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Online Module: Chiari Malformations

Online Module: Chiari Malformations. About the term. To say “Chiari malformations” is slightly misleading. The Chiari malformations actually consist of four defined types of hindbrain abnormalities, each distinct from the others.

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Online Module: Chiari Malformations

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  1. Online Module:Chiari Malformations

  2. About the term • To say “Chiari malformations” is slightly misleading. The Chiari malformations actually consist of four defined types of hindbrain abnormalities, each distinct from the others. • The first two types, especially type 1, will be briefly reviewed here. The 3rd and 4th types of Chiari malformations are exceedingly rare and will only briefly be mentioned.

  3. The Chiari Malformation • The term usually refers to “Type 1 Chiari Malformation,” which is classically described as “adult-onset Chiari” (avg. age of presentation is ~40 yrs) with downward displacement of the cerebellar tonsils through the foramen magnum. • Although this is a common radiographic feature of the condition, it is not a prerequisite for diagnosis.

  4. Type 1 Chiari • Most common presenting complaint is suboccipital headache • Neck pain, subjective weakness, numbness, loss of temperature sensation also ~40-60% incidence • Most common presenting sign is hyperactive lower extremity reflexes • “Cape”-like sensory loss, nystagmus (downbeat), gait disturbance, upper extremity weakness, etc., are also all very common (~30-50% incidence of each).

  5. Type 1 Chiari – why should you care? • Patients who are identified early and receive early treatment have the best response to surgical intervention. Because of the variable constellation of signs and symptoms associated with type 1 Chiari, it is regularly missed or misdiagnosed.

  6. Type 1 Chiari - imaging • MRI is diagnostic test of choice • Can show compression of brain stem at foramen magnum (common, and significant finding) • Hydrocephalus can be present • Syringomyelia • Descent of cerebellar tonsils through foramen magnum • Importance probably related to brainstem compression at foramen magnum; nevertheless, this is classic finding associated with type 1 Chiari.

  7. Type 1 Chiari malformation • T1 weighted MRI w/o contrast, sagittal view; in this outstanding picture of a patient with type 1 Chiari, you see: • (1) cerebellar tonsils well below the foramen magnum • (2) syringomyelia • (3) compression of brainstem (3)→ ←(1) (2)→

  8. Type 1 Chiari – cerebellar tonsils • The cerebellar tonsils normally ascend as we age; in normal adults, the tonsils usually do not descend through the foramen magnum (or descend a very small amount), but in Chiari 1 patients descent is the norm. Lack of tonsillar descent is an extremely sensitive marker; therefore, in a patient with presentation that can be consistent with type 1 Chiari and cerebellar tonsil protrusion through the foramen magnum > 3mm, the pt. needs neurosurgery referral.

  9. Operative Results • The most commonly-performed surgery is suboccipital craniectomy (essentially opens up the foramen magnum), with or without C1 laminectomy and dural graft patch. • Patients with pain as primary complaint respond best to surgery; weakness less responsive, but overall ~80% of patients report favorable results. • Presence of muscle atrophy, ataxia, and duration of symptoms >2 yrs all associated with poorer outcome.

  10. Type 2 Chiari • Type 2 Chiari malformation is also referred to as “Arnold-Chiari malformation.” • Presents in childhood • Usually the younger it presents, the more severe the condition. • Usually associated with myelomeningocele!!! (The USMLE loves this)

  11. Arnold-Chiari malformation • Signs/symptoms secondary to brainstem and lower cranial nerve dysfunction. • Findings (best seen on MRI): • Caudal displacement of posterior fossa structures, including cervicomedullary junction, pons, medulla, 4th ventricle, and cerebellar tonsils. Classically, the cervicomedullary junction is described as having a “kink-like deformity.”

  12. Arnold-Chiari malformation • Associated findings • Hydrocephalus (VERY common – requires shunt) • Syringomyelia • Agenesis/dysgenesis of corpus callosum • Operative goals similar to type 1, but these patients do not do as well (and the younger their presentation, the worse the general outcome).

  13. Others • Type 3 Chiari - Rare and severe (usually not compatible with life); basically, posterior fossa structures end up everywhere except where they should be. • Type 4 Chiari – Cerebellar hypoplasia without herniation.

  14. Summary • For USMLE purposes, it’s good to understand the major characteristics of and differences between type 1 Chiari and type 2 Chiari, as it seems to show up a lot (probably because people mix them up a lot). • Understand that type 1 Chiari malformation has an extremely variable presentation. If you keep it on your radar in patients who present with these symptoms, you can be a patient’s hero!

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