This unresponsive patient has vital signs consistent with Cushing triad (ie, hypertension, bradycardia, irregular respirations), fixed pupils, and extensor posturing of the extremities, a combination of findings that is most concerning for brain herniation.

Brain herniation occurs when a rapid increase in intracranial pressure (ICP) (eg, due to intracranial hemorrhage) leads to protrusion of portions of the brain through openings in the dural folds or the foramen magnum.  This patient likely has central herniation, a type of herniation where the diencephalon and midbrain become caudally displaced through the tentorium cerebelli.  This herniation not only compresses the brainstem but can stretch and rupture the paramedian branches of the basilar artery, resulting in further ischemic injury.  Common findings of central herniation include:

• Unconsciousness due to dysfunction of the ascending reticular activating system responsible for arousal

• Midsized, fixed pupils due to impairment of both parasympathetic and sympathetic nerve fibers innervating the pupil

• Abnormal limb posturing due to disruption of descending tracts (eg, rubrospinal, vestibulospinal) typically involved in reflexive and postural movements.  This commonly begins with decorticate (flexor) posturing with damage above the level of the midbrain (ie, red nucleus) but often changes to decerebrate (extensor) posturing as brainstem injury progresses caudally.

Treatment focuses on decreasing ICP and correcting the underlying etiology (eg, intracranial hemorrhage evacuation) to prevent progression to tonsillar herniation, which can compress the medulla and result in cardiac and respiratory arrest.

(Choice A)  Acute dystonia (eg, antipsychotic-induced) can cause involuntary muscle contractions and spasms.  However, these are typically intermittent (vs fixed).  In addition, acute dystonia does not typically affect consciousness or alter pupillary response.

(Choice C)  Toxins from scorpion stings can cause neuronal membrane hyperexcitability that leads to uncontrolled, repetitive firing of axons.  However, this commonly results in fasciculations and jerking of the extremities rather than fixed, extensor posturing; furthermore, patients typically remain awake and complain of intense pain at the site of the bite.

(Choices D)  Strychnine, an ingredient in rodenticide and some illicit drugs, blocks inhibitory (glycine) neurotransmission within the spinal cord and can result in powerful, uncontrollable muscle contractions.  However, it classically causes episodic contractions appearing like tonic-clonic activity in a fully awake patient (sometimes termed an "awake seizure").

(Choices E)  Tetanus can block inhibitory interneuron neurotransmission within the spinal cord and lead to painful muscle contractions, often beginning in the head and neck (eg, trismus) and progressing to the rest of the body (eg, opisthotonus).  However, patients are typically awake, and the pupils are not involved.

Educational objective:
Central herniation occurs when the diencephalon and midbrain are displaced caudally due to increased intracranial pressure (eg, intracranial hemorrhage).  This causes damage to the brainstem, which results in Cushing triad, unconsciousness, midsized and fixed pupils, and abnormal posturing (decorticate vs decerebrate).