76 5 Topical Diagnosis and Differential Diagnosis of Neurologic Syndromes

Disturbances of Consciousness

Anatomical substrate. Intact consciousness requires normal functioning of the cortex of both cerebral hemispheres. The “driving force” of cortical activity, however, is located in a lower center, i. e., in a collective of neurons in the brainstem called the reticular formation, which sends impulses toward the cerebral cortex by way of the intralaminar thalamic nuclei. The reticular

Table 5.6 Causes of impaired consciousness

Clinical situation	Possible causes
Impaired consciousness or	various metabolic disorders
coma without focal signs or		hypoor hyperglycemia
meningism (purely toxic/metab-		uremia
olic or anoxic coma)		liver failure
		endocrine dysfunction
		electrolyte disturbances
		metabolic acidosis or alka-
		losis
	intoxications (e. g., alcohol,
	medications, carbon mon-
	oxide)
	acute heart failure or
	diminution of the circulating
	blood volume (anoxic
	encephalopathy)
		myocardial infarction
		atrial fibrillation
		cardiac tamponade
		hypovolemic shock
		cardiorespiratory arrest etc.
	Focal neurological signs are
	absent only if cerebral
	ischemia and hypoxia have not
	yet caused irreversible struc-
	tural damage in the CNS
Impaired consciousness or	meningitis
coma without focal signs, with	subarachnoid hemorrhage
meningism
Impaired consciousness or	supratentorial lesions (acute:
coma with focal signs (struc-	infarct, trauma, intracranial
tural lesion)	hemorrhage, subdural/
	epidural hematoma; subacute/
	chronically progressive: infec-
	tions, tumors); impairment of
	consciousness is often due to
	edema in the brain tissue
	around the lesion and the
	resulting intracranial hyperten-
	sion and capillary hypoxia,
	sometimes accompanied by
	midline shift and herniation,
	with secondary brainstem
	damage
	infratentorial lesions (acute:
	infarct, trauma, hemorrhage;
	subacute/chronically progres-
	sive: infections, tumors)
Transient impairment of	generalized epileptic seizure
consciousness, possibly accom-
panied by involuntary motor
phenomena

formation and its ascending projections are known collectively as the ascending reticular activating system.

Impairments of consciousness may thus be caused either by the simultaneous impairment of function of both cerebral hemispheres, or by damage to the reticular formation in the brainstem, and/or to its ascending projections (uncoupling of the cortex from the activating input of the reticular formation). Depending on their severity, impairments of consciousness are termed somnolence, stupor, or coma (Table 3.9, p. 40). Coma, the most severe impairment of consciousness, can also be more finely graded, according to any of several semiquantitative schemes that have been proposed. The best known of these is the Glasgow Coma Scale (GCS, Table 6.6, p. 88).

Causes. Consciousness may be impaired either by a structural lesion of brain tissue, or else indirectly by a systemic disturbance of some kind (metabolic, toxic, or anoxic coma; see below). If a structural lesion is the cause, there are often accompanying focal neurological deficits whose presence enables the clinician to infer the probable site of the lesion. The direct cause of the impairment of consciousness is often not the lesion itself, but rather the cerebral edema surrounding it (cf. Table 5.6). Focal neurological signs are usually absent in purely metabolic, toxic, or anoxic coma.

Bilateral cortical dysfunction can also be the result of an epileptic seizure or an infectious/inflammatory process such as meningitis or encephalitis (in which case meningism is usually present). Finally, there are also purely psychogenic states (psychogenic stupor) that may superficially resemble an organic impairment of consciousness. The more important causes of impaired consciousness are listed in Table 5.6. Here we have classified all impairments of consciousness into four basic clinical situations and listed the common etiologies for each.

Differential diagnosis. We will now briefly describe three other types of disturbance of consciousness that must be distinguished from coma.

Apallic syndrome/coma vigil (“persistent vegetative state”). This condition is usually due to severe and extensive brain damage. It is characterized by a complete uncoupling of midbrain and diencephalic activity from cortical activity and thus by a complete dissociation of wakefulness and consciousness. (The term “apallic” signifies “without cerebral cortex.”) The vegetative functions (breathing, cardiovascular regulation, sleep−wake cycle) are preserved, though possibly abnormal to some extent. Cognitive or goal-directed motor activity is entirely lacking. Unlike the comatose patient, the apallic patient lies in bed with eyes open, staring blankly into the distance, not fixating the gaze on anything, and not responding to verbal or noxious stimuli. At other times, the patient is in a sleeplike state, with eyes closed. Muscle tone is elevated, and automatisms and primitive reflexes are sometimes observed in the perioral area. Common types of autonomic dysfunction seen in apallic

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