Online Module Chiari Malformations About the term To

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. n The first two types, especially type 1, will be briefly reviewed here. The 3 rd and 4 th types of Chiari malformations are exceedingly rare and will only briefly be mentioned. n

The Chiari Malformation n 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. n Although this is a common radiographic feature of the condition, it is not a prerequisite for diagnosis.

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

Type 1 Chiari – why should you care? n 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.

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

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

Type 1 Chiari – cerebellar tonsils n 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 > 3 mm, the pt. needs neurosurgery referral.

Operative Results n The most commonly-performed surgery is suboccipital craniectomy (essentially opens up the foramen magnum), with or without C 1 laminectomy and dural graft patch. n Patients with pain as primary complaint respond best to surgery; weakness less responsive, but overall ~80% of patients report favorable results. n Presence of muscle atrophy, ataxia, and duration of symptoms >2 yrs all associated with poorer outcome.

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

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

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

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

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). n 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! n