I HAVE A PATIENT WITH TRANSIENT LOSS OF CONSCIOUSNESS
How do I determine the cause?
Mr. M is a 23-year-old medical student who lost consciousness this morning after entering his anatomy lab for the first time. He is quite alarmed (and embarrassed).
What is the differential diagnosis of transient loss of consciousness? How would you frame the differential?
CONSTRUCTING A DIFFERENTIAL DIAGNOSIS
Transient loss of consciousness may be caused by trauma, intoxication, seizures, hypoglycemia, subarachnoid hemorrhage, cerebrovascular disease (involving the brainstem), or syncope. Although syncope is often incorrectly assumed to be synonymous with transient loss of consciousness, syncope actually refers to that subset of patients with transient loss of consciousness due to transient global cerebral hypoperfusion, which in turn is virtually always caused by transient profound hypotension. Therefore, the first pivotal step in the evaluation of patients with transient loss of consciousness is to distinguish syncope from nonsyncopal causes of transient loss of consciousness. Three critical characteristics help distinguish patients with syncope: Syncope is (1) abrupt in onset, (2) brief in duration, and (3) recovery is complete and spontaneous (Figure 31-1). The explanation for this is straightforward; because syncope is due to transient global cerebral hypoperfusion, the loss of consciousness is abrupt. Furthermore, restoration of blood flow must occur quickly or the patient would die rather than present with syncope. Therefore, syncope is brief. Finally, when blood flow is restored, spontaneous recovery occurs promptly. Patients with other features (eg, a prolonged recovery period) should be evaluated for nonsyncopal causes that may masquerade as syncope (eg, seizures, hypoglycemia). A useful question is to ask the patient what is the next thing the patient remembers after losing consciousness. Any significant persistence of confusion beyond a minute or two is critical because this would suggest a nonsyncopal etiology of the transient loss of consciousness (such as a postictal period from a seizure).
The second pivotal step in patients with syncope identifies the likely category of their syncope: reflex, orthostatic, or cardiac syncope (Figure 31-2). This narrows the differential diagnosis, since each of these categories is associated with specific underlying diseases. Importantly, this key step also helps identify patients with cardiac syncope who are at a substantially increased risk for sudden death. Sudden death may occur if the underlying cardiac process that caused the syncope (arrhythmias or obstruction [eg, aortic stenosis]) is prolonged rather than brief.
Distinguishing cardiac, reflex and orthostatic syncope.
Patients with syncope should be carefully evaluated to determine if they have cardiac syncope and are at increased risk for sudden cardiac death.