In the 1890’s, the Austrian pathologist Hans Chiari described four different congenital hindbrain anomalies. These were later termed the Chiari Malformations. In recent years, two additional types have been described, but these are not clearly distinct lesions—instead they attempt to describe a continuum of the Type 1 malformation. Classically, the four anomalies are congenital in nature, however, certain intracranial conditions that result in raised intracranial pressure can cause hindbrain herniation similar to that seen in the Chiari Malformation type 1. Despite a similar radiographic appearance, these “acquired” Chiari malformations are treated differently than the classic malformations. Types 1 and 2 are the most commonly encountered of the malformations, so they will be the focus of the remainder of this manuscript.
Chiari Malformation type 1 (CM1) is the most common of the classic malformations. Radiographically, the cerebellar tonsils are ectopic, or descend through the foramen magnum at least 5mm into the cervical spinal canal. The 5mm cutoff is somewhat arbitrary, as many patients with cerebellar tonsillar descent greater than 5mm are asymptomatic.
When symptomatic, CM1 most commonly presents as headache. The headache is specifically located in the occipital or cervical region (back of the head), and is worsened by actions such as laughing, coughing, running or sneezing that transiently raise intracranial pressure.
Patients may also have motor and sensory symptoms, that make up a spinal cord syndrome. This syndrome consists of motor or sensory loss, most commonly in the hands, altered reflexes, and scoliosis. The reason for the spinal cord syndrome has to do with either direct cord compression from the malformation, or with the formation of a spinal cord syrinx, which is an expansion or dilatation of the central canal of the spinal cord. There are many theories as to how a syrinx forms, but most agree that there is some alteration of flow of cerebrospinal fluid at the cranial-cervical junction that causes fluid to be forced into the central canal of the spinal cord, leading to a dilatation or ballooning of the central canal. A syrinx can form anywhere in the spinal cord, but is most commonly seen in the cervical and thoracic cord.
Patients may also present with a cerebellar syndrome, which results in alterations in trunk or limb coordination.
Also, in very young patients, the CM1 may manifest itself as a brainstem syndrome, with respiratory irregularities, apnea, aspiration and general difficulties associated with swallowing, due to lower cranial nerve dysfunction.
Symptomatic CM1 can only be treated surgically, as there is no medication that can cause the cerebellar tonsils to assume their normal position. However, there have been case reports of spontaneous resolution of CM1 symptoms and radiological findings. In cases where treatment is indicated, the operation consists of removal of bone from the base of the skull to widen the foramen magnum. It may also be necessary to remove the posterior portion of the C1 lamina, depending on the level of tonsillar descent. There is debate among neurosurgeons regarding the necessity of opening the dura and placing a patch graft in an affort to provide even more decompression. There is also debate regarding the handling of the cerebellar tonsils, including simply removing scar tissue, coagulating and shrinking the tonsils, or physically removing them. Variations on the aggressiveness of the decompression are reported in the literature, with many groups presenting good outcomes and symptom relief.
In general, there is a reported 80-85% likelihood of resolution of symptoms—most notable occipital/cervical headaches. There has been a reported up to 90% decrease in size or resolution of symptomatic syringomyelia. Of the 10% that do not respond to the decompression, a shunt may need to be placed in the syrinx to divert flow to a different space, usually the pleural space or the peritoneal cavity.
Chiari Malformation type 2 (CM2) is always associated with open spina bifida, or myelomeningocele. Similar to CM1 there is a malformation of the cerebellum. However, in CM2 the cerebellar vermis herniates downward into the cervical spinal canal. Unlike CM1, CM2 also has associated with it brainstem anomalies, and frequently there is brainstem herniation along with the cerebellar vermis. Brainstem anomalies include tectal “beaking,” due to fusion of the superior and inferior colliculi of the midbrain, and a medullary kink that occurs inferior to the herniation of the vermis on the cervical spinal cord. There are also associated anomalies of the remainder of the brain, including interdigitating gyri (secondary to an incomplete falx cerebri) and a large massa intermedia (between the thalami). Also, unlike CM1, many patients with CM2 will also have hydrocephalus due to obstruction of flow of cerebral spinal fluid, and require placement of a permanent ventriculoperitoneal shunt.
Symptoms from the CM2 occur characteristically within age groups. Newborns are usually asymptomatic. Infants tend to present with findings consistent with brainstem compression, such as stridor (high pitched sound produced during inhalation) secondary to vocal cord paralysis, central sleep apnea, aspiration, and difficulty swallowing. Nasogastric regurgitation, in which an infant chokes on formula or milk followed by the liquid then coming out of the nose is a classic presentation of brainstem compression. Signs of brainstem compression will occur in approximately 1/3 of patients. Older children tend to present similar to those children with CM1, with occipital/cervical pain, hand weakness, poor coordination and scoliosis. Syrinx formation also occurs commonly in these children.
Similar to CM1, definitive treatments tend to be surgical. CM2 can become symptomatic secondary to hydrocephalus, so the first line of treatment in cases where hydrocephalus is present is to place a shunt. The shunt can be placed in the front or back of the head, and a tube is positioned into the ventricular system (the fluid space in the brain the produces cerebrospinal fluid), which is then connected to a valve, and then this is connected to a tube that drains into the abdominal cavity (most commonly). In certain circumstances, it is necessary to drain into either the pleural cavity (lung space) or the heart or vena cava, through the jugular vein, or another deep vein.
In a child with a functioning shunt and a symptomatic CM2, the treatment consists of performing a decompression somewhat similar to that done for CM1. However, due to large differences in anatomy between the two malformations, the decompression for CM2 is based mostly on the spinal lamina in the midline, with very little, if any removal of bone at the base of the skull. Again, there is some debate regarding opening the dura, however, most pediatric neurosurgeons do so. Once the dura is opened, it is important to locate the cerebellar vermis and distinguish it from the brainstem, as it may be possible to remove scar tissue, allowing access to the outlet of cerebrospinal fluid.
Outcome after surgery is related to age of presentation, with very young infants and neonates having poor outcomes, including death, in up to 23% of cases. Infants three months and older and young children have better outcomes, with a reported up to 77% recovery of brainstem function.
3 year old female with headaches, MRI scan of the brain shows the Chiari I malformation with cerebellar tonsils herniating downward through the foramen magnum (opening at the bottom of the skull) and into the cervical spinal canal (white arrow).
14 year old boy who presented with loss of sensation in his right arm, from the shoulder to the fingertips. MRI scan of the brain and cervical-thoracic spinal cord shows Chiari I malformation with cerebellar tonsils herniating downward to the cervical spinal canal (upper arrow). There is also a large spinal cord syrnix extending from C1-2 downward through the thoracic spinal cord (lower arrow).
14 year old boy from above figure, six months after undergoing suboccipital decompression and C1 laminectomy. Cerebellar tonsils are no longer herniating downward (upper arrow), and the spinal cord syrinx has nearly completely resolved (lower arrow).
13 year old female with myelomeningocele (spina bifida) and a Chiari II malformation. The cerebellar vermis is shown herniating through the foramen magnum into the cervical spinal canal (white arrow). Also of note is the medullary kink inferior to the cerebellar vermis (blue arrow), as well as the “beaking” of the midbrain tectum (yellow arrow).
Same 13 year old female as above with myelomeningocele and Chiar II malformation, shown here is severe scoliosis due to a spinal cord syrinx (arrow).
Written by: Matthew A. Adamo, MD., Assistant Professor, Division of Neorosurgery @ AlbanyMedical Center