15.7. Arousal
The interplay between the ascending reticular activating system and posterior thalamic nuclei interact to determine the state of wakefulness. The ascending reticular activating system starts in the lateral medulla. It receives projections from the spinothalamic system and all major special senses. The dorsal pons, periaqueductal grey and intralaminar nuclei of the thalamus are its major subcortical components. The intralaminar nuclei project bilaterally to the cerebral cortex. Each cortical area is activated by a reticular activating system projection. Disruption of this anatomic system is primarily related to disorders of arousal.
Major Anatomical Areas that Compromise Arousal
- Lateral > medial reticular formation
- Dorsal pons
- Periaqueductal gray of the midbrain
- Intralaminar thalamic nuclei
- Bilateral cerebral cortex
- There is full alertness with one functioning hemisphere
Disorders of Arousal
- Central uncal herniation
- Vascular disease:
- MCA stem or carotid occlusion
- Basal ganglionic hemorrhage
- Thalamic hemorrhage
- Epidural/subdural hematoma
- Brain tumor
- Rare metabolic causes:
- Acute hepatic failure
- Poisoning that decrease the production of ATP
- Abscess
- Bilateral thalamic infarction
- Top of the basilar syndrome:
- One thalamic peduncle (of Percheron) that supplies both thalami
- Dorsomedial and intralaminar nuclei are the most important thalamic structures for arousal.
- Anterior thalamic lesions may cause waxing and waning of alertness
Cerebral Herniation that Causes Lethargy to Stupor
- Cingulate gyrus herniated under the falx:
- Compromises venous outflow and is a major cause of central herniation
- Usual lesions are structural above the tentorium
- Transtentorial diencephalic:
- Compresses the midbrain RAS at the periaqueductal grey
- Becomes terminal with rupture of Duret's pontine and midline mesencephalic arteries with hemorrhage
- Unilateral uncal herniation:
- Unilateral temporal lobe swelling from tumor, trauma, or vascular event
- IIIrd nerve palsy is an early sign
- Upward Herniation:
- Posterior fossa lesions
- Most often in children with posterior fossa tumors
- Extraocular movement paralysis occurs prior to pupillary involvement
- Compression of the collicular plate by the superior vermis
General Causes of Disorders of Arousal
- Thalamic/subthalamic infarction
- Top of the basilar embolus
- Vertebrobasilar infarction
- Acute agitated delirium:
- Caudate nucleus infarction
- Bilateral temporal lobe dysfunction
- Right sided temporo-parietal occipital lesions
- Orbitofrontal cortex
- Hyper alertness or insomnia
- IIIrd ventricular lesions
- Anterior thalamic lesions
- Sleep disturbances:
- Sleep apnea
- Locus ceruleus lesions
- Pedunculopontine nuclear lesions
- Basal forebrain lesions
- Delirium (drugs, toxins, metabolic disorders)
- Chronic vegetative state
- Akinetic mutism
- Locked in syndrome
Akinetic mutism is a state of disordered arousal in which the patient neither initiates nor responds to any verbal stimulus. Patients may have visual tracking movements that appear to be purposeful. Lesions may be in the basal forebrain of the frontal lobe and are from trauma, anterior cerebral artery aneurysm, or tumor.
Abulic patients resemble those suffering from akinetic mutism but differ in that with intense verbal stimulation they may be induced to respond or to move. Difficulties with initiation of speech or movement predominate. Lesions are similar to those with akinetic mutism.
Chronic Vegetative State: patients are not conscious and do not respond to verbal or painful stimuli. They may have reflexive visual tracking movements. Lesions may be in the dorsal forebrain, dorsal pons, periaqueductal gray or lateral reticular activating system and cortex. Trauma, basilar artery stroke and increased intracranial pressure with herniation are causative.
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