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The cerebellum overlies the pons, It is connected by three pairs of cerebellar peduncles. The fourth ventricle is found between the dorsal surface of the pons and the cerebellum. The ventral surface of the pons is dominated by fibers, which form a large ventral enlargement that carries fibers from pontine nuclei to the cerebellum in the middle cerebellar peduncle.
Though spastic paresis indicates involvement of the pyramidal tracts from the cerebrum on down, in this case, because of the 6th nerve injuryat the level ofthe pons is indicated. In the pons the pyramidal tracts are in the basis pontis, and in this case the side opposite the weakness, i.e., the right side. Ataxic gait, vibratory and positional deficits on the left suggest injury to the medial lemniscus, which lies near the midline in the ventral tegmentum, on the right. The ataxia could also have a cerebellar component due to injury of the basis pontis and the pontine nuclei. Normal pain and temperature perception indicate that the lesion was more limited to the midline rather than lateral where the spinothalamic and 5th nerve components lie. Gaze paralysis to the right and internal strabismus of the right eye indicate weakness of the right lateral rectus and injury to the fibers of the right abducens nerve. If the 6th nucleus had been involved the medial rectus of the left eye would have shown signs as well, due to involvement of the nearby PPRF. Since convergence was preserved and only the lateral rectus of the right eye was paralyzed this was a lesion involving only the fibers of the 6th nerve.This constellation of symptoms is consistent with the midline distribution of the paramedian branches of the basilar artery and occlusion of its branches in the caudal pons.
Anatomy of the pons
Anatomy of the PonsBrief overview of Pontine syndromes Daniel Vela-Duarte, MD Department of Neurology Loyola University Medical Center June 2012
Functional Neuroanatomy The pons is located between the medulla (caudally) and the midbrain (rostrally). The corticospinal tracts are more diffuse in the pons The medial lemniscus is still situated near the midline The Spinothalamic tract and the descending hypothalamic fibers continue to course together in the lateral pons The lateral lemniscus (An ascending auditory pathway), is lateral and dorsal to the medial lemniscus. It carries the bulk of ascending auditory fibers from both cochlear nuclei to the inferior colliculus of the midbrain. The medial longitudinal fasciculus (MLF) is located near the midline, beneath the fourth ventricle.
The cerebellum overlies the pons, It is connected by three pairs of cerebellar peduncles.The fourth ventricle is found between the dorsal surface of the pons and the cerebellum. The ventral surface of the pons is dominated by fibers, which form a large ventral enlargement that carries fibers from pontine nuclei to the cerebellum in the middle cerebellar peduncle.
Vascular supply to the Pons The Pons is supplied by the; Basilar artery, contributions of this main artery can be further subdivided; Paramedian branches, to medial pontine region Short circumferential branches, supply anterolateral pons Long circumferential branches, run laterally over the anterior surface of the Pons to anastomose with branches of the anterior inferior cerebellar artery (AICA). Some reinforcing contributions by the anterior inferior cerebellar and superior cerebellar arteries
Blood supply Additional branches from the Basilar artery:Anterior Inferior cerebellar Artery(AICA), first branch of the basilararteryIt supplies anterior inferior surface(Inferior pons) Superior cerebellar artery Emerges from the basilar artery, rostrally. It supplies cerebellar cortex, white matter and central nuclei
Blood supply Labyrynthine artery Variable in origin, supplies the inner ear. Divides into two branches; a. anterior vestibular b. common cochlear•It could emerge from:Wende et. al., 1975, (sample size of 238)1. Basilar (16%)2. AICA (45%)3. Superior cerebellar (25%)4. PICA (5%)5. Remaining 9% were of duplicateorigin
Blood SupplyThe paramedian branches of the Basilar artery supplies the paramedianregions of the Pons, including: corticospinal fibers the medial leminiscus, abducens nerve and nucleus (CN VI) pontine reticular area, periaquaductal gray areas
Blood Supply to the PonsThe paramedian branchesof the Basilar arterysupplycorticospinal fibers,the medialleminiscus, abducensnerve and nucleus (cranialnerve VI) ,pontine reticular area,periaquaductal gray areas
Medial Pontine Syndrome/ Middle Alternating Hemiplegia Paramedian branches of basilar artery occlusionClinical picture Where’s the lesion ? contralateral hemiplegia of arm (corticospinal fibers in & leg basilar pons) contralateral loss/decrease of (medial lemniscus) proprioception, vibration, discriminative touch ipsilateral lateral rectus muscle (abducens nerve fibers or n paralysis (paramedian pontine paralysis of conjugate gaze reticular formation/pontine toward side of lesion gaze center)ucleus—CN 6)
Blood Supply to the PonsObstruction of the paramedian pontine arteries will produce amiddle alternating hemiplegia (also termed medial pontinesyndrome)which is characterized by;1. Hemiplegia of the contralateral arm and leg, due to damage tothe corticospinal tracts2. Contralateral loss of tactile discrimination, vibratory and positionsense, due to damage to the medial lemniscus3. Ipsilateral lateral rectus muscle paralysis, due to damage to theabducens nerve or tract (can cause diplopia “double vision”)
Blood Supply to the PonsOcclusions of long branches circumferential branches of the basilarartery produce a lateral pontine syndrome, characterized by;1. Ataxia, due to damage to the cerebral peduncles (middle and superior)2. Vertigo, nausea, nystagmus, deafness, tinitus, vomiting, due todamage to vestibular and cochlear nuclei and nerves3. Ipsilateral pain and temperature deficits from face, due to damage tothe spinal trigeminal nucleus and tract4. Contralateral loss of pain and temperature sense from the body,due to damage to the anterolateral system (spinothalamic)5. Ipsilateral paralysis of facial muscles and masticatory muscles, dueto damage to the facial and trigeminal motor nuclei (cranial nervesVII and V)
Case # 1. A 48 year old man, right handed, suffered a sudden weakness of his left arm and leg which caused him to fall while shaving. He was helped to his feet but his left arm and leg felt stiff. In addition, he complained of seeing "double".On exam normal mental status. There was no evidence of increased intracranial pressure though his blood pressure was 200/95. There was a spastic paresis with extensor plantar response in the left extremities and loss of vibratory and positional sense on the left. The patient walked with an ataxic gait. Pain and temperature sensations were normal. There was diplopia when the patient looked toward the right side. At rest , the right eye deviated toward the nose (internal strabismus or squint) while the left eye looked straight ahead. There was a paralysis of conjugate gaze toward the right (i.e, the right eye did not move laterally toward the right though the left eye did) Ocular convergence was normal. Temple University School of Medicines Department of Anatomy and Cell Biology
Case # 2 A 55 year old man was brought to the hospital after suddenly falling to the ground unable to move his right arm and leg. The neurologic exam revealed that the limbs on the right side had markedly diminished strength, heightened deep tendon reflexes, ankle clonus, Babinski and increased resistance to passive stretch. The left arm and leg had near normal strength but performed in an uncoordinated manner on the finger-to-nose test and the heel-to-shin test. Cranial nerve examination was significant in that the upon smiling the man did not elevate his mouth on the right side and could not blow out his right cheek; he could tightly close his eyelids on both sides. Temple University School of Medicines Department of Anatomy and Cell Biology