In neurology, the Chiari malformation ( ; CM) is a structural defect in the cerebellum, characterized by a downward displacement of one or both cerebellar tonsils through the foramen magnum (the opening at the base of the skull).
CMs can cause headaches, difficulty swallowing, vomiting, dizziness, neck pain, unsteady gait, poor hand coordination, numbness and tingling of the hands and feet, and speech problems. Less often, people may experience ringing or buzzing in the ears, weakness, slow heart rhythm, fast heart rhythm, curvature of the spine (scoliosis) related to spinal cord impairment, abnormal breathing such as in central sleep apnea, and, in severe cases, paralysis. CM can sometimes lead to non-communicating hydrocephalus as a result of obstruction of cerebrospinal fluid (CSF) outflow. The CSF outflow is caused by phase difference in outflow and influx of blood in the vasculature of the brain.
The malformation is named after the Austrian pathologist Hans Chiari. A type II CM is also known as an Arnold–Chiari malformation after Chiari and German pathologist Julius Arnold.
Signs and symptoms Findings are due to brainstem and lower cranial nerve dysfunction. The onset of symptoms is less likely to be present during adulthood in most patients. Younger children generally have a substantially different presentation of clinical symptoms from older children. Younger children are more likely to have a more rapid neurological degeneration with profound brainstem dysfunction over several days.
Headache is the most common symptom in those with Chiari malformation type 1 (in which only the cerebellar tonsils descend below the foramen magnum). This headache is usually occipital or sub-occipital in location (but may also present in other cranial areas), is usually dull or throbbing and is characteristically associated with Valsalva maneuvers (such as bearing down, coughing, sneezing, bending over, or forcefully exhaling against a closed airway). A tension headache is also a common repetitive sign before diagnosis.
Symptoms that may be due to Chiari malformations include:
Neurogenic dysphagia: Difficulty swallowing. Cyanosis: Bluish discoloration of skin while feeding. Weak crying Facial weakness Aspiration Headaches aggravated by Valsalva maneuvers Tinnitus (ringing in the ears) Lhermitte's sign (electrical sensation that runs down the back and into the limbs) Vertigo (dizziness) Nausea Schmahmann's syndrome Nystagmus (irregular eye movements; typically, so-called "downbeat nystagmus") Facial pain Muscle weakness Impaired gag reflex Dysphagia (difficulty swallowing) Restless leg syndrome Sleep apnea Sleep disorders Impaired coordination Severe cases may develop all the symptoms and signs of a bulbar palsy Paralysis due to pressure at the Cervico-medullary junction may progress in a so-called "clockwise" fashion, affecting the right arm, then the right leg, then the left leg, and finally the left arm; or the opposite way around. Papilledema on fundoscopic exam due to increased intracranial pressure Pupillary dilation Dysautonomia: tachycardia (rapid heart), syncope (fainting), polydipsia (extreme thirst), chronic fatigue Apnea: Sudden pause of breathing, usually during sleep. Opisthotonos: Spasm of the head which causes the head to arch backwards. More common in infants than in adults. Stridor
The blockage of cerebrospinal fluid (CSF) flow may also cause a syrinx to form, eventually leading to syringomyelia. Central cord symptoms such as hand weakness, dissociated sensory loss, and, in severe cases, paralysis may occur.
Syringomyelia
Syringomyelia is most often a chronic progressive degenerative disorder characterized by a fluid-filled cyst located in the spinal cord. However, there can also be cases where the syrinx, in terms of size and extent of symptoms, actually stays stable throughout a lifetime. Syringomyelia symptoms include pain, weakness, numbness, and stiffness in the back, shoulders, arms or legs. Other symptoms include headaches, the inability to feel changes in temperature, sweating, sexual dysfunction, and loss of bowel and bladder control. It is usually seen in the cervical region but can extend into the medulla oblongata and pons, or it can reach downward into the thoracic or lumbar segments. Syringomyelia is often associated with type I Chiari malformation and is commonly seen between the C-4 and C-6 levels. The exact development of syringomyelia is unknown, but many theories suggest that the herniated tonsils in type I Chiari malformations cause a "plug" to form, which does not allow an outlet of CSF from the brain to the spinal canal. Syringomyelia is present in 25% of patients with type I Chiari malformations.
Pathophysiology
The most common pathophysiological mechanism by which Chiari type I malformations occur is due to a congenitally small posterior fossa. Other pathophysiological mechanisms involve increased intracranial pressure above the foramen magnum, which causes a downward pressure against the cerebellum, thus causing the cerebellar tonsils to displace below the foramen magnum. Such causes include hydrocephalus (an accumulation of cerebrospinal fluid [CSF] around the brain), space occupying lesions in the brain such as tumors, subdural hematomas or other subdural fluid collections, arachnoid cysts, craniosynostosis (early closure of the cranial sutures)(especially of the lambdoid suture), hyperostosis (an excessive growth of bone) (such as craniometaphyseal dysplasia, osteopetrosis). Another pathophysiological mechanism by which Chiari malformations form is by negative pressure or a pulling force from below the foramen magnum, which pulls against the brain, causing the cerebellar tonsils to herniate past the foramen magnum. Causes of this negative or pulling pressure include a tethered cord or an excessively tense Filum terminale, a cerebrospinal fluid leak creating a negative pressure around the spinal cord as the fluid surrounding the cord leaks out, or a CSF-venous fistula, in which the CSF leaks into a nearby vein.
Traumatic brain injury may cause delayed acquired Chiari malformation, but the pathophysiology of this is unknown. This is due to the fact that the condition can go asymptomatic for years and a diagnosis is often led through testing for a variety of symptoms. Additionally, ectopia may be present but asymptomatic until a whiplash injury causes it to become symptomatic. Other conditions linked to Chiari malformations include X-linked vitamin D-resistant rickets, and neurofibromatosis type I.
Diagnosis Diagnosis is made through a combination of patient history, neurological examination, and medical imaging. Magnetic resonance imaging (MRI) is considered the preferred imaging modality for Chiari malformation. The MRI visualizes neural tissue such as the cerebellar tonsils and spinal cord as well as bone and other soft tissues. CT and CT myelography are other options and were used prior to the advent of MRI, unfortunately the resolution of CT based modalities do not characterize syringomyelia and other neural abnormalities as well.
By convention, the cerebellar tonsil position is measured relative to the basion-opisthion line, using sagittal T1 MRI images or sagittal CT images. The selected cutoff distance for abnormal tonsil position is somewhat arbitrary, as not every person will be symptomatic at a certain amount of tonsil displacement, and the probability of symptoms and syrinx increases with greater displacement; however, greater than 5 mm is the most frequently cited cutoff number, though some consider 3–5 mm to be "borderline"; pathological signs and syrinx may occur beyond that distance. One study showed little difference in cerebellar tonsil position between standard recumbent MRI and upright MRI for patients without a history of whiplash injury. Neuroradiological investigation is used to rule out any intracranial condition that could be responsible for tonsillar herniation first. Neuroradiological diagnostics evaluate the severity of crowding of the neural structures within the posterior cranial fossa and their pressure against the foramen magnum. Chiari 1.5 is a term used when both brainstem and tonsillar herniation through the foramen magnum are present.
The diagnosis of a Chiari II malformation can be made prenatally, through ultrasound.
Classification In the late 19th century, Austrian pathologist Hans Chiari described seemingly related anomalies of the hindbrain, the so-called Chiari malformations I, II and III. Later, other investigators added a fourth (Chiari IV) malformation. The scale of severity is rated I–IV, with IV being the most severe. Types III and IV are very rare. Since Dr. Chiari's original descriptions Chiari 0, 1.5, 3.5, and 5 have been described in the medical literature.
Types of Chiari malformation Type Presentation Clinical Features0Syringohydromyelia in the absence of cerebellar tonsillar herniation.Back pain, leg pain, hand numbness/weakness IHerniation of cerebellar tonsils. Tonsillar ectopia below the foramen magnum, with greater than 5 mm below as the most commonly cited cutoff value for abnormal position (although this is considered somewhat controversial). Syringomyelia of cervical or cervicothoracic spinal cord can be seen. Sometimes, the medullary kink and brainstem elongation can be seen. It can be congenital or acquired through trauma. When congenital, it may be asymptomatic during childhood, but often manifests with headaches and cerebellar symptoms. Syndrome of occipitoatlantoaxial hypermobility is an acquired Chiari I malformation in patients with hereditary disorders of connective tissue. Patients who exhibit extreme joint hypermobility and connective tissue weakness as a result of Ehlers–Danlos syndrome or Marfan syndrome are susceptible to instabilities of the craniocervical junction; thus they are at risk for acquiring a Chiari malformation. Headache, neck pain, unsteady gait usually during childhood1.5In addition to tonsillar ectopia, patients with this entity also have caudal descent of the brainstem. A significant proportion of these patients require a second surgery as a result of persistent syringomyelia.Headache and neck pain, similar to Chiari I IIThis is the only type, also known as an "Arnold–Chiari" malformation. As opposed to the less-pronounced tonsillar herniation seen with Chiari I, there is a larger cerebellar vermian displacement. Low-lying torcular herophili (confluence of sinuses), tectal beaking, and hydrocephalus with consequent clival hypoplasia are classic anatomic associations. Usually accompanied by a lumbar or lumbosacral myelomeningocele, with tonsillar herniation below the foramen magnum. Colpocephaly may be seen due to the associated neural tube defect.Paralysis below the spinal bifida defect III Associated with an occipital encephalocele containing a variety of abnormal neuroectodermal tissues as well as possible herniation of elements of cerebellum, brainstem, and occipital lobe. Syringomyelia, tethered cord, and hydrocephalus may also be seen.Abundant neurological deficits3.5In 1894, Giuseppe Muscatello described what is believed to be the only case of an occipitocervical encephalocele with a communication to the stomach.Not compatible with life. IV Characterized by a lack of cerebellar development, in which the cerebellum and brainstem lie within the posterior fossa with no relation to the foramen magnum. Equivalent to primary cerebellar agenesis.Not compatible with lifeVCharacterized by the absence of cerebellar development and occipital lobe ectopia into the foramen magnum. This rare entity has two described cases in the medical literature, both associated with myelomeningocele. This has sparked controversy, given the proposed mechanism of this syndrome. Experts in neurosurgery disagree if this is truly a separate entity or just a part of the spectrum of Chiari 2 malformations.