Department of Neurosurgery, Boston Children’s Hospital, Harvard Medical School, Boston, Massachusetts 02115, USA.
J Neurosurg. 2012 Dec;117(6):1189-96. doi: 10.3171/2012.9.JNS121227. Epub 2012 Oct 12.
The pathophysiology of normal pressure hydrocephalus (NPH), and the related problem of patient selection for treatment of this condition, have been of great interest since the description of this seemingly paradoxical condition nearly 50 years ago. Recently, Eide has reported that measurements of the amplitude of the intracranial pressure (ICP) can both positively and negatively predict response to CSF shunting. Specifically, the fraction of time spent in a "high amplitude" (> 4 mm Hg) state predicted response to shunting, which may represent a marker for hydrocephalic pathophysiology. Increased ICP amplitude might suggest decreased brain compliance, meaning a static measure of a pressure-volume ratio. Recent studies of canine data have shown that the brain compliance can be described as a frequency-dependent function. The normal canine brain seems to show enhanced ability to absorb the pulsations around the heart rate, quantified as a cardiac pulsation absorbance (CPA), with properties like a notch filter in engineering. This frequency dependence of the function is diminished with development of hydrocephalus in dogs. In this pilot study, the authors sought to determine whether frequency dependence could be observed in humans, and whether the frequency dependence would be any different in epochs with high ICP amplitude compared with epochs of low ICP amplitude.
Systems analysis was applied to arterial blood pressure (ABP) and ICP waveforms recorded from 10 patients undergoing evaluations of idiopathic NPH to calculate a time-varying transfer function that reveals frequency dependence and CPA, the measure of frequency-dependent compliance previously used in animal experiments. The ICP amplitude was also calculated in the same samples, so that epochs with high (> 4 mm Hg) versus low (≤ 4 mm Hg) amplitude could be compared in CPA and transfer functions.
Transfer function analysis for the more "normal" epochs with low amplitude exhibits a dip or notch in the physiological frequency range of the heart rate, confirming in humans the pulsation absorber phenomenon previously observed in canine studies. Under high amplitude, however, the dip in the transfer function is absent. An inverse relationship between CPA index and ICP amplitude is evident and statistically significant. Thus, elevated ICP amplitude indicates decreased performance of the human pulsation absorber.
The results suggest that the human intracranial system shows frequency dependence as seen in animal experiments. There is an inverse relationship between CPA index and ICP amplitude, indicating that higher amplitudes may occur with a reduced performance of the pulsation absorber. Our findings show that frequency dependence can be observed in humans and imply that reduced frequency-dependent compliance may be responsible for elevated ICP amplitude observed in patients who respond to CSF shunting.
自近 50 年前描述这种看似矛盾的病症以来，正常压力脑积水（NPH）的病理生理学以及相关的治疗选择问题一直备受关注。最近，Eide 报道称，颅内压（ICP）幅度的测量既可以正面预测，也可以负面预测 CSF 分流的反应。具体来说，处于“高幅度”（>4mmHg）状态的时间分数可预测分流反应，这可能代表脑积水病理生理学的标志物。ICP 幅度增加可能表明脑顺应性降低，即压力-容积比的静态测量。最近对犬类数据的研究表明，脑顺应性可以描述为频率相关的函数。正常犬类大脑似乎具有增强吸收心率周围脉动的能力，表现为心脏搏动吸收率（CPA），其性质类似于工程中的陷波滤波器。这种功能的频率依赖性随着犬类脑积水的发展而减弱。在这项初步研究中，作者试图确定在人类中是否可以观察到频率依赖性，以及与 ICP 幅度较低的时期相比，在 ICP 幅度较高的时期是否会观察到任何不同。
对 10 例接受特发性 NPH 评估的患者的动脉血压（ABP）和 ICP 波形进行系统分析，以计算时变传递函数，该函数揭示了频率依赖性和 CPA，即以前在动物实验中使用的频率相关顺应性的度量。在同一样本中还计算了 ICP 幅度，以便可以在 CPA 和传递函数中比较高（>4mmHg）与低（≤4mmHg）幅度的时期。
对于幅度较低的更“正常”时期的传递函数分析显示，在心率的生理频率范围内出现了一个凹陷或陷波，这在人类中证实了以前在犬类研究中观察到的脉动吸收器现象。然而，在高幅度下，传递函数中的凹陷消失。CPA 指数与 ICP 幅度之间存在明显的负相关关系，具有统计学意义。因此，ICP 幅度升高表明人类脉动吸收器的性能降低。
结果表明，人类颅内系统表现出与动物实验中相同的频率依赖性。CPA 指数与 ICP 幅度之间存在负相关关系，表明较高的幅度可能与脉动吸收器性能降低有关。我们的发现表明，在人类中可以观察到频率依赖性，并暗示 CSF 分流反应患者中观察到的升高的 ICP 幅度可能是由于频率相关顺应性降低所致。
