Geometrical Complexity of Cortical Microvascularization in Moyamoya Disease

Fractal Analysis of the Cerebrovascular System Pathophysiology

The cerebrovascular system is characterized by parameters such as arterial blood pressure (ABP), cerebral perfusion pressure (CPP), and cerebral blood flow velocity (CBFV). These are regulated by interconnected feedback loops resulting in a fluctuating and complex time course. They exhibit fractal characteristics such as (statistical) self-similarity and scale invariance which could be quantified by fractal measures. These include the coefficient of variation, the Hurst coefficient H, or the spectral exponent α in the time domain, as well as the spectral index ß in the frequency domain. Prior to quantification, the time series has to be classified as either stationary or nonstationary, which determines the appropriate fractal analysis and measure for a given signal class. CBFV was characterized as a nonstationary (fractal Brownian motion) signal with spectral index ß between 2.0 and 2.3. In the high-frequency range (>0.15 Hz), CBFV variability is mainly determined by the periodic ABP variability induced by heartbeat and respiration. However, most of the spectral power of CBFV is contained in the low-frequency range (<0.15 Hz), where cerebral autoregulation acts as a low-pass filter and where the fractal properties are found. Cerebral vasospasm, which is a complication of subarachnoid hemorrhage (SAH), is associated with an increase in ß denoting a less complex time course. A reduced fractal dimension of the retinal microvasculature has been observed in neurodegenerative disease and in stroke. According to the decomplexification theory of illness, such a diminished complexity could be explained by a restriction or even dropout of feedback loops caused by disease.

Development of cortical microvascularization in Moyamoya disease using the maximum intensity projection method from three-dimensional rotational angiography

BACKGROUND

Neurosurgeons often experience increased cortical microvascularization in Moyamoya disease (MMD). However, there are no previous reports that radiologically evaluated preoperative cortical microvascularization. We investigated the development of cortical microvascularization and clinical characteristics of MMD using the maximum intensity projection (MIP) method.

METHODS

We enrolled 64 patients at our institution, including patients with MMD (n = 26), intracranial atherosclerotic disease (ICAD; n = 18), and unruptured cerebral aneurysms (n = 20) as the control group. All patients underwent three-dimensional rotational angiography (3D-RA). The 3D-RA images were reconstructed using partial MIP images. Cortical microvascularization was defined as the vessels that branched off from the cerebral arteries and were classified as grade 0-2 depending on their development.

RESULTS

Cortical microvascularization observed in patients with MMD was classified into grade 0 (n = 4, 8.9%), grade 1 (n = 17, 37.8%), and grade 2 (n = 24, 53.3%). The development of cortical microvascularization was more common in the MMD group than in the other groups. The inter-rater reliability measured using weighted kappa was 0.68 (95% confidence interval = 0.56-0.80). There were no significant differences in cortical microvascularization according to the onset type and hemispheres. Cortical microvascularization correlated with periventricular anastomosis. Most patients with Suzuki classifications 2-5 developed cortical microvascularization.

CONCLUSION

Cortical microvascularization was characteristic of patients with MMD. These findings developed in the early stages of MMD and may act as a bridge to the development of periventricular anastomosis.

Ivy sign: a diagnostic and prognostic biomarker for pediatric moyamoya

OBJECTIVE

Ivy sign is a radiographic finding on FLAIR MRI sequences and is associated with slow cortical blood flow in moyamoya. Limited data exist on the utility of the ivy sign as a diagnostic and prognostic tool in pediatric patients, particularly outside of Asian populations. The authors aimed to investigate a modified grading scale with which to characterize the prevalence and extent of the ivy sign in children with moyamoya and evaluate its efficacy as a biomarker in predicting postoperative outcomes, including stroke risk.

METHODS

Pre- and postoperative clinical and radiographic data of all pediatric patients (21 years of age or younger) who underwent surgery for moyamoya disease or moyamoya syndrome at two major tertiary referral centers in the US and Israel, between July 2009 and August 2019, were retrospectively reviewed. Ivy sign scores were correlated to Suzuki stage, Matsushima grade, and postoperative stroke rate to quantify the diagnostic and prognostic utility of ivy sign.

RESULTS

A total of 171 hemispheres in 107 patients were included. The median age at the time of surgery was 9 years (range 3 months-21 years). The ivy sign was most frequently encountered in association with Suzuki stage III or IV disease in all vascular territories, including the anterior cerebral artery (53.7%), middle cerebral artery (56.3%), and posterior cerebral artery (47.5%) territories. Following surgical revascularization, 85% of hemispheres with Matsushima grade A demonstrated a concomitant, statistically significant reduction in ivy sign scores (OR 5.3, 95% CI 1.4-20.0; p = 0.013). Postoperatively, revascularized hemispheres that exhibited ivy sign score decreases had significantly lower rates of postoperative stroke (3.4%) compared with hemispheres that demonstrated no reversal of the ivy sign (16.1%) (OR 5.5, 95% CI 1.5-21.0; p = 0.008).

CONCLUSIONS

This is the largest study to date that focuses on the role of the ivy sign in pediatric moyamoya. These data demonstrate that the ivy sign was present in approximately half the pediatric patients with moyamoya with Suzuki stage III or IV disease, when blood flow was most unstable. The authors found that reversal of the ivy sign provided both radiographic and clinical utility as a prognostic biomarker postoperatively, given the statistically significant association with both better Matsushima grades and a fivefold reduction in postoperative stroke rates. These findings can help inform clinical decision-making, and they have particular value in the pediatric population, as the ability to minimize additional radiographic evaluations and tailor radiographic surveillance is requisite.