====== Posterior fossa lesion ======

===== Etiology =====


Etiologies of [[infratentorial mass]]es:

1. vascular: [[brainstem infarction]] (including [[basilar artery occlusion]]), [[cerebellar infarction]] or [[cerebellar hemorrhage]]

2. inflammatory: [[cerebellar abscess]], [[central pontine myelinolysis]], [[brainstem encephalitis]].

3. [[neoplasm]]s: primary or metastatic, see [[posterior fossa tumor]]

4. traumatic: [[epidural hematoma]] or [[subdural hematoma]].

===== Clinical findings =====

Symptoms: 1. sensory
a) craniocervical pain: usually a nearly symptom, commonly in neck and occiput.Aching nature. ↑with head movement

b) sensory findings: usually occur later.Numbness and tingling of the fingers

2. motor

a) spastic weakness of the extremities:weakness usually starts in the ipsilateral UE,then the ipsilateral LE, then the contralateral LE, and finally the contralateral UE (“rotating paralysis”).

Signs:
1. sensory

a) dissociated sensory loss:loss ofpain and temperature contralateral to lesion with preservation of tactile sensation
b) lossofpositionandvibratorysense,greaterintheupperthanthelowerextremities

2. motor
a) spastic weakness of the extremities

b) atrophy of the intrinsic hand muscles:a lower motor nerve finding

c) cerebellar findings may rarely be present with extensive intracranial extension

3. long tract findings

a) brisk muscle stretch reflexes(hyperreflexia,spasticity)

b) loss of abdominal cutaneous reflexes

c) neurogenic bladder: usually a very late finding.

4. ipsilateral Horner syndrome: due to compression of cervical sympathetics

5. nystagmus: classically [[downbeat nystagmus]] but other types can occur It had been postulated that long tract findings were due to direct compression at the cervicomedullary junction, and that lower motor nerve findings in the upper extremities were due to central necrosis of the gray matter as a result of compression of arterial blood supply. Anatomic study suggests that it is actually venous infarction at lower cervical levels (C8–1) that is responsible for the lower motor neuron findings.

===== Complications =====
==== Hydrocephalus ====

Infratentorial masses can produce [[obstructive hydrocephalus]] by compressing the [[Sylvian aqueduct]] and/or [[fourth ventricle]] 

==== Upward cerebellar herniation ====

see [[Upward cerebellar herniation]].


==== Tonsillar herniation ====

see [[Tonsillar herniation]].