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. 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 3 rd and 4 th types of Chiari malformations are exceedingly rare and will only briefly be mentioned. 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.

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

Type 1 Chiari Most common presenting complaint is suboccipital headache Most common presenting complaint is suboccipital headache Neck pain, subjective weakness, numbness, loss of temperature sensation also ~40-60% incidence Neck pain, subjective weakness, numbness, loss of temperature sensation also ~40-60% incidence Most common presenting sign is hyperactive lower extremity reflexes 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). “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? 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. 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 MRI is diagnostic test of choice MRI is diagnostic test of choice Can show compression of brain stem at foramen magnum (common, and significant finding) Can show compression of brain stem at foramen magnum (common, and significant finding) Hydrocephalus can be present Hydrocephalus can be present Syringomyelia Syringomyelia Descent of cerebellar tonsils through foramen magnum 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. Importance probably related to brainstem compression at foramen magnum; nevertheless, this is classic finding associated with type 1 Chiari.

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: 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 (1) cerebellar tonsils well below the foramen magnum (2) syringomyelia (2) syringomyelia (3) compression of brainstem (3) compression of brainstem ← (1) (2) → (3) →

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. 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.

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. 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. 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. Presence of muscle atrophy, ataxia, and duration of symptoms >2 yrs all associated with poorer outcome.

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

Arnold-Chiari malformation Signs/symptoms secondary to brainstem and lower cranial nerve dysfunction. Signs/symptoms secondary to brainstem and lower cranial nerve dysfunction. Findings (best seen on MRI): Findings (best seen on MRI): 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.” 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 Associated findings Associated findings Hydrocephalus (VERY common – requires shunt) Hydrocephalus (VERY common – requires shunt) Syringomyelia Syringomyelia Agenesis/dysgenesis of corpus callosum 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). 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. 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. Type 4 Chiari – Cerebellar hypoplasia without herniation.

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). 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! 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!