Chapter 119 Chiari Malformations and Syringomyelia
History
John Cleland (1835–1925) was among the first to describe hindbrain herniation in a patient with myelodysplasia.1–3 In 1883, Cleland published “Contribution to the study of spina bifida, encephalocele and anencephalus,” which included an illustration and a description of patients with hindbrain herniation, in the Journal of Anatomy and Physiology. Julius Arnold (1835–1915) also discussed a patient with hindbrain herniation and myelodysplasia in 1894.1,4–7 In 1891 and 1896, Hans Chiari (1851–1916) provided descriptions of hindbrain herniation in postmortem specimens in which variations I through IV were described.5,8 Because Chiari performed the majority of the early work describing the condition, his name remains associated with the condition today.
The surgical treatment of Chiari malformations began with the technique described by Penfield and Coburn, who performed a posterior fossa decompression. In the case they described, the patient died, and subsequent early surgeries had a high rate of complications and death.9 Gardner et al.10–15 noted five deaths among 74 patients undergoing posterior fossa decompression. Williams16–18 reported 5 deaths in 41 cases and also described increased neurologic deficits, increased ventricular size, and arachnoiditis in his patients. Despite these inauspicious beginnings, modern surgical techniques make Chiari surgery much safer, with the chance of a serious, irreversible injury less than 2%.
Pathophysiology
The various types of Chiari malformations are difficult to explain with a unifying theory that accounts for all the cerebral and spinal anomalies and addresses the occurrence of hindbrain herniation. Instead, the various Chiari types likely result from different causative factors but share similar radiographic and symptomatic expression. It remains unclear whether defects are a result of embryonic missteps or of other pathologic processes. While clear associations can be made with Chiari II, myelomeningocele, and folate deficiency, few other clear associations can be garnered. Few studies have demonstrated a genetic predisposition to the condition.19
Although causation remains unclear, mechanistic theories abound to describe Chiari I and II malformations and the common finding of syringomyelia. Gardner11,12 posited the hydrodynamic theory, which attributes the development of hydromyelia to a “water hammer” effect caused by blockage of the foramen of Magendie whereby cerebrospinal fluid (CSF) transits the potential space of the central canal in the spinal cord, causing slow progressive dilation. Oldfield20,21 suggested that the downward pistoning of the cerebellar tonsils seen on cine-mode MRI (cine-MRI) forces CSF via perivascular and interstitial spaces into the spinal cord, resulting in a syrinx. Regarding Chiari II, McLone et al.22–28 posited the unified theory that CSF loss at the myelomeningocele site reduces the volume of CSF needed to distend the developing ventricular system, leading to caudal displacement of hindbrain structures. Because Chiari types I through IV represent a spectrum of conditions with various causations, research continues in an effort to understand the exact pathophysiologic processes that lead to these malformations and associated symptoms.
Signs, Symptoms, and Imaging: Pathologic Features
Chiari I
Chiari I malformation involves caudal herniation of the cerebellar tonsils more than 5 mm below the level of the foramen magnum without brainstem descent or hydrocephalus (Table 119-1) and has been associated with numerous conditions (Box 119-1). Patients typically present with nondermatomal pain in the occipital or cervical region that is exacerbated by the Valsalva maneuver.29 Pain in younger children may manifest as irritability, crying, or failure to thrive. Sleep apnea is another common finding in younger patients. Various other symptoms are described in association with Chiari I malformation, including motor and sensory alterations, clumsiness, dysphagia, dysarthria, ataxia, and incontinence (Box 119-2). Signs on examination can include nystagmus, hyperreflexia of lower extremities, diminished upper extremity reflexes, cerebellar signs, and lower cranial nerve dysfunction, including dysarthria, palatal weakness, and decreased gag reflex. Scoliosis also can be seen in some patients, especially in the setting of an underlying spinal cord syrinx.
| Type | Description |
|---|---|
| Chiari I | >5 mm tonsillar herniation below McRae line |
| No hindbrain abnormalities | |
| Syringomyelia possible | |
| Chiari II | Herniation of brainstem, cerebellar vermis, fourth ventricle through foramen magnum |
| Associated with myelomeningocele | |
| Hydrocephalus common | |
| Syringomyelia common | |
| Chiari III | Foramen magnum/high cervical encephalocele |
| Chiari IV | Cerebellar hypoplasia or aplasia |
| Chiari 1.5 | Low brainstem and fourth ventricle |
| Caudal displacement of cerebellar tonsils | |
| No associated myelomeningocele | |
| Chiari 0 | Crowded posterior fossa without hindbrain herniation |
| Syringomyelia |
Multiple findings are demonstrated on imaging and at autopsy in patients with Chiari I malformation (Fig. 119-1). Abnormalities of the skull base and craniocervical junction, including a small posterior fossa, empty sella, platybasia, basilar impression, Klippel-Feil syndrome, and atlantoaxial assimilation, are seen in approximately 50% of patients. MRI is used to demonstrate cerebellar tonsils below the level of the foramen magnum, and cine-MRI routinely may show decreased flow posteriorly at the craniocervical junction.19,30 Imaging also may demonstrate scoliosis with a leftward convexity, in contrast to the right convexity curve usually seen in idiopathic scoliosis. The fourth ventricle can be elongated, and hydrocephalus is present in 5% to 10% of cases. The cerebellar tentorium is elevated, but other brain abnormalities common in Chiari II malformation often are absent. A spinal cord syrinx is a common feature, occurring in 50% to 75%, or even more, of patients. Syrinx formation usually is seen in the lower cervical and upper thoracic cord; however, this may vary, and holochord syrinx is possible. Volumetric analysis has demonstrated reduced posterior fossa volumes and upward of 40% CSF volumes with normal brain volume.31–33
Chiari II
Chiari II malformation is characterized by caudal herniation of the cerebellar vermis, the brainstem, and the fourth ventricle in the setting of myelomeningocele. Hydrocephalus is common in patients with this condition, along with multiple skeletal and intracranial abnormalities (Box 119-3). After closure of the myelomeningocele and the shunting that usually is necessary, patients may display symptoms of irritability or apnea as the first sign of a Chiari II malformation. Aspiration can lead to recurrent pneumonia, and problems with dysphagia and dysarthria may be evident. Hindbrain anomalies, usually secondary to respiratory insufficiency, are the leading cause of death in myelodysplastic patients. Findings on physical examination include down-beating nystagmus, quadriparesis with hypotonia, ataxia, ocular motility defects, diminished gag reflex, and stridor (see Box 119-2). Symptom onset in older children usually indicates spinal cord tethering, shunt malfunction, or the development of spinal cord syrinx.
Among the imaging and autopsy findings in patients with Chiari II malformations, the brain may show complete or partial agenesis of the corpus callosum and septum pellucidum, prominent anterior commissure, polygyria, interdigitation of the occipital and parietal lobes, partial or complete agenesis of the olfactory bulb/tracts, enlargement of the massa intermedia, or fusion of the colliculi (i.e., tectal beaking). In addition, the cranial nerve nuclei can be malformed. The cerebellum is reduced in size, there can be dysplasia with absent folia, the medulla can be elongated and flattened with the classic medullary kink, and the cranial and upper cervical nerves can course upward. Luckenschadel, a beaten copper or fenestrated appearance on imaging, can characterize the skull. Scalloping of the petrous bone can occur. In these patients, the posterior fossa is small, and basilar impression/assimilation can be seen. Spine anomalies can include Klippel-Feil deformities and enlargement of the cervical canal. Hydrocephalus is a common feature, seen in upward of 90% of patients with Chiari II malformation, and the ventricles may demonstrate colpocephaly. The tentorium usually is low-lying, creating a more vertical straight sinus and low-lying torcula. In addition to the myelodysplasia, associated spinal cord abnormalities include split cord malformations and syrinx formation (Fig. 119-2).34,35
Chiari 1.5
Patients with Chiari 1.5 malformation have a condition that falls somewhere between Chiari I and Chiari II. Chiari 1.5 malformation is characterized by Chiari I–type tonsillar herniation but with elongation of the brainstem and fourth ventricle.36
Chiari 0
Conversely, Chiari 0 malformation is characterized by syrinx formation without hindbrain herniation. It is thought that dysregulation of CSF equilibrium at the craniocervical junction contributes to the Chiari 0 malformation, and that operative findings such as arachnoid veils and adhesions account for obstruction at the foramen of Magendie or crowding at the foramen magnum.37
