Approach to Toxic, Metabolic, Degenerative, and CSF Disorders

Main Text

Preamble

Metabolic disorders are relatively uncommon but important diseases in which imaging can play a key role in early diagnosis and appropriate patient management. Drug and alcohol abuse are increasing around the world, and the list of environmental toxins that can affect the CNS continues to increase. Recognizing toxic and metabolic-induced encephalopathies has become a clinical and imaging imperative. The two etiologies are often linked because many toxins induce metabolic derangements and some systemic metabolic diseases have a direct toxic effect on the brain.

With rapidly increasing numbers of aging people, the prevalence of dementia and brain degeneration is also becoming a global concern. Brain scans in older patients with mental status changes are now some of the most frequently requested imaging examinations, so normal and pathologic age-related CNS changes are discussed in this section.

Anatomy and Physiology of the Basal Ganglia and Thalami

Physiologic Considerations

By weight and volume, the brain is a small structure. However, relative to its size, the brain is one of the most metabolically active of all organs. It normally receives ~ 15% of total cardiac output, consumes ~ 20% of blood oxygen, and metabolizes up to 20% of blood glucose.

Because of its high intrinsic metabolic demands, the brain is exquisitely sensitive to processes that decrease delivery or utilization of blood, oxygen, and glucose. A variety of toxic substances do exactly that.

Two areas of the brain are especially susceptible to toxic and metabolic damage: The deep gray nuclei and the cerebral white matter (WM). The basal ganglia (BG) are highly vascular, rich in mitochondria, and loaded with neurotransmitters. The BG—especially the putamen and globus pallidus (GP)—are particularly susceptible to hypoxia or anoxia and are also commonly affected by toxins and metabolic derangements. The cerebral WM is particularly vulnerable to lipophilic toxic substances.

Normal Gross Anatomy

The BG are symmetric paired subcortical (deep gray matter) nuclei that form the core of the extrapyramidal system and control motor activity. The BG consist of (1) the caudate nucleus (CNuc), (2) the putamen, and (3) the GP (34-1).

The thalami are the largest and most prominent of the deep gray matter nuclei but are generally not included in the term “basal ganglia.”

Normal Imaging Anatomy

NECT

T1WI

The CNuc, putamina, and thalami are isointense with cortex on T1 scans (34-2). As the site of both physiologic calcification and age-related iron deposition, the GP segments vary in signal intensity. Calcification may cause T1 shortening and mild hyperintensity in the medial segment.

T2WI

The CNuc, putamina, and thalami are isointense with cortical gray matter on T2 scans (34-3). Increasing iron deposition occurs with aging, and the putamen becomes progressively more hypointense. A “dark” putamen is normal by the seventh or eighth decade of life.

T2*

The GP is hypointense relative to cortex on GRE or SWI imaging. By the seventh or eighth decade of life, iron deposition in the putamen “blooms,” and the lateral putamen appears hypointense relative to the thalami but not as intensely hypointense as the GP.

Toxic and Metabolic Disorders

Preamble

Many toxic, metabolic, systemic, and degenerative diseases affect the BG and thalami in a strikingly symmetric fashion (34-15). Lesions are most often secondary to diffuse systemic or metabolic derangements (34-11). Patchy, discrete, focal, and asymmetric lesions are more commonly infectious, postinfectious, traumatic, or neoplastic in origin (34-12).

Differential Diagnoses of Bilateral Basal Ganglia Lesions

The most common bilateral BG lesions are normal variants (e.g., physiologic calcification and prominent perivascular spaces). Vascular disease, hypoxic-ischemic insults, and common metabolic disorders, such as chronic liver failure, are the most frequent causes (34-11). Infection, toxins, drug abuse, or metabolic disorders, such as osmotic demyelination and Wernicke encephalopathy, are less common causes of bilateral BG lesions (34-12).

Putamen Lesions

Toxic, metabolic, and hypoxic-ischemic events and degenerative disorders account for the vast majority of symmetric putamen lesions. In general, the putamina are less commonly affected than either the GPs or thalami. The most common lesion to affect the putamen is hypertensive hemorrhage. Acute hypertensive bleeds are usually unilateral, although T2* scans often disclose evidence of prior hemorrhages.

Bilateral symmetric putamen lesions usually occur with more generalized BG involvement. However, there are some lesions that predominantly or almost exclusively involve the putamina (34-22).

Globus Pallidus Lesions

The globus pallidus (GP) is the part of the BG that is most sensitive to hypoxia. The GPs are rich in mitochondria, vascular supply, and neurotransmitters. Their high metabolic activity makes them especially vulnerable to numerous metabolic abnormalities and systemic/generalized disease processes.

The vast majority of symmetric GP lesions are secondary to hypoxic, toxic, or metabolic processes that diminish blood flow, oxygen supply, &/or energy availability. Most cause bilateral symmetric abnormalities on imaging studies (34-15) (34-16) (34-17).

COMMON GLOBUS PALLIDUS LESIONS

Normal Variant

• Physiologic calcification

Medial GP

Hypoxic-Ischemic Encephalopathy

• Anoxia, hypoxia (near drowning, cerebral hypoperfusion)

• Neonatal HIE (profound acute)

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