Chiari I Malformation
Incidental finding noted on brain scan done for head trauma. No other associated findings are noted.
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| Descent > 10 mm. |
Chiari I malformation is the most common variant of the Chiari malformations, and it is characterised by a caudal descent of the cerebellar tonsil through the foramen magnum. It was first described in 1891 by Hans Chiari. Chiari I malformations are more frequently encountered in females.
Clinical presentation
Chiari I malformations often remain asymptomatic until adulthood. Symptoms, if present, include headache and those associated with syrinx.
The likelihood of becoming symptomatic is proportional to the degree of descent of the tonsils. All patients who have greater than 12 mm of descent were symptomatic, whereas approximately 30% of those whose descent measured between 5 and 10 mm were asymptomatic. Chiari I malformations are often isolated abnormalities; however, sometimes they are associated with cervical cord syrinx, hydrocephalus and other skeletal anomalies like basilar invagination, atlanto-occipital assimilation, sprengel deformity and klippel feil syndrome.
Differential Diagnosis:
Chiari I needs to be distinguished from:
- incidental tonsillar ectopia: <5 mm
- Chiari 1.5 Malformation - Caudal herniation of some portion of brainstem and cerebellar tonsil.
- Chiari II malformation - Myelomeningocoele and a small posterior fossa with descent of the brainstem and cerebellar tonsils.
- acquired tonsillar ectopia
- lumbar puncture
- lumboperitoneal shunt
- basilar invagination
- raised intracranial pressure
Imaging features:
The diagnosis is suspected on axial images when the medulla is embraced by the tonsils and little if any CSF is present. This is referred to as a Crowded Foramen Magnum.
The diagnosis is suspected on axial images when the medulla is embraced by the tonsils and little if any CSF is present. This is referred to as a Crowded Foramen Magnum.
Crowded Foramen Magnum
The diagnosis is made by measuring how far the tonsils protrude below the margins of the foramen magnum. The distance is measured by drawing a line from the inner margins foramen magnum (basion to opisthion), and measuring the inferior most part of the tonsils.
- above foramen magnum: normal
- <5 mm: benign tonsillar ectopia
- >5 mm: Chiari 1 malformation
In neonates, the tonsils are located just below the foramen magnum and descend further during childhood, reaching their lowest point somewhere between 5 and 15 years of age. As the individual ages further the tonsils usually ascend coming to rest at the level of the foramen magnum. A 5mm descent in a child is most likely normal.
Treatment and prognosis:
Treatment is usually reserved only for symptomatic patients or those with a syrinx. It consists of decompressing the posterior fossa, by removing part of the occipital bone, and posterior arch of C1 as well as performing a duroplasty.
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