffness and decreased opening of the jaw. Which of the following is the most likely path taken by the agent responsible for this patient's neurologic symptoms?
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This patient with painful muscle spasms and neck/jaw contraction likely has tetanus, which is caused by the spore-forming anaerobic bacterium Clostridium tetani. C tetani spores are found in soil worldwide and are typically transmitted to humans during a puncture injury (eg, dirty needle use, splinter). The low oxygen tension of a puncture wound allows spores to germinate into vegetative rod-shaped bacteria that proliferate locally and produce tetanospasmin (tetanus toxin).
Although the pathogen is noninvasive (it stays localized to the wound site), the toxin enters the presynaptic terminals of the lower motor neurons and travels by retrograde axonal transport to the CNS. In the spinal cord, at the level of the anterior horn cells, the toxin blocks the release of neurotransmitters from inhibitory interneurons, leading to spasmodic muscle contraction.
Classic findings include jaw stiffness due to masseter muscle spasm (trismus/lockjaw) and sustained contraction of the facial muscles, producing a bizarre "smiling" appearance (risus sardonicus). Spasmodic contraction of the back and neck can cause opisthotonos. Intermittent muscle spasms are frequently triggered by sensory stimuli (eg, loud noises, light, touch). Respiratory muscle involvement can lead to respiratory failure.
(Choice A) Rabies virus is transmitted via the saliva of an infected animal during a bite wound. The virus spreads by retrograde axonal transport through peripheral nerves and enters the spinal cord/CNS. Patients with rabies usually develop painful pharyngeal spasms, hydrophobia, fever, and hyperactivity. However, this patient has no history of an animal bite; he likely developed tetanus due to a puncture wound with a contaminated needle.
(Choice B) Food-borne botulism is marked by the ingestion of preformed botulism toxin with subsequent spread through the systemic circulation. The toxin blocks cholinergic presynaptic nerve terminals, which results in symmetric, descending weakness.
(Choice C) Bacteria (eg, Staphylococcus aureus) from a wound can spread through the bloodstream to the brain leading to brain abscess. Symptoms vary but the most common manifestation is unilateral headache.
(Choice D) Diphtheria is associated with the formation of a pseudomembranous pharyngeal exudate and the elaboration of diphtheria toxin into the systemic circulation. The toxin causes cardiac (myocarditis) and neurologic toxicity.
(Choice E) Herpes simplex virus type 1 causes orolabial lesions and is the most frequent cause of sporadic encephalitis, which occurs when the virus spreads via the trigeminal nerve or olfactory tract to the temporal lobe. Patients usually present with confusion, fever, focal neurologic deficits, and seizures.
Educational objective:
Tetanus is caused by infection with toxigenic strains of the anaerobic bacterium Clostridium tetani. Transmission typically occurs when spores are inoculated into the skin via a puncture wound. The bacteria germinate, replicate locally, and elaborate tetanospasmin (tetanus toxoid). The toxin spreads in a retrograde fashion through the lower motor neurons to the spinal cord, where it blocks inhibitory interneurons and causes spasmodic muscle contraction (eg, trismus, risus sardonicus).