Trismus is NOT an Expected Feature of Rabies
Trismus (lockjaw) is not a characteristic symptom of rabies and should not be expected in patients with this disease. In fact, the presence of trismus should prompt consideration of alternative diagnoses, particularly tetanus.
Key Clinical Distinctions
Rabies Does NOT Present with Trismus
- Rabies manifests with hydrophobia, which is pathognomonic for the disease—this involves terror and violent spasms of the inspiratory muscles, larynx, and pharynx precipitated by attempts to drink water or even by the sight or sound of running water 1, 2
- These spasms in rabies involve inspiratory muscles contracting with swallowing attempts, not sustained jaw muscle contraction 3
- The brainstem is preferentially involved in rabies, but there are notably no clinical signs of brainstem dysfunction that would produce trismus 4
Clinical Forms of Rabies
Furious (Encephalitic) Rabies (80% of cases):
- Intermittent episodes of agitation, hyperexcitability, and bizarre behavior alternating with periods of lucidity 1
- Hydrophobia and aerophobia are the hallmark features 5, 2
- Hypersalivation, tachycardia, and hyperpyrexia occur 2
Paralytic (Dumb) Rabies (20% of cases):
- Ascending flaccid paralysis resembling Guillain-Barré syndrome, starting from the bitten extremity 1
- Progressive lower motor neuron weakness without muscle rigidity 3
Critical Differential Diagnosis Consideration
- Tetanus is the primary differential diagnosis when trismus is present and must be distinguished from rabies 2
- If a patient presents with suspected rabies but has trismus, tetanus should be strongly considered as the actual diagnosis
- Hysterical pseudo-hydrophobia, other encephalitides, and delirium tremens are other important differentials for furious rabies 2
Clinical Pitfall to Avoid
Do not confuse the pharyngeal/laryngeal spasms of hydrophobia in rabies with the sustained masseter muscle contraction (lockjaw) of tetanus. These are fundamentally different neuromuscular phenomena affecting different muscle groups through different pathophysiologic mechanisms.