Brainstem and Cerebellum

Several groups of lower motor neurons (LMNs) are present in the lower brainstem, including those supplying the tongue (CN XII), the pharynx and larynx (nucleus ambiguus), and the face (CN VII). Neurodegeneration of these brainstem LMNs can occur in bulbar polio, amyotrophic lateral sclerosis, and other LMN diseases. The affected muscles are atrophic and flaccid. Such a condition is called bulbar palsy (or progressive bulbar paralysis), an LMN disorder, accompanied by loss of movement, tone, and reflexes. The tongue is weak and atrophic, and the patient cannot speak or vocalize (dysarthria or anarthria, not aphasia) and cannot swallow (dysphagia); as a consequence, the patient may aspirate in an attempt to swallow. This LMN condition must be distinguished from upper motor neuron (UMN) lesions, which, when bilateral, can also result in dysphonia, dysphagia, and weakened bulbar muscles. This UMN condition is called pseudobulbar palsy or spastic bulbar palsy. In this condition, the muscles are not atrophic, and reflexes (jaw jerk and facial reflexes) are brisk. In amyotrophic lateral sclerosis, both LMN and UMN degeneration may occur progressively during the course of the disease. Because the LMNs are the final common pathway to the muscles, the LMN state usually progresses; once the LMNs have degenerated, continuing UMN damage does not make a difference functionally.

The medulla is supplied with blood by the paramedian and circumferential branches of the anterior spinal artery and the vertebral arteries. A major circumferential branch of the vertebral artery, the posterior inferior cerebellar artery (PICA), supplies a lateral wedge of medulla with blood. A brainstem stroke or an infarct in a vertebral artery or in the PICA produces a complex of symptoms called the lateral medullary syndrome (Wallenberg syndrome), which is caused by damage to an array of nuclei and tracts. The patient can demonstrate (1) loss of pain and temperature sensation on the ipsilateral side of the face (descending nucleus and tract of V) and the contralateral side of the body (spinothalamic/spinoreticular system); (2) dysphagia and dysarthria (paralysis of ipsilateral pharyngeal and laryngeal muscles resulting from damage to the ipsilateral nucleus ambiguus); (3) ataxia of the limbs and falling to the ipsilateral side (inferior cerebellar peduncle and its afferent tracts); (4) vertigo with nausea, vomiting, and nystagmus (vestibular nuclei); and (5) ipsilateral Horner’s syndrome, with ptosis, miosis, and anhidrosis (descending axons from the hypothalamus to the T1–T2 intermediolateral cell column of the spinal cord).

Occlusion of a paramedian branch of the basilar artery in the lower pons results in medial inferior pontine syndrome . This vascular syndrome causes (1) contralateral hemiparesis (corticospinal system) and drooping of the contralateral lower face (corticobulbar fibers); (2) loss of fine, discriminative touch, vibratory sensation, and joint position sense on the contralateral body that is more severe in the upper extremity (medial lemniscus); (3) limb ataxia and gait ataxia (pontine nuclei and bilateral crossing connections going into the middle cerebellar peduncles); (4) paralysis of lateral gaze by the ipsilateral eye, with resultant diplopia (abducens nerve, nucleus); (5) paralysis of conjugate gaze to ipsilateral side, with preservation of convergence (parapontine reticular formation); and (6) diplopia on attempted lateral gaze to the contralateral side, called internuclear ophthalmoplegia (medial longitudinal fasciculus).

The pons is a common site for a hemorrhagic stroke. A pontine hemorrhage is commonly large and lethal. When not fatal, such a hemorrhage may result in the rapid progression of (1) total paralysis (quadriplegia); (2) decerebrate posturing (extensor posturing) caused by UMN damage to the corticospinal and rubrospinal systems, thereby disinhibiting the lateral vestibular nuclei; (3) coma; (4) paralysis of ocular movements; and (5) small but reactive pupils. A pontine hemorrhage that results in coma is commonly lethal. A large infarct in the basilar artery may produce the same clinical picture. Some small, lacunar infarcts also may occur in the pons; these infarcts may produce purely motor symptoms (contralateral UMN paresis at base of pons), ataxia, or both (cerebellar peduncles, pontine nuclei).

A vascular lesion of circumferential branches of the basilar artery or the anterior inferior cerebellar artery can cause lateral pontine syndrome , which is characterized by (1) contralateral loss of sensation in the body, both epicritic and protopathic (medial lemniscus and anterolateral system); (2) loss of pain and temperature sensation on the contralateral face (ventral trigeminal lemniscus, located on dorsal surface of the medial lemniscus); (3) loss of fine, discriminative touch (main sensory nucleus of CN V) or impaired general sensation (CN V fibers) on the ipsilateral face; (4) ipsilateral paralysis of muscles of mastication (motor nucleus of CN V); (5) limb ataxia (middle and superior cerebellar peduncles); (6) paralysis of conjugate gaze to the ipsilateral side (parapontine reticular formation and its connections); and (7) other possible ipsilateral brainstem problems, depending on the extent of the vascular involvement, such as deafness or tinnitus (auditory nuclei or nerve fibers), vertigo and nystagmus (vestibular nuclei or nerve fibers), facial palsy (CN VII nucleus or nerve fibers), and Horner’s syndrome (descending hypothalamo-spinal sympathetic connections).

A space-occupying lesion in the forebrain, such as a bleed (epidural or subdural hematoma), a tumor, or increased intracranial pressure resulting from a variety of causes, can cause herniation of the fore-brain through the tentorium cerebelli. This transtentorial herniation displaces the thalamus and upper midbrain in a downward direction and causes a variety of changes in brain function. These changes are characterized by functions attributable to the remaining intact lower midbrain and more caudal structures, with loss of function of the upper midbrain and more rostral structures. Most conspicuous is a progressive deterioration of the state of consciousness, rapidly going from drowsiness to stupor to an unarousable state of coma; consciousness requires an intact brainstem reticular formation and at least one functioning cerebral hemisphere. When both hemispheres are nonfunctional, coma ensues. With the loss of activity in the corticospinal system and the rubrospinal system and removal of cortical influence on the other UMN pathways, a state of decerebration occurs (called decerebrate rigidity, although it is really spasticity, not true rigidity). The neck is extended (opisthotonus), the arms and legs are extended and rotated inward, and the hands, fingers, feet, and toes are flexed. Plantar responses are extensor. Cheyne-Stokes respiration is seen (crescendo-decrescendo breathing), followed at a slightly later stage of damage by shallow hyperventilation. The pupils are midsized and usually unresponsive because of compression of the third nerves against the free edge of the tentorium. Caloric testing or the doll’s-eye maneuver shows no vertical eye movements (visual tectal damage), and the eyes do not move in a conjugate fashion.