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 10 days ago while working in her garden. On examination, she has opisthotonus and generalized muscle rigidity with preserved consciousness. Blood pressure is 140/90 mmHg and heart rate is 110 bpm. What is the pathophysiological mechanism of the muscle rigidity in this patient?

Explanation

## Clinical Diagnosis: Tetanus The patient presents with classic features of tetanus (lockjaw, risus sardonicus, opisthotonus) following a contaminated puncture wound. This is a clinical case of *Clostridium tetani* infection. ## Pathophysiology of Muscle Rigidity **Key Point:** Tetanospasmin (tetanus toxin) is a zinc-dependent endopeptidase that cleaves SNARE proteins (specifically synaptobrevin) on the presynaptic membrane of inhibitory interneurons. ### Mechanism of Action 1. **Toxin Entry**: Tetanospasmin enters the motor neuron terminal via endocytosis after binding to polysialogangliosides. 2. **Retrograde Transport**: The toxin undergoes retrograde axonal transport to the spinal cord. 3. **SNARE Protein Cleavage**: In the spinal cord, the heavy chain releases the light chain, which cleaves synaptobrevin, preventing the release of GABA and glycine (inhibitory neurotransmitters). 4. **Loss of Inhibition**: Without inhibitory neurotransmitter release, there is unopposed excitation of motor neurons, leading to sustained muscle contraction and rigidity. **High-Yield:** The rigidity is due to **loss of inhibitory control**, not excessive excitation. This is why consciousness remains intact — the sensory and cognitive pathways are unaffected. ## Clinical Correlation **Clinical Pearl:** The characteristic features of tetanus reflect this pathophysiology: - **Trismus** (lockjaw): masseter muscle rigidity - **Risus sardonicus**: facial muscle contraction - **Opisthotonus**: severe back arching from unopposed extensor muscle contraction - **Preserved consciousness**: cerebral cortex is not affected ## Comparison with Other Clostridial Toxins | Toxin | Source | Mechanism | Effect | |-------|--------|-----------|--------| | Tetanospasmin | *C. tetani* | Blocks inhibitory NT release | Muscle rigidity, spasms | | Botulinum toxin | *C. botulinum* | Blocks acetylcholine release | Flaccid paralysis | | Alpha toxin | *C. perfringens* | Phospholipase C | Gas gangrene, myonecrosis | **Mnemonic:** **GABA-Glycine Gone** = Tetanus toxin blocks inhibitory transmitters, leading to unopposed excitation and rigidity.