This patient most likely has an epidural hematoma. Traumatic epidural hematoma classically presents with precipitous neurologic decline after head trauma. Most patients with this diagnosis have a skull fracture with associated rupture of an underlying artery, typically the middle meningeal artery. Blood under arterial pressure accumulates between the inner table of the skull and the dural membranes. The most common symptoms are severe headache and vomiting. Impairment of consciousness may develop immediately or after a lucid interval. Uncal or subfalcine brain herniation can occur and is characterized by ipsilateral occulomotor nerve (cranial nerve III) palsy, contralateral paresis, and stupor or coma. Hypertension with bradycardia (the Cushing response) can be another sign of increased intracranial pressure. A CT scan of the head confirms the diagnosis, and immediate surgical evacuation is required. Mortality rates are commonly reported to be 10% to 20%.

Dissection of the left internal carotid artery typically results in ipsilateral Horner syndrome with ptosis, miosis, and anhidrosis but not oculomotor nerve (cranial nerve III) palsy. Contralateral hemiparesis could result if a secondary stroke were to occur in the left frontal lobe after the dissection, but rapidly declining consciousness would be unexpected.

Postconcussion syndrome is defined by a constellation of neurologic, psychological, and constitutional symptoms without significant abnormalities on physical examination. Minor neurologic findings noted on the examination of a patient with mild traumatic brain injury may include ocular convergence insufficiency or mild ataxia, but typically examination findings are normal. This patient's clinical findings do not fit this pattern.

Seizures occur in approximately 5% of persons hospitalized for acute head trauma. They may be classified as “immediate” if occurring within the first 24 hours, “early” if noted within the first week, or “late” if occurring more than 1 week after the injury. Half of the seizures occurring within the first week will occur in the first 24 hours, and the risk decreases with time. Some correlation between the severity of injury and the risk of posttraumatic seizures exists. This patient shows no signs of involuntary motor activity, so convulsive status epilepticus is not present. Nonconvulsive status epilepticus might manifest as stupor, but the presence of focal cranial nerve and motor deficits in this patient is more indicative of a progressive structural lesion.