Carotid ultrasounds are frequently ordered for the evaluation of syncope. However, there is little anatomic, physiologic, or empiric evidence for its utility in this setting.
Background of conciousness
Consciousness is maintained in the brain through a structure called the reticular activating formation (RAF). This is a network of neurons that originates in the brainstem and project into bilateral thalami and diffusely into the cortices. Significant disruption of the RAF can lead to impaired consciousness and arousal.
A focal lesion in the brainstem can disrupt the RAF and impair consciousness. Outside the brainstem, however, large bilateral insults are required to impaired consciousness. This can occur with cardioembolic strokes, or global cerebral hypoperfusion, but not carotid stenosis.
Carotid stenosis is mostly a static lesion that may contribute to cerebral hypoperfusion, but is not the primary cause. If a carotid plaque were unstable causing cerebral ischemia, clinical symptoms would include focal neurologic deficit, i.e. a stroke. Carotid stenosis also cannot affect the brainstem RAF as the basilar artery supplies this territory.
Even severe bilateral carotid stenoses does not cause syncope. Bilateral simultaneous plaque rupture could theoretically impair consciousness, but this would be extremely uncommon and should have other neurologic deficit.
There is a scenario where unilateral carotid stenosis can be associated with impaired consciousness. If a patient has preexisting neurologic disease such as an advanced dementia or large remote contralateral stroke, they may lose consciousness in the setting of an acute CVA. However, this type of event is best evaluated through neuroimaging such as a head CT or MRI, and not carotid ultrasound.
If the patient has focal neurologic signs or symptoms, syncope may be part of a larger stroke syndrome, especially if the patient has significant cardiovascular risk factors, carotid bruit, or preexisting neurologic disease such as advanced dementia or remote contralateral stroke.
Results from cardiac enzymes tests, CT scans, echocardiography, carotid ultrasonography, and electroencephalography all affected diagnosis or management in less than 5% of cases and helped determine the etiology of syncope less than 2% of the time. Postural blood pressure (BP) recording, performed in only 38% of episodes, had the highest yield with respect to affecting diagnosis (18–26%) or management (25–30%) and determining etiology of the syncopal episode (15–21%). 
Neurovascular ultrasonography had a low yield for diagnosing vascular lesions that contributed to the pathophysiology of syncope. However, in patients with focal signs or symptoms or carotid bruits, it detected incidental lesions that typically require treatment or follow-up. In patients with syncope, neurovascular ultrasonography should be reserved for this subset.
The Centers for Medicare and Medicaid Services (CMS) applies stroke core measure criteria to all patients with carotid stenosis with or without infarct. In other words, CMS believes that all patients with carotid stenosis > 70% should be on the stroke protocol whether they came in for stroke or syncope. Thus, unless we address rehab services or give the patient stroke education or address TPA (if they arrived in the 3 hours window), all things that are not routinely done for syncope patients.
- Due to the anatomic arrangement of the reticular activating formation, it is extremely unlikely for carotid stenosis to cause isolated syncope, even in the setting of bilateral disease.
- It may be appropriate to evaluate for carotid stenosis in the setting of focal neurologic signs or symptoms, carotid bruit, severe cerebrovascular risk factors, or preexisting neurologic disease. However, in this scenario, you are essentially evaluating for an acute stroke causing syncope as part of a stroke syndrome, and initial evaluation should consist of neuroimaging such as a head CT or MRI, not neurovascular imaging.
- There is no evidence to recommend carotid ultrasound in the evaluation of typical syncope.
- There is significant financial disincentive to the patient and the hospital in routinely ordering carotid ultrasound without strong clinical indication. If the carotid ultrasound shows an incidental stenosis, then we either add financial burden to the patient in applying stroke protocols to satisfy CMS, or we fall out of stroke core measures and lose reimbursement.
- Yield of diagnostic tests in evaluating syncopal episodes in older patients. Arch Intern Med. 2009 Jul 27;169(14):1299–305 ↩
- Diagnostic yield and utility of neurovascular ultrasonography in the evaluation of patients with syncope. Mayo Clin Proc. 2005 Apr;80(4):480–8 ↩
- The American Academy of Neurology’s Top Five Choosing Wisely recommendations ↩