Superficial temporal artery–middle cerebral artery (STA-MCA) bypass surgery is an important therapy for symptomatic moyamoya disease. Its success depends on bypass function, which may be impaired by primary or secondary bypass insufficiency. Catheter angiography is the current gold standard to assess bypass function, whereas the diagnostic value of ultrasonography (US) has not been systematically analyzed so far.
The authors analyzed 50 STA-MCA bypasses in 39 patients (age 45 ± 14 years [mean ± SD]; 26 female, 13 male). Bypass patency was evaluated by catheter angiography, which was performed within 24 hours after US. The collateral circulation through the bypass was classified into 4 types as follows: the bypass supplies more than two-thirds (type A); between one-third and two-thirds (type B); or less than one-third (type C) of the MCA territory; or there is bypass occlusion (type D). The authors assessed the mean blood flow velocity (BFV), the blood volume flow (BVF), and the pulsatility index (PI) in the external carotid artery and STA by duplex sonography. Additionally, they analyzed the flow direction of the MCA by transcranial color-coded sonography. US findings were compared between bypasses with higher (types A and B) and lower (types C and D) capacity.
Catheter angiography revealed high STA-MCA bypass capacity in 35 cases (type A: n = 22, type B: n = 13), whereas low bypass capacity was noted in the remaining 15 cases (type C: n = 12, type D: n = 3). The BVF values in the STA were 60 ± 28 ml/min (range 4–121 ml/min) in the former and 12 ± 4 ml/min (range 6–18 ml/min) in the latter group (p < 0.0001). Corresponding values of mean BFV and PI were 57 ± 21 cm/sec (range 16–100 cm/sec) versus 22 ± 8 cm/sec (range 10–38 cm/sec) (p < 0.0001) and 0.8 ± 0.2 (range 0.4–1.3) versus 1.4 ± 0.5 (range 0.5–2.4) (p < 0.0001), respectively. Differences in the external carotid artery were less distinct: BVF 217 ± 71 ml/min (range 110–425 ml/min) versus 151 ± 41 ml/min (range 87–229 ml/min) (p = 0.001); mean BFV 47 ± 17 cm/sec (range 24–108 cm/sec) versus 40 ± 7 cm/sec (range 26–50 cm/sec) (p = 0.15); PI 1.5 ± 0.4 (range 1.0–2.5) versus 1.9 ± 0.4 (range 1.2–2.6) (p = 0.009). A retrograde blood flow in the MCA was found in 14 cases (9 in the M1 and M2 segment; 5 in the M2 segment alone), and all of them showed a good bypass function (type A, n = 10; type B, n = 4). The best parameter (cutoff value) to distinguish bypasses with higher capacity from bypasses with lower capacity was a BVF in the STA ≥ 21 ml/min (sensitivity 100%, negative predictive value 100%, specificity 91%, positive predictive value 83%).
Duplex sonography is a suitable diagnostic tool to assess STA-MCA bypass function in moyamoya disease. Hemodynamic monitoring of the STA by US provides an excellent predictor of bypass patency.