Laboratory Assessment of Neurologic Function

The electroencephalogram (EEG) is useful for identifying seizure activity, diagnosing isoelectric brain death, and monitoring therapy during therapeutic barbiturate coma. Otherwise, in most neurologic diseases, the EEG is so nonspecific that it is uninterpretable. Brainstem evoked potentials (auditory, visual, and somatosensory) have gained acceptance as a means of neurologic assessment in a comatose patient. Evoked potentials define marginally functioning neurons that cannot be detected by clinical examination. Asymmetry, abnormality, or absence of conduction of auditory, visual, or tactile stimuli through the brainstem to the cortex can provide useful information regarding the progress of neurologic injury or recovery. Although evoked potentials are only mildly affected by barbiturates, a flat response during therapeutic coma cannot be regarded as diagnostic of brain death.

Computed tomography (CT) remains an invaluable tool for rapid evaluation of a neurologically impaired child for surgical lesions, the extent of structural injury, and non-invasive assessment of ICP. Cranial ultrasound is an effective bedside method of assessing ventricular size and intracranial anatomy in an infant whose cranial sutures have not yet fused. MRI is playing an increasingly important role in the evaluation of acute brain injury. Qualities that make MRI invaluable in the examination of intraorbital and ocular injuries and brainstem and spinal cord disorders are its multiplanar imaging capabilities, exquisite soft tissue contrast resolution, and lack of bone interference.^[191] The major drawbacks of MRI are the length of time required to complete the examination and the relative inaccessibility of the patient in the scanner. The logistics of safely obtaining an MRI scan in a patient who has significant cardiorespiratory compromise are formidable.

Doppler ultrasound is gaining popularity as a method of assessing cerebral blood flow (CBF) velocity in the ICU. Though incapable of measuring CBF directly, it is proving useful as a noninvasive bedside tool for guiding therapy to effect cerebral perfusion. Cerebral angiography is playing an increasingly smaller role with the introduction and development of these less invasive tests. CBF scans remain the gold standard for the diagnosis of brain death during barbiturate coma.

Methods of ICP monitoring include placement of a catheter in a lateral ventricle, a subarachnoid screw, or a transducer in the epidural space or cerebral parenchyma. The ventricular catheter is inserted through a drill hole placed in the area of the coronal suture in line with the ipsilateral pupil. If the ventricle is large enough, a catheter can be placed easily after penetrating the brain substance. It produces an accurate waveform and has the advantage that cerebrospinal fluid (CSF) can be removed as treatment of increased ICP.

The subarachnoid screw is placed transcranially by means of a drill hole through which the dura and

2856

The epidural transducer is placed by way of a bur hole or through a formal craniotomy incision at surgery. It is relatively easy to place and does not require incising the dura; once placed, however, it cannot be recalibrated and is therefore of only relative value.