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Acquired Toxic/Metabolic Disorders Overview
Approach to Toxic and Metabolic Disorders
Acquired toxic and metabolic disorders of the brain result from a wide variety of agents, including toxic exposures, substance abuse, radiation, and chemotherapy, as well as metabolic alterations, including hypertension, hepatic failure, hypoglycemia, and osmotic demyelination. The vast majority of toxic and metabolic disorders of the brain involve the deep gray nuclei (basal ganglia and thalamus) or the cerebral white matter. Typically, there is symmetric abnormality of the involved structures, which can provide a clue to the correct diagnosis. MR shows damage at both early and delayed phases of toxic and metabolic disorders. DWI and FLAIR are extremely valuable in differentiating among the various pathologies in this group and can help arrive at a correct diagnosis. Of course, clinical history of possible exposure or substance abuse is often the key to the patient's diagnosis.
Imaging Anatomy
Basal Ganglia
The basal ganglia (BG) are symmetric paired deep gray matter nuclei in cerebral hemispheres and form the core of the extrapyramidal system and regulate motor activity. The BG consists of caudate nucleus, putamen, and globus pallidus (GP). Caudate nucleus and putamen form the corpus striatum. The putamen and BG together are called the lentiform nuclei.
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Caudate nucleus is a C-shaped nucleus with a large head, tapered body, and down-curving tail. The head forms the floor and lateral wall of the anterior horn of the lateral ventricle. The caudate body parallels the lateral ventricles. The anterior limb of the internal capsule separates the caudate head from the putamen and GP.
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Putamen is the outermost part of the BG located lateral to GP and is separated by the lateral or external medullary lamina.
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GP consists of 2 segments, lateral (external) and medial (internal), separated by a thin layer of myelinated axons, the internal medullary lamina.
Thalamus
The thalamus is comprised of paired ovoid nuclear complexes that act as relay stations for most of the sensory pathways. The thalami extend from the foramen of Monro to the quadrigeminal plate of the midbrain. The medial aspect of the thalami form the 3rd ventricle lateral walls. The posterior limb of the internal capsule forms the lateral border of the thalami. The thalamus is subdivided into several nuclear groups, the anterior, medial, lateral, medial geniculate nuclei (part of the auditory system), lateral geniculate nuclei (part of the visual system), and pulvinar. These nuclear groups are further subdivided into 10 additional nuclei. The pulvinar is easily identified, as it is the most posterior of the thalamic nuclei and overhangs the superior colliculus. The subthalamic nucleus is small, lens-shaped, lies superolateral to the red nucleus, and is rarely involved in toxic or metabolic disorders.
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