A 21-year-old woman comes to the office due to a long history of episodic headaches, dizziness, and gait imbalance. The headaches are located in the occipital region, are characterized as dull, and last for several hours. Physical examination reveals gait ataxia but no focal weakness or sensory loss. MRI of the brain shows low-lying cerebellar tonsils extending below the plane of the foramen magnum into the vertebral canal. Which of the following is the most likely cause of this patient's condition?
Chiari malformations are a group of congenital disorders caused by underdevelopment of the posterior fossa. The small size of the fossa causes parts of the cerebellum and medulla to herniate through the foramen magnum. There are 2 types of Chiari malformations. Chiari type I is the most common and benign, and is characterized by low-lying cerebellar tonsils that extend below the foramen magnum into the vertebral canal. Patients typically present during adolescence/adulthood with paroxysmal occipital headaches (due to meningeal irritation) and cerebellar dysfunction (eg, dizziness, ataxia) due to compression of the cerebellar tonsils.
Chiari type II, Arnold-Chiari malformation, is more severe and typically becomes evident during the neonatal period. It is characterized by downward displacement of the cerebellum (vermis, tonsils) and medulla through the foramen magnum. Non-communicating hydrocephalus almost always occurs due to aqueductal stenosis. Compression of the medulla may result in dysphagia, stridor, and apnea. Patients also often have an associated lumbar myelomeningocele, which can cause lower limb paralysis.
(Choice A) Multiple sclerosis is an autoimmune demyelinating disorder of the central nervous system that typically presents in young adult women with neurologic deficits disseminated in space and time. Neuroimaging typically shows periventricular white matter lesions.
(Choice C) Friedreich ataxia is an autosomal recessive trinucleotide repeat disorder that presents during childhood with ataxia, loss of vibration/proprioception sense, and skeletal abnormalities (eg, kyphoscoliosis, pes cavus).
(Choice D) Cerebellar neoplasms (eg, medulloblastoma, pilocytic astrocytoma) often present in children with symptoms of increased intracranial pressure (eg, headache, vomiting) and cerebellar dysfunction. However, the absence of a cerebellar tumor on neuroimaging makes this unlikely.
(Choice E) Traumatic brain injury can cause headache and neurologic impairment; however, this patient's MRI findings are highly characteristic of Chiari type I malformation.
(Choice F) Cerebellar stroke may present with acute-onset occipital headache, dizziness, and ataxia. However, this patient's young age, absence of cardiovascular risk factors, and long history of episodic symptoms make this less likely.
Educational objective:
Chiari malformations are congenital disorders that result from underdevelopment of the posterior fossa, causing parts of the cerebellum and medulla to herniate through the foramen magnum. Chiari type I is relatively benign and presents during adulthood with occipital headache and cerebellar dysfunction. Chiari type II is a more severe form that affects neonates and is often associated with lumbar myelomeningocele and hydrocephalus.