Chapter 18: Studies for Neurologic Evaluation

INTRODUCTION

Although continued improvements in neuroimaging and neuromonitoring have added insight into the developing brain and have helped the clinician to identify infants at risk for poor neurologic outcome, available techniques continue to have limited accuracy in predicting neurodevelopmental outcomes. Moreover, given the enormous plasticity of the neonate’s brain, even significant detectable defects may result in “normal” neurodevelopmental outcomes. Nevertheless, imaging and monitoring modalities hold future promise in assisting clinicians to better identify and refer patients at risk for neurodevelopmental sequelae.

I. NEUROIMAGING

1. Ultrasonography

   1. Definition. Using the bone window of a fontanelle, sound waves are directed into the brain and reflected according to the echodensity of the underlying structures. The reflected waves are used to create 2- and 3-dimensional images.

   2. Indication. Ultrasonography is preferred for identification and observation of germinal matrix/intraventricular hemorrhage and hydrocephalus. Ultrasound is valuable in detecting midline structural abnormalities, hypoxic ischemic injury, periventricular leukomalacia, subdural and posterior fossa hemorrhage, ventriculitis, periventricular calcifications, tumors, cysts, and vascular abnormalities.

   3. Method. A transducer is placed over the anterior fontanelle, and images are obtained in coronal and parasagittal planes. The posterior or mastoid fontanelle is the preferred acoustic window for the imaging of the infratentorium, including the fourth ventricle, brainstem, and cerebellum. Ultrasonography’s advantages include high resolution, portability, safety (no sedation, contrast material, or radiation), noninvasiveness, and low cost. Disadvantages include the lack of visualization of non-midline structures, especially in the parietal regions, and the lack of differentiation between gray and white matter.

   4. Results. The integrity of the following structures may be evaluated with ultrasonography: all ventricles, the choroid plexus, caudate nuclei, thalamus, septum pellucidum, and corpus callosum.

2. Doppler ultrasonography

   1. Definition. Doppler ultrasonography also uses a bone window to direct sound waves into the brain. The sound waves from the transducer are reflected by red blood cells in the vessel, and their frequency travels proportionally to the velocity of the circulating red blood cells. These changes are measured and expressed as the pulsatility index and resistance index (RI). The angle of the probe in relation to the flow affects the Doppler shift and requires exact standards for serial measurements.

   2. Indication. Doppler ultrasonography of the anterior cerebral artery is a sensitive and specific tool for measuring cerebral blood flow (CBF) and resistance in the neonatal period.

      CBF (cm³/time) = CBF velocity (cm/time) × area (cm²).

      Doppler ultrasonography is of clinical value in states of cessation of CBF (eg, brain death or cerebrovascular occlusion), states of altered vascular resistance (eg, hypoxic ischemic encephalopathy, hydrocephalus, or arteriovenous [AV] malformation), and ductal steal syndrome.

   3. Method. Combined with conventional ultrasonography to identify the blood vessel, Doppler ultrasonography produces a color image indicating flow (red = toward the transducer; blue = away from the transducer). CBF velocity is measured as the area under the curve of velocity waveforms. Small body weight and low gestational ages negatively influence the success rate in visualizing intracranial vasculature. Contrast-enhanced ultrasound ...