Chiari malformations

• Patho/Etiology
  • Structural defects in the posterior fossa, leading to downward displacement of cerebellar and/or brainstem structures through the foramen magnum.
  • Believed to result from a mismatch between the size of the developing cerebellum and the volume of the posterior fossa.
  • Obstruction of normal cerebrospinal fluid (CSF) flow is a major cause of symptoms.
• Types & Presentation
  • Chiari I Malformation (Usually asymptomatic in childhood, manifests in adulthood)
    • Anatomy: Low-lying cerebellar tonsils (>5 mm below foramen magnum).
    • Presentation: Often asymptomatic in childhood, presents in adolescence/adulthood.
      • Occipital headache and neck pain, classically exacerbated by coughing or Valsalva maneuver.
      • Cerebellar symptoms (e.g., ataxia, nystagmus) and cranial nerve dysfunction (e.g., dysphagia, hoarseness).
    • Key Association: Syringomyelia (fluid-filled cavity in the spinal cord), which can cause a “cape-like” distribution of lost pain and temperature sensation in the upper extremities.
  • Chiari II Malformation (Arnold-Chiari Malformation) (More severe than Chiari I, usually presents early in life)
    • Anatomy: Herniation of cerebellar vermis and tonsils, plus brainstem (medulla, pons) and a low-lying 4^th ventricle through the foramen magnum.
    • Presentation: Presents in infancy or early childhood.
      • Symptoms are often due to brainstem compression and hydrocephalus, including stridor (a harsh vibrating noise when breathing), apnea, dysphagia, and limb weakness.
    • Key Association: Almost universally associated with myelomeningocele (a severe form of spina bifida). Non-communicating hydrocephalus is also very common.
• Diagnosis
  • MRI is the gold standard imaging modality to visualize the herniated structures, assess for hydrocephalus, and identify a syrinx.
• Management
  • Asymptomatic Chiari I patients are typically monitored.
  • Symptomatic patients often require surgery.
    • Posterior fossa decompression (craniectomy) is the most common procedure, often with a laminectomy of C1, to relieve pressure and restore CSF flow.
    • A ventriculoperitoneal (VP) shunt may be necessary to treat associated hydrocephalus.

Chiari malformation vs Dandy-Walker malformation

Feature	Chiari Malformation	Dandy-Walker Malformation
Pathophysiology	Cerebellum herniates through foramen magnum due to small posterior fossa.	Agenesis of cerebellar vermis allows 4th ventricle to enlarge into a cyst.
Posterior Fossa	Small / Normal	Enlarged
Key Abnormality	Tonsillar (Type I) or vermis + tonsillar (Type II) herniation.	Absent cerebellar vermis.
Hydrocephalus	Common, due to CSF outflow obstruction.	Very common, due to cystic 4th ventricle.
Key Associations	- Type I: Syringomyelia. - Type II: Myelomeningocele.	Hydrocephalus, Spina Bifida, Agenesis of corpus callosum.
Typical Presentation	- Type I: Adult w/ headaches. - Type II: Infant w/ brainstem dysfunction.	Infant w/ macrocephaly.

Chiari畸形：过度拥挤的房间

请把后颅窝想象成一个小卧室，把小脑和脑干想象成里面的家具。

• 问题所在： 在Chiari畸形中，这个房间建得太小了，无法容纳所有它需要放置的家具。
• 后果： 为了把所有东西都塞进去，一部分家具（小脑扁桃体）被挤了出来，并从唯一的开口——门道（枕骨大孔）——伸了出去。
• 结果： 这些堵在门口的家具阻碍了交通（脑脊液流动），导致房间内压力积聚（脑积水），并影响到走廊的墙壁（脊髓空洞症）。

核心：Chiari是一个_拥挤_问题。

Dandy-Walker畸形：缺失的帐篷支柱

请把后颅窝想象成一个大帐篷，把小脑蚓部想象成它主要的中央支撑柱。

• 问题所在： 在Dandy-Walker畸形中，这根中央帐篷支柱缺失了（蚓部发育不全）。
• 后果： 没有了支柱的支撑，帐篷的顶部（第四脑室）无法正常成型。它会塌陷并被“雨水”（脑脊液）充满，形成一个巨大的、充满液体的囊肿。
• 结果： 这个不断膨胀的水囊肿将帐篷壁向外推（后颅窝扩大），并堵住了拉链出口（脑脊液流出道），将所有的水都困在里面，导致整个帐篷都肿胀起来（脑积水）。

核心：Dandy-Walker是一个_结构缺陷_问题。