The issue of whether to image patients under the influence of alcohol or other substances to also consider other etiologies as a possible explanation for mental status change frequently arises in Ed practice. The approach is discussed in detail in Chap,.,221 and CMp..,24.7. Generally, patients with Glasgow Coma Score (GCS) of 13 or less, those that fail to improve with time, or those in whom alcohol or drug levels are not in keeping with the clinical findings, require imaging.
An initial NECT can be performed in patients with mental status changes who also have a known primary tumor, or who have a suspected metastases or primary brain tumor. This will suffice in detecting lesions resulting in mental status change, such as marked cerebral edema, herniation, and intracranial hemorrhage, that require emergent intervention. However, the optimal choice of imaging is based on the availability of imaging resources as well as the clinical suspicion. For example, contrast MRI is the more sensitive test and is typically required for patients in which clinical concern persists, even if the initial CT is normal. However, if MRI resources were immediately available, it would be more appropriate in instances of high suspicion to proceed directly to MRI and avoid unnecessary time delay and additional imaging costs to the patient. Contrast-enhanced CT subjects patients to the small, but unnecessary, risk of iodinated contrast, and is less sensitive than MRI.
Proceeding directly to imaging with MRI when available, is also indicated with certain cerebral infections. Encephalitis presents as acute mental status change and fever. Herpes type 2 encephalitis is a particularly devastating form, but can have a good clinical outcome if diagnosed and treated early in the course of the disease.
Herpes often has a characteristic appearance in which the temporal lobes are first effected ( Fig 22.9-4.). Due to the difficulties assessing parenchyma in direct apposition to bone structures, detection of the subtle early findings of cytotoxic edema involving the temporal lobes can be difficult when utilizing NECT in the first three days. Sensitivity is not increased by the addition of intravenous contrast. Conversely, MR is highly sensitive to the early manifestations of encephalitis. It is often positive in the first 24 h and can be helpful in expeditious diagnosis.
FIG. 229-4. Axial CT scan demonstrates decreased attenuation in the right temporal lobe ( straight arrow). In a young patient with acute onset mental status change, this is highly suggestive of herpes encephalitis. Note the normal left temporal horn ( curved arrow) as compared to the effaced right temporal horn.
The approach to imaging the HIV-positive patient with mental status change is greatly debated (see Chap 139). There is a tendency to provide an emergent screening examination with NECT followed by nonemergent performance of contrast-enhanced MRI as indicated. Using this approach, processes creating vasogenic edema and those requiring immediate intervention, such as toxoplasmosis and lymphoma, will undoubtedly be detected. However, further investigation using contrast MRI is often requested to better characterize these lesions and for the detection of HIV encephalopathy and progressive multifocal leukoencephalopathy. A cost-effective approach of proceeding directly to MRI has also been recommended. This largely depends on the institutional availability of MRI and the management practices of clinicians.
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