Purpose of Transcranial Doppler (TCD) in Subarachnoid Hemorrhage
The primary purpose of transcranial Doppler (TCD) testing in patients with subarachnoid hemorrhage is to measure blood flow velocity to assess for vasospasm and predict delayed cerebral ischemia.
Primary Clinical Application
TCD serves as the standard noninvasive screening tool for detecting cerebral vasospasm after subarachnoid hemorrhage by measuring blood flow velocities in the basal cerebral arteries. 1, 2 The test demonstrates 90% sensitivity and 92% negative predictive value for predicting delayed cerebral ischemia (DCI), making it reasonable for routine monitoring in at-risk populations. 1, 2
How TCD Works for Vasospasm Detection
TCD measures mean flow velocity (MFV) in cerebral arteries, with elevated velocities indicating arterial narrowing from vasospasm. 3, 4 Normal MCA velocities are typically 50-80 cm/s in adults. 5
Flow velocities >200 cm/s predict high likelihood of severe vasospasm, corresponding to ≥50% arterial narrowing on angiography. 1, 2, 3
The Lindegaard ratio (MCA velocity/extracranial ICA velocity) helps distinguish true vasospasm from hyperemia, with ratios of 5-6 indicating severe spasm requiring treatment. 1, 5
Timing and Monitoring Strategy
Daily bedside TCD screening is recommended during days 3-14 post-SAH, when vasospasm risk is highest, with maximal narrowing typically occurring at 5-14 days. 1, 2
Early velocity increases (>50 cm/sec per 24 hours during the first few days) predict which patients will develop delayed ischemic deficits, allowing for prophylactic intervention. 6
Prolonged TCD screening beyond day 10 post-SAH does not appear to increase detection of delayed cerebral ischemia. 1
Critical Limitations
TCD should NOT be used alone for surgical decision-making or definitive diagnosis of vasospasm. 2 The test has several important constraints:
10-20% of patients lack adequate temporal bone acoustic windows, making TCD impossible in these individuals. 2, 5
Accuracy diminishes significantly for distal vessels and posterior circulation, with sensitivity dropping to 55-80% for these territories. 5
Only 50% of patients with severe angiographic vasospasm develop clinical symptoms, and conversely, DCI can occur without elevated TCD velocities. 1, 7
Despite high sensitivity for vasospasm detection, no high-quality literature demonstrates that TCD monitoring improves patient outcomes. 1, 2
Integration with Other Modalities
TCD functions best as an initial screening tool, followed by confirmatory imaging with CTA or CT perfusion when velocities become elevated or neurological examination is limited. 1, 2, 5 CTA has 91% sensitivity for detecting central vasospasm and provides anatomic detail that TCD cannot. 1
What TCD Does NOT Do
TCD does not identify venous thrombosis - this requires CT venography or MR venography. 1
TCD does not directly assess extent of hemorrhage - this requires CT or MRI imaging. 1
TCD does not predict extent of neurologic damage - while it predicts DCI risk, overall neurologic outcome depends on multiple factors including initial hemorrhage severity, rebleeding, hydrocephalus, and treatment complications. 1, 8