A 32-year-old woman from rural Maharashtra presents to the emergency department with a 3-day history of progressive muscle rigidity, trismus, and risus sardonicus. She sustained a puncture wound to her left foot from a rusty nail while working in the fields 10 days ago. On examination, she has opisthotonus and generalized muscle spasms triggered by minimal sensory stimuli. Her vital signs are stable and she is afebrile. Cerebrospinal fluid analysis is normal. Which of the following best explains the pathophysiology of her clinical presentation?

Explanation

## Pathophysiology of Tetanus **Key Point:** Tetanospasmin (tetanus toxin) acts on spinal interneurons, NOT the neuromuscular junction, blocking the release of inhibitory neurotransmitters (GABA and glycine), resulting in unopposed excitatory motor neuron activity. ### Mechanism of Tetanospasmin Action 1. **Toxin Entry & Transport**: Tetanospasmin is produced locally at the wound site by *Clostridium tetani* and is taken up by motor nerve terminals. 2. **Retrograde Axonal Transport**: The toxin travels retrogradely along motor neurons to the spinal cord. 3. **Synaptic Blockade**: Once in the spinal cord, tetanospasmin cleaves SNARE proteins (specifically VAMP/synaptobrevin), preventing vesicular release of GABA and glycine from inhibitory interneurons. 4. **Loss of Inhibition**: Without inhibitory tone, motor neurons fire unopposed, causing sustained muscle contraction. ### Clinical Correlates of the Mechanism | Feature | Explanation | | --- | --- | | **Trismus** | Masseter muscles contract unopposed; jaw muscles are among the first affected | | **Risus sardonicus** | Facial muscle rigidity creates a characteristic "sardonic smile" | | **Opisthotonus** | Extensor muscles are stronger than flexors; unopposed contraction causes severe back arching | | **Spasm on stimuli** | Any sensory input triggers motor neuron firing without inhibitory brake | | **Normal CSF** | No CNS inflammation; toxin acts at synaptic level, not parenchymal | **High-Yield:** The classic triad of tetanus—trismus, risus sardonicus, and opisthotonus—all result from loss of inhibitory control of motor neurons, NOT from increased acetylcholine production or direct CNS invasion. **Clinical Pearl:** Tetanus is a medical emergency requiring airway management, muscle relaxants, and supportive care, because the toxin's effects are irreversible at the synaptic level; recovery depends on new nerve terminal sprouting over weeks to months. **Mnemonic: GABA-GONE** — Tetanospasmin blocks GABA (and Glycine) release, leaving motor neurons unGONE (unopposed).