Commentary on “Midline Shift Greater than 3 mm Independently Predicts Outcome After Ischemic Stroke”

Excerpt

When large regions of the brain are subject to prolonged cerebral ischemia, for example, in the setting of large vessel occlusion, tissue infarction triggers a biologic cascade that results in brain water accumulation [1]. This cerebral edema involves the infarct region as well as surrounding tissue. The main consequence of stroke-related edema is an increase in hemispheric volume resulting in a compartmental pressure gradient between the two hemispheres, as well as the supratentorial and infratentorial spaces. The rise in pressure is initially attenuated by compensatory mechanisms that attempt to maintain the stability of total intracranial volume; these include the displacement of blood and cerebrospinal fluid (CSF) from the affected hemisphere. This can be visualized early after stroke by effacement of sulci along the hemispheric convexity, and later by effacement and displacement of the ventricular system. In fact, the global volumetric effects of edema can be quantified by measuring the total CSF displaced from the supratentorial space in the hours to days after stroke [2]. Progression of cerebral edema after hemispheric stroke will result in midline shift when this capacity for compensation is exhausted and compartmental pressure rises. Ultimately, this pressure gradient can result in herniation with severe clinical deterioration. Greater intracranial reserve (CSF volume as a ratio of cranial volume), a measure of this capacity for compensation, has been shown (similar to older age) to be protective against complications relating to edema formation [3]. Those patients with stroke who develop midline shift and deterioration have less reserve and greater CSF displacement measurable by the 24-h mark, a time when midline shift is often still minimal [4]. Furthermore, given that the majority of edema and consequent CSF displacement occurs in the hemisphere ipsilateral to the stroke, the ratio of CSF remaining in that hemisphere relative to the volume in the contralateral hemisphere can be used as a dynamic early biomarker of edema that is significantly lower in those destined to develop midline shift [5]. …