Neuroimaging Predictors

MRI has better resolution than CT for subtle brain injury but, overall, has not provided much prognostic value beyond the neurologic examination. In the acute period after CA, MRI is often

Adults with anoxic coma n= 1136

SSEP abnormal n = 310

22% awaken CI 17-26%

SSEP normal n = 491

52% awaken CI 48-56%

Figure 2 Outcome after anoxic coma, depending upon results of median nerve SSEPs. Abbreviations: SSEPs, somatosensory-evoked potentials; CI, Confidence interval. Source: Adapted from Ref. 24.

normal, but in the subacute period ( < 2 weeks), MRI may show abnormal signal in the deep gray matter (GM) nuclei and obscured gray-white matter (WM) distinction (25). Some cases may show cortical laminar necrosis as a strip of high-signal intensity on the Tl-weighted images, but this is usually seen in the chronic phase (more than a few weeks after CA) (25,26).

Diffusion-weighted MRI reveals ischemic brain as areas of restricted diffusion in stroke patients within several hours of onset of symptoms. Similar findings have been reported in patients studied within the first few days of CA (25). One study reported restricted diffusion (confirmed by measurements of the mean apparent diffusion coefficient) predominantly in the WM; diffusion weighted imaging (DWI) images showed diffuse, symmetric high signal in the periventricular WM (Fig. 3) (27). Another reported diffuse signal abnormality on DWI in both cortical and WM regions in the majority of patients (28). Widespread abnormality involving cortex, thalamus, and cerebellum on DWI seemed to correlate with poor prognosis, although the number of patients studied was small (n = 9) (28).

Figure 3 MRI studies of leukoencephalopathy after anoxic brain injury. MRI, for example, of 2 levels of a single patient showing the following sequences (from left to right): fluid-attenuated inversion recovery (FLAIR), diffusion-weighted imaging (DWI), apparent diffusion coefficient (ADC) map, T2-weighted image. Note confluent white matter hyperintensity on FLAIR, DWI, and T2, with dark signal on ADC, confirming restricted diffusion. Source: Adapted from Ref. 27.

Figure 3 MRI studies of leukoencephalopathy after anoxic brain injury. MRI, for example, of 2 levels of a single patient showing the following sequences (from left to right): fluid-attenuated inversion recovery (FLAIR), diffusion-weighted imaging (DWI), apparent diffusion coefficient (ADC) map, T2-weighted image. Note confluent white matter hyperintensity on FLAIR, DWI, and T2, with dark signal on ADC, confirming restricted diffusion. Source: Adapted from Ref. 27.

Magnetic resonance spectroscopy after CA demonstrates an elevation in brain lactate and a decrease in the neuronal marker N-acetylaspartate (29). The usefulness of this technique over other prognostic methods has not been demonstrated. Given the logistic difficulties of obtaining MRI studies in critically ill patients, the use of MRI for prognosis is not widespread.

Despite its limitations in terms of resolution, CT has the advantage of much greater availability. CT may reveal focal ischemia in some patients after CA but is primarily used to investigate other neurologic causes for coma, such as intracerebral or subarachnoid hemorrhage. Loss of gray-white distinction is a well-known early sign of ischemia on CT. Torbey et al. described a simple method of measuring the CT density in Hounsfield units in the GM of the basal ganglia and in the WM of the internal capsule and calculating a GM/WM ratio (30). In control subjects, this ratio was 1.45, with all controls having a ratio of > 1.30; however, in comatose patients with CA, the median value was 1.18. In this series, all comatose patients who had a ratio of < 1.18 died, whereas the survival rate was 46% among those with a ratio of > 1.18. The investigators went on to propose a Brain Arrest Neurological Outcome Scale, which combines scores for duration of CA, a reversed GCS, and scores of the GM/WM index (31). Prognostic cutoffs were derived from a series of 32 comatose patients, but another study should validate these results prospectively.

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