All That Glitters Is Not Scar.
All That Glitters Is Not Scar.
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DOI：
10.1161/circep.122.011173
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发表时间：
2022-07
影响因子：
8.4
通讯作者：
Nazarian, Saman
中科院分区：
文献类型：
--
作者：
Zghaib, Tarek;Markman, Timothy M.;Nazarian, Saman
心脏磁共振（CMR）成像已成为心肌病或心律失常患者的宝贵工具。除了提供结构和功能信息外，CMR还提供了对心肌组织特征的深入了解，并使用不同组织成分中钆基药物的注射后时间依赖性浓度差异来识别病灶变化。钆是一种细胞外造影剂，在健康心肌中的分布体积有限，但在细胞外间隙扩张的情况下分布体积增加。这沿着来自异常组织的造影剂的延迟洗脱，导致CMR上的像素图像强度增加，标记为晚期钆增强（LGE）。即使在缺血性心肌病的背景下，被认为是一个相对简单和同质的条件，显着比例的可视化LGE意味着脂肪瘤化生，而不是疤痕。非缺血性心肌病（NICM）是一个总括性术语，指多种疾病，每种疾病都是由于不同的肌病过程导致心肌功能障碍，其异质性要大得多。尽管NICM患者LGE的各种分布模式经常被标记为纤维化或瘢痕化，但缺乏组织学和影像学相关性，事实可能更复杂。例如，肥厚型心肌病的心肌肥大和紊乱导致肌细胞蛋白质结构紊乱，导致钆（钆累积的非纤维化病灶）扩张。LGE也可能是由于异常蛋白质浸润（淀粉样变性）、肉芽肿性沉积（结节病）或局灶性炎症（心肌炎）引起的心包非胶原性扩张。然而，这些NICM亚型仅代表少数病例。在多达一半的NICM患者中，可以在中层心肌中观察到LGE（图），这种LGE的原因和组织病理学组成尚未确定。T1标测可以测量整体弥漫性纤维化的负担，但目前的临床CMR序列缺乏空间分辨率来可视化间质纤维化的区域变化。鉴于特发性NICM的潜在病理生理学的不确定性，我们必须考虑另一种基于心肌纤维层的解剖方向和机械相互作用的LGE机制假设。心室肌由三层不同取向的纤维组成。定量地，心内膜下纤维具有正螺旋角（定义为肌纤维相对于左心室短轴平面的倾斜角），而心外膜下纤维具有负螺旋角。1中心肌纤维在短轴平面上呈环状分布。在收缩期，显著的剪切力沿着心肌片的解理面，特别是在心肌内膜下和心肌中层之间。2这导致肌纤维的延伸和径向重新定向，促进心肌增厚和血液喷射。当这些纤维分离时产生的应变可能导致细胞外空间增加。在细胞外间隙持续胶原性扩张以及由于左心室扩张引起的重塑导致肌纤维重新定向的NICM患者中，该过程可能会被夸大。由此产生的细胞外空间的扩张可能导致钆积聚，特别是在中层心肌中。这种现象的另一个例子是在右心室间隔插入部位，来自两个心室的心肌纤维交叉导致紊乱的心肌纤维。
Cardiac magnetic resonance (CMR) imaging has emerged as an invaluable tool for patients with cardiomyopathy or arrhythmia. In addition to providing structural and functional information, CMR offers insights into myocardial tissue characteristics and identifies focal changes using postinjection differential time-dependent concentration of gadolinium-based agents in different tissue compositions. Gadolinium is an extracellular contrast agent with a limited volume of distribution in healthy myocardium but increased volume of distribution in conditions with expansion of extracellular space. This, along with delayed washout of contrast from abnormal tissues, leads to an increase in pixel image intensity on CMR, labeled as late-gadolinium enhancement (LGE). Even in the setting of ischemic cardiomyopathy, thought to be a relatively simple and homogenous condition, a significant proportion of visualized LGE signifies lipomatous metaplasia rather than scar. The heterogeneity in the setting of nonischemic cardiomyopathy (NICM), an umbrella terminology that refers to multiple conditions, each due to a different myopathic processes resulting in myocardial dysfunction, is far greater. Although various distribution patterns of LGE in patients with NICM are frequently labeled as fibrotic or scarred, histologyimaging correlations are lacking, and the truth is likely more complicated. For example, myocardial hypertrophy and disarray in hypertrophic cardiomyopathy lead to disorganized myocyte protein apparatus with resultant expansion of interstitium, a nonfibrotic nidus for gadolinium accumulation. LGE may also result from noncollagenous expansion of the interstitium due to infiltration with abnormal protein (amyloidosis), granulomatous deposits (sarcoidosis), or focal inflammation (myocarditis). These subtypes of NICM, however, only represent a minority of cases. In up to half of patients with NICM, LGE can be seen in the midmyocardium (Figure), and the cause and histopathologic composition of this LGE have not been established. T1 mapping can measure the burden of global diffuse fibrosis, but current clinical CMR sequences lack the spatial resolution to visualize regional variations in interstitial fibrosis. Given the uncertainty in the underlying pathophysiology of idiopathic NICM, we must consider another hypothesis for the mechanism of LGE, based upon the anatomic orientation and mechanical interaction of myocardial fiber layers. The ventricular myocardium consists of three layers of differentially oriented fibers. Quantitatively, subendocardial fibers have a positive helix angle (defined as the inclination angle of a myofiber relative to the left ventricular short-axis plane), whereas subepicardial fibers have a negative helix angle. 1 Midmyocardial fibers are circumferential in the short-axis plane. During systole, significant shear forces develop along the myocardial sheets cleavage planes, particularly between subendocardial and midmyocardial layers. 2 This leads to extension and radial reorientation of myofibers, facilitating myocardial thickening and ejection of blood. The strain created as these fibers separate may lead to increased extracellular space. This process may be exaggerated in patients with NICM who have ongoing collagenous expansion of the extracellular space as well as myofiber reorientation due to remodeling in response to left ventricular dilatation. The resultant expansion of the extracellular space may result in gadolinium accumulation, particularly in the midmyocardium. Another example of this phenomenon is at the right ventricular septal insertion sites where the intersection of myocardial fibers from both ventricles leads to disorganized …