ACUTE NEUROLOGICAL DYSFUNCTION
Altered consciousness can be a manifestation of a variety of disease processes. It is commonly indicative of life-threatening illness that requires urgent stabilization, diagnosis, and institution of disease-specific therapy to prevent ongoing neurologic injury and long-term neurodevelopmental disability. Although the anatomic structures related to consciousness have been defined, depressed consciousness is usually poorly localized to specific nervous system pathways, and often represents the consequence of a failed non-neurologic organ system.
The goals of this chapter are to review the pathophysiology and age-specific epidemiology of altered mental status, to delineate the current framework used to assess the presentation of depressed consciousness, to give a practical overview of its differential diagnosis, and finally to outline a strategy for stabilization, diagnostic evaluation, and emergent treatment options. A full discussion of the clinical approach to altered consciousness, including a comprehensive review of the interpretation of clinical signs, can be found in standard texts on coma.
PATHOPHYSIOLOGY AND EPIDEMIOLOGY
Consciousness has 2 components, wakefulness and awareness (of self and of the surrounding environment). Awareness cannot occur without wakefulness but wakefulness can occur without awareness. Consciousness is dependent on the function of the reticular activating system (RAS) that promotes widespread cortical activation. The core areas for maintaining wakefulness are thought to be ascending glutamatergic and cholinergic neurons in the dorsal tegmentum of the midbrain and pons. These activate the central thalamus and basal forebrain, which in turn activate the cortex.
The level of arousal is a function of the overall state of activity of the brain. Awareness is a more complex and integrated process involving the cerebral cortex and thalamus. Coma is a state in which a child has eyes closed and is unaware and unresponsive to any external stimuli, except for reflex responses. It results from either damage to the RAS or profound global cortical dysfunction. While lesions of the RAS can result in coma, detailed knowledge of this anatomic pathway is generally not required as global cortical dysfunction is by far the most common cause of altered consciousness. Focal lesions that affect the RAS do occur, especially those that produce pressure in the posterior fossa.
Regulation of Cerebral Blood Flow
Cerebral blood flow is tightly matched to the metabolic need of brain tissue, which is dependent on a continuous supply of oxygen and glucose for obligate-aerobic metabolism. Neither excessive nor inadequate cerebral blood flow can be tolerated for prolonged duration without causing brain injury. Multiple layers of adaptive mechanisms work simultaneously to adjust and constrain cerebral blood flow to an optimized level during disrupted physiological states such as shock or disturbed delivery of substrate.
The first adaptive mechanism to preserve cerebral blood flow is the systemic vasoconstrictor response to shock or hypotensive states, which is characteristically associated with neurohormonal activation. However, vessels in the brain ...