What is Chiari I malformation?

Chiari malformation is an umbrella term to describe the congenital condition where an individual's cerebellum is pushed through the foramen magnum into the spinal canal, which can impact cerebrospinal fluid flow around the brain and the spinal cord. 

Figure 1. My first brain MRI, indicating Chiari I malformation with herniation of the cerebellar tonsils (blue) into the foramen magnum with compression of the brainstem (pink)

The obstruction that impacts cerebrospinal fluid flow can occur as a result of many anatomical causes, including:

• Cerebellum being too large
• Foramen magnum being too small
• Posterior fossa being too small [8]

Figure 2. Sagittal (side) view of the skull with the cerebellum denoted in dark blue and the area of the foramen magnum denoted with a dark orange arrow

Figure 3. Inferior (bottom-up) view of the skull, with the posterior fossa denoted with light purple and foramen magnum outlined with the dark purple circle

Many theories exist trying to explain what causes Chiari, but our best current understanding is that most Chiari malformations occur sporadically during fetal development and are not inherited [2]. So while there are theories that the condition may have a genetic component (like a spontaneous mutation or deletion during development) [8], passing Chiari to your children remains possible, but its likelihood is uncertain [2]. 

Four main types of Chiari are most widely recognized, with a few types and subtypes currently being debated. The most common types are Chiari I and II, so even though I'll be focusing on Chiari I, I wanted to describe both of them: 

Chiari malformation I (CM-I): The cerebellar tonsils are displaced ≥5 mm below the level of the foramen magnum, though the amount of tonsillar descent is not necessarily indicative of symptom severity [5]. This means, for example, someone with 6 mm tonsillar descent may present with more severe symptoms than someone with 12 mm tonsillar descent. CM-I is the type I have and the type that many who have it are not diagnosed with until adolescence or adulthood. 

Figure 4. My second brain MRI- with the horizontal green line denoting the location of the foramen magnum and the 1.12 cm approximation indicating the 11.2 mm herniation of my cerebellar tonsil into my foramen magnum

• Chiari I is slightly more common in females and has an estimated prevalence of 1-3.6% [10], which is higher than previously suspected with the increased use of MRI in clinical settings and the number of patients that are often asymptomatic.
• Even though many people with Chiari I can be asymptomatic, and Chiari I may be found incidentally on their MRI and may not be the root cause of their symptoms, it is important to trust yourself and advocate for yourself to your doctors if you think Chiari I is impacting your life. You know your body. One of the first doctors I talked to told me that my Chiari was an incidental finding. If I had not sought additional opinions from other doctors, I may have never found out I had a syrinx and my condition would have only deteriorated further.

Chiari malformation II (CM-II) (older textbooks may also call it Arnold-Chiari): The cerebellar tonsils and vermis, as well as the medulla oblongata, are displaced below the level of the foramen magnum. Most patients with Chiari II also have myelomeningocele, so this is usually detected before or immediately after birth [8]. Chiari II is also associated with spina bifida [7]. 

Most common Chiari I manifestations

Since there are so many conditions that may be related to CM-I, and there is still so much we don't know about the disorder itself, it can be difficult to nail down exactly what symptoms can be attributed to Chiari I. But some of the more common signs may include [1, 8, 9]:

• Cerebellar dysfunction signs: cerebellar dysarthria, muscle weakness, nystagmus 
• Brainstem compression signs: hiccups, hoarseness, oscillopsia, sleep apnea
• Feeding problems in infants 
• Headache: the most common sign- can be constant or episodic, most often localized to the back of the head and worsened with acts of straining (also known as Valsalva maneuvers- including coughing, laughing, sneezing)
• Progressive scoliosis: seen in up to 30% of children that present with Chiari I and often associated with a syrinx
• Syringomyelia: cerebrospinal fluid-filled cyst in the spinal cord [12] 
• See post: "What is syringomyelia?"

Other Associated Conditions

Some additional conditions that have been reflected across studies to be associated with Chiari I include: 

• Ehler-Danlos Syndrome [3]
• Hydrocephalus [8]
• Neurofibromatosis type 1 [11]
• Noonan syndrome [4]
• Robin sequence [6]
• Tethered cord [8]

Treatment Options

See post: "I have Chiari, now what?"

For additional information as a good starting point, my favorite lecture that I've seen describing Chiari I is this lecture from Dr. Alan Cohen, the director of pediatric neurosurgery at Johns Hopkins.

Glossary

Brainstem: the stalk that connects the cerebrum and spinal cord- made up of the midbrain, pons, and medulla oblongata

Cerebellar dysarthria: a speech disorder caused by damage to the cerebellum, which may present as slowed or monotonous speech, increased variability in pitch and volume, or other irregular speech patterns

Cerebellum: Latin for "little brain"- a part of the brain separate from the cerebrum that usually sits above or only ≤3 mm below the foramen magnum- it is made up of the vermis, peduncles, tonsil, and flocculonodular lobe and controls balance, coordination, and movement

Cerebrospinal fluid (CSF): the clear, protective, nutrient-rich fluid that flows around the brain and spinal cord

Cerebrum: the largest part of the brain which is comprised of two hemispheres and four lobes, when we imagine the brain this is usually the part we're thinking of

Congenital: a condition someone is born with

Foramen magnum: Latin for "large hole"- the hole in the skull that connects the brain to the spinal cord

Incidental finding: an extra discovery made during medical imaging not related to the original reason for the test

MRI (Magnetic Resonance Imaging): a medical imaging technique that uses radio waves and strong magnetic currents to generate images of the body

Myelomeningocele: a serious birth defect where the neural tube (an embryonic structure that becomes the brain and spinal cord) doesn't close during fetal development, resulting in a protruding sac of cerebrospinal fluid and nerves visible on the baby's back

Nystagmus: involuntary, rapid, and repetitive eye movements- most often initially tested in a doctor's office using the H-Test and considered an issue if sustained (temporary nystagmus can be a normal finding)

Oscillopsia: visual issue where your vision may blur and your surroundings may feel like they're jumping or vibrating

Posterior fossa: the small space in the skull that contains the cerebellum and the brainstem, with the foramen magnum located in the middle 

References

1. Ciaramitaro P, Ferraris M, Massaro F, Garbossa D. Clinical diagnosis-part I: what is really caused by Chiari I. Childs Nerv Syst. 2019 Oct;35(10):1673-1679. doi: 10.1007/s00381-019-04206-z. Epub 2019 Jun 3. PMID: 31161267.
   
   
2. Dhawan A, Dhawan J, Sharma AN, Azzam DB, Cherry A, Fehlings MG. Familial Chiari malformation: a systematic review and illustrative cases. J Neurosurg Spine. 2024 Apr 12;41(1):105-114. doi: 10.3171/2024.1.SPINE231277. PMID: 38608294.
   
   
3. Henderson FC Sr, Austin C, Benzel E, Bolognese P, Ellenbogen R, Francomano CA, Ireton C, Klinge P, Koby M, Long D, Patel S, Singman EL, Voermans NC. Neurological and spinal manifestations of the Ehlers-Danlos syndromes. Am J Med Genet C Semin Med Genet. 2017 Mar;175(1):195-211. doi: 10.1002/ajmg.c.31549. Epub 2017 Feb 21. PMID: 28220607.
   
   
4. Holder-Espinasse M, Winter RM. Type 1 Arnold-Chiari malformation and Noonan syndrome. A new diagnostic feature? Clin Dysmorphol. 2003 Oct;12(4):275. doi: 10.1097/00019605-200310000-00013. PMID: 14564218.
   
   
5. Lawrence BJ, Urbizu A, Allen PA, Loth F, Tubbs RS, Bunck AC, Kröger JR, Rocque BG, Madura C, Chen JA, Luciano MG, Ellenbogen RG, Oshinski JN, Iskandar BJ, Martin BA. Cerebellar tonsil ectopia measurement in type I Chiari malformation patients show poor inter-operator reliability. Fluids Barriers CNS. 2018 Dec 17;15(1):33. doi: 10.1186/s12987-018-0118-1. PMID: 30554565; PMCID: PMC6296028.
   
   
6. Lee J, Hida K, Seki T, Kitamura J, Iwasaki Y. Pierre-Robin syndrome associated with Chiari type I malformation. Childs Nerv Syst. 2003 Jun;19(5-6):380-3. doi: 10.1007/s00381-003-0740-2. Epub 2003 May 14. PMID: 12748800.
   
   
7. Northrup H, Volcik KA. Spina bifida and other neural tube defects. Curr Probl Pediatr. 2000 Nov-Dec;30(10):313-32. doi: 10.1067/mpp.2000.112052. PMID: 11147289.
   
   
8. Sarnat HB. Disorders of segmentation of the neural tube: Chiari malformations. Handb Clin Neurol. 2008;87:89-103. doi: 10.1016/S0072-9752(07)87006-0. PMID: 18809020.
   
   
9. Steinbok P. Clinical features of Chiari I malformations. Childs Nerv Syst. 2004 May;20(5):329-31. doi: 10.1007/s00381-003-0879-x. Epub 2004 Feb 14. PMID: 14966660.
   
   
10. Strahle J, Muraszko KM, Kapurch J, Bapuraj JR, Garton HJ, Maher CO. Chiari malformation Type I and syrinx in children undergoing magnetic resonance imaging. J Neurosurg Pediatr. 2011 Aug;8(2):205-13. doi: 10.3171/2011.5.PEDS1121. PMID: 21806364.
    
    
11. Tubbs RS, Rutledge SL, Kosentka A, Bartolucci AA, Oakes WJ. Chiari I malformation and neurofibromatosis type 1. Pediatr Neurol. 2004 Apr;30(4):278-80. doi: 10.1016/j.pediatrneurol.2003.09.013. PMID: 15087107.
    
    
12. Urbizu A, Toma C, Poca MA, Sahuquillo J, Cuenca-León E, Cormand B, Macaya A. Chiari malformation type I: a case-control association study of 58 developmental genes. PLoS One. 2013;8(2):e57241. doi: 10.1371/journal.pone.0057241. Epub 2013 Feb 21. PMID: 23437350; PMCID: PMC3578784.