A 16-year-old girl with a noncontributory medical history presented with a 10-month history of unremitting headaches, which interfered with her ability to attend school. She describes them as constant throughout the day; sleep was the only time she had relief. They are located at the left temporal and occipital area; they are moderate to severe in intensity and tension type in character. She denies any numbness/tingling, snoring, and swallowing difficulties. Her neurological examination is normal. A pediatric neurologist recommended a brain MRI for the workup of the headaches. The MRI (Fig. 21.2) showed that the cerebellar tonsils extend below the foramen magnum by approximately 9 mm. There was narrowing of the subarachnoid space at the craniovertebral junction. On phase-contrast flow studies, there is minimal flow in the dorsal subarachnoid space at the craniovertebral junction.

A dural sparing suboccipital decompression was performed with an uncomplicated postoperative course. At the 3-month follow-up visit, the headaches had almost completely resolved, and the patient was excited with the result. A post-op MRI (Fig. 21.2) was obtained at that time that showed resolution of the tonsillar compression with reconstitution of the subarachnoid space at this region. Phase-contrast studies showed an improved CSF flow at the dorsal subarachnoid space of the craniovertebral junction.

The definition of Chiari malformation encompasses a series of posterior fossa malformations that were first described by the Austrian Professor Hans Chiari. Recently, some new variants have been added, and the increased availability of the MRI has increased the rate of diagnosis of these malformations. The pathophysiology of the symptom production is complex and probably multifactorial. Headache and neck pain are not uncommon symptoms in a general medical pediatric practice. Therefore, as Chiari malformations are not rare, it is likely that symptomatic cases might be underdiagnosed, while at the same time many children may be receiving treatment for what may be presumed incorrectly to be a symptomatic radiographic finding. The prevalence of headache and Chiari and the unknown natural history of the asymptomatic malformation highlight the significance of the primary care physician’s role. The role of the primary care physician is important in identifying cases that might be symptomatic and refer for a neurosurgical evaluation prior to initiating medical treatment. The following chapter aims to present in a comprehensive yet concise manner the disorder and to acquaint the pediatrician with the diagnosis of Chiari and its surgical management.

Hans Chiari provided us with a comprehensive description of hindbrain malformations in 1891. Cruveilhier, Arnold, and Cleland should also be credited for the recognition of the hindbrain anomalies [1]. The original description by Hans Chiari [2] included the following four types of malformations:

Chiari 1 (Fig. 21.3): The cerebellar tonsils herniate below the level of the foramen magnum. An arbitrary limit of 5 mm has been set in order to differentiate between Chiari malformation and tonsillar ectopia (<5 mm). Emphasis has traditionally been placed on the degree of tonsillar herniation; however, in clinical practice, the actual caudal displacement of the tonsils often does not correlate with the presence of symptoms. Hydrocephalus is rare, but syringomyelia is often present. Syringomyelia is the presence of a fluid-filled cavity inside the spinal cord.

Chiari II (Fig. 21.4): This type is characterized by protrusion of the cerebellar vermis, medulla, and fourth ventricle through the foramen magnum into the cervical spinal canal and is associated with supratentorial anomalies [3] and almost always a myelomeningocele. It is usually diagnosed in the prenatal investigation of a fetus with a neural tube defect. In this type, it is not unusual for the cerebellar tonsils to reach the level of fifth cervical vertebra. Some rare cases of Chiari II without spinal dysraphism have been reported [4]. Although the Chiari II malformation is associated with other pathologies such as the spina bifida and hydrocephalus, the leading cause of mortality in young patients with this malformation constellation is the Chiari II malformation [5].

Chiari 3 : This is a very rare form that can be considered as an encephalocele with associated cervical spina bifida and prolapse of the cerebellum into the spinal canal.

Chiari 4 : This malformation is essentially a cerebellar hypoplasia/aplasia, and apart from being a hindbrain malformation, it doesn’t appear to share any other characteristics with the other types of Chiari malformation.

Chiari types 3 and 4 are very rare disorders, which are diagnosed usually in the prenatal period or in neonates and are treated in specialized centers. These disorders are outside the scope of this manuscript and will not be analyzed. More recently, some new types of Chiari malformations have been introduced in the literature.

Chiari 0 : This type of malformation involves minimal or null tonsillar herniation, but it is characterized by a crowding of the neural structures at the foramen magnum. This crowding obliterates the normal subarachnoid spaces in the region and effectively blocks CSF flow. This type is uncommon, accounting for 3.7 % of pediatric Chiari patients, and is often associated with intraoperative findings of arachnoid veils and adhesions at the foramen magnum [6]. Consequently, the symptomatology of blocked CSF pulsation might develop as well as syringomyelia.

Chiari 1 .5 (Fig. 21.5): This is a recently introduced type of Chiari and is essentially a Chiari 1 with an added caudal displacement of the obex, below the level of the foramen magnum. The obex is the caudalmost point of the fourth ventricle that can be recognized in the MRI. This caudal displacement of the obex denotes a caudal displacement of the brainstem. It is not clear whether this patient population has any differences compared with the typical Chiari 1 cases . There is ongoing research trying to delineate whether these patients present with a different constellation of symptoms or need an alternative management plan.

Chiari 5 : This is the most recently proposed type of hindbrain malformation with absence of the cerebellum and herniation of the occipital lobes through the foramen magnum [7].

The blockage of CSF flow is considered an important pathophysiological mechanism in the Chiari malformations. In our experience, the blockage of CSF flow can be caused by arachnoid septations in the subarachnoid space, perpendicular to the long axis of the neuraxis and the CSF flow direction. These septations can block the CSF flow and cause Chiari-like symptoms. These septations can also be seen along with typical for Chiari tonsillar herniations and might be the cause for symptom recurrence following certain decompressive surgeries in which only the bone is removed. We feel that special attention should be placed in this condition, given that it can be diagnosed by newer neuroimaging techniques. Treating this completely requires a more extensive surgery that includes intra-arachnoidal exploration.

It is important to note that the various Chiari types are essentially a classification scheme of hindbrain malformations with the most common (but not always present) feature being the hindbrain herniation. It is not a grading scheme of a common disorder with different degrees of severity, and as Tubbs et al. stated “no single theory could explain all forms of the Chiari malformation, and that this malformation might be a heterogeneous entity” [8].

The pathogenesis of Chiari 1 is probably multifactorial. The causative factor might be a small posterior fossa, a pressure gradient at the foramen magnum due to spinal hypotension or intracranial hypertension, the traction of spinal cord by tethering [9], cerebellar dysgenesis [10], secondary to space-occupying intracranial lesion, or occipitoatlantoaxial joint instability and cranial settling [11]. While the variety of pathology is extensive, for the purposes of this review chapter, we will focus on the most common types of presentations without delving into the minutiae of subtle variants.

Regardless of the causative mechanism of tonsillar herniation, the main theories of symptom production focus on the traction/compression of neural structures/blood vessels/pain-sensitive structures and on the obstruction of normal CSF pulsation through the foramen of Monro. The cranial cavity is nonelastic. During the systolic phase of heart cycle, the inflow of blood causes an increase of the intracranial blood volume because the venous outflow cannot match the rapid arterial inflow. The contents of the cranial cavity are the brain, the blood, and the CSF (Monro-Kellie doctrine). Since the brain cannot move out of the cranial cavity, it is the CSF that exits the cranial cavity through the foramen magnum to counteract the increase of blood volume. In the diastolic phase, the whole process is reversed. In the Chiari malformation, the flow of the CSF is blocked at the foramen magnum due to the blockage of the subarachnoid space by the herniating cerebellar tonsils . This blockage probably creates an instantaneous cranial hypertension during the systolic phase that is at least partially responsible for the symptoms. Exertion and other conditions that cause an increase in the heart rate may reproduce and worsen symptoms [12]. Valsalva maneuvers worsen symptoms because the spinal subarachnoid pressure is increased, causing further reduction of an already diminished CSF flow through the foramen magnum.

Chiari is probably caused by a hypoplastic occipital bone that leads to posterior fossa crowding of neural structures and subsequent herniation of the relatively mobile cerebellar tonsils through the opening of the foramen magnum [13]. The blockage of CSF flow might also be involved in the pathophysiology of syringomyelia formation, as it leads to a pressure gradient between the spinal cord and the subarachnoid space [14].

Controversy exists on the contribution of congenital and acquired factors for Chiari 1 development. The normalization of hindbrain anatomic anomalies of Chiari 1 malformation following decompressive surgery suggest that even if there is a congenital predisposition, symptoms may be reversible, but there is also likely an acquired pathophysiological component [15]. On the other hand, an underlying genetic basis is suggested by the familial cases [16–18]. Recently, susceptibility to Chiari 1 malformation was attributed to specific genes involved in somitogenesis and fetal vascular development [19].

The prevalence of asymptomatic Chiari I in the general pediatric population is unknown. A study in a general pediatric population who had MR imaging for symptoms related either to the Chiari I or to other diagnoses revealed a 1 % prevalence of Chiari malformation with tonsillar ectopia of 5 or more mm [20]. A borderline tonsillar ectopia was revealed in further 0.4 % of the patients, 74 % of whom were symptomatic. In a mixed pediatric and adult patient population, tonsillar herniation greater than 5 mm was identified in 0.77 % of patients [21]. Familial cases of pediatric Chiari 1 can account for 3 % of cases [22].

The gold standard examination is the MRI, due to the ability to obtain sagittal and coronal views and higher resolution of neural structures. The hallmark of the Chiari malformation (types 1, 1.5, and 2) is the caudal displacement of the cerebellar tonsils in relation to the foramen magnum. A line is drawn from the anteriormost point of foramen magnum (basion) to the posteriormost point (opisthion), and the maximum herniation of the cerebellar tonsils is measured from this line. The basion-opisthion (or McRae) line and the line that measures the maximum tonsillar herniation should be perpendicular. MRI does not depict bony anatomy as clearly as CT, and the accurate identification of the cortical bone margins is sometimes challenging . Tonsillar herniation greater than 5 mm has historically been used as the cutoff point for Chiari malformation from a neuroradiologic perspective. It is important to note that tonsils ascend with age, so in the first decade of life, over 6 mm (rather than 5 mm) of displacement is usually considered pathologic [64]. Pediatric Chiari 1 can be associated with hydrocephalus, focal cerebral heterotopia with epilepsy, or partial agenesis of the corpus callosum [65]. Less obvious herniation may be exceedingly symptomatic whereas more profound herniation may present asymptomatically; thus, strict adherence to these measurements alone as surgical criteria is not advocated.

Chiari malformations are often associated with presyrinx or syringomyelia (Fig. 21.6)—with the distinction between the two entities being the presence of frank spinal cord cavitation in the latter [66] or hydromyelia (distention of the central canal). If a syrinx or typical Chiari symptoms are present in the absence of tonsillar herniation, then consideration should be given to obtaining CINE MRI or MRI in the upright position. The former might reveal arachnoid velum that blocks CSF flow, while the latter might reveal tonsillar herniation that occurs only in the upright position [67]. In the case of an asymmetric tonsillar herniation, an eccentric syrinx is likely to be deviated to the side of the greater tonsillar herniation and is common to have scoliosis with the convex side similar to the side of the syrinx and the greater tonsillar herniation [68].

Chiari malformation might be associated with hereditary or spontaneous connective tissue disorders most commonly seen in association with Ehlers-Danlos syndrome. Retro-odontoid pannus can suggest ligamentous laxity and craniocervical hypermobility [69]. Basilar invagination (BI) can be associated with Chiari (Fig. 21.7), and the combination of instability and BI leads to a diagnosis of complex Chiari syndrome requiring even more specialized evaluation and management, usually surgical.

The most common neuroimaging study in the neonates is the sonogram through the acoustic window of the anterior fontanel, a technique that is not optimal for depicting anatomic relationships in the posterior fossa and craniocervical junction. Chiari II malformation can be revealed or compared with the prenatal neuroimaging studies either with brain MRI or a sonogram through the acoustic window of the foramen magnum [70].