Chiari Malformations

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CHAPTER 179 Chiari Malformations

R. Shane Tubb, Jeffrey A. Pugh, W. Jerry Oakes

The Chiari malformations are a collection of hindbrain abnormalities ranging from simple herniation of the cerebellar tonsils through the foramen magnum to complete agenesis of the cerebellum. There is likely no unifying pathophysiology underlying these findings, which are becoming more frequently recognized with the increased use of magnetic resonance imaging (MRI). That said, our ability to treat patients with symptomatic Chiari malformations focuses on restoring normal cerebrospinal fluid (CSF) dynamics across the craniocervical junction. There is great variability in the clinical presentation, imaging findings, and technical aspects of decompression. As such, careful patient selection is perhaps most important to achieve successful outcomes for this population.

History

In the early 1890s, Dr. Hans Chiari, professor of pathologic anatomy at the German University in Prague, used autopsy specimens to describe four congenital anomalies later termed the Chiari malformations (types I to IV) (Table 179-1). Chiari was not the first to observe and report the type II malformations. In Observationes Medicae, written by the Dutch physician and anatomist Nicholas Tulp (1593-1674), reference is made to hindbrain herniation in a myelodysplastic individual.¹ John Cleland (1835-1925) of Scotland reported a single myelodysplastic patient with hindbrain herniation and hydrocephalus in 1883. Temporally, Julius Arnold (1835-1915), professor of anatomy at Heidelberg, described a single myelodysplastic patient with hindbrain herniation and no hydrocephalus. Although the term Arnold-Chiari malformation has been used specifically in reference to hindbrain herniation in myelodysplastic patients, it was Chiari who described and attempted to delineate the pathophysiology of these posterior fossa abnormalities. As such, it is most appropriate to refer to this abnormality as the Chiari II malformation.²

TABLE 179-1 The Chiari Malformations

CHIARI TYPE	FEATURES
I	Tonsillar herniation >5 mm inferior to the plane of the foramen magnum (basion-opisthion line) No associated brainstem herniation or supratentorial anomalies Hydrocephalus uncommon Syringomyelia common

Herniation of the cerebellar vermis, brainstem, and fourth ventricle through the foramen magnum

Associated with myelomeningocele and multiple brain anomalies

Hydrocephalus and syringomyelia very common

III High cervical or occipital encephalocele containing herniated cerebellar and brainstem tissue IV Hypoplasia or aplasia of the cerebellum and tentorium cerebelli

Terminology

Chiari I Malformation

The Chiari I malformation (CIM) consists of caudal displacement of the cerebellar tonsils into the upper cervical spinal canal (Fig. 179-1). The most common associated findings are cervical syringomyelia and on occasion hydrocephalus. Multiple associations have been cited in the medical literature regarding this malformation. The full foramen magnum potentially compresses the herniated cerebellar tissue and restricts normal CSF flow across the craniocervical junction.

FIGURE 179-1 Cadaveric image (posterior view) of a Chiari I malformation.

Chiari II Malformation

The Chiari II malformation (CIIM) is seen almost exclusively in the setting of myelodysplasia and hydrocephalus. In type II malformations, the structures herniating through the foramen magnum include the cerebellar vermis, brainstem, and fourth ventricle (Fig. 179-2). In addition to these neural structures, the accompanying choroid plexus and the associated basilar artery and posterior inferior cerebellar arteries may also be caudally displaced. The posterior fossa is often small and the foramen magnum expanded, and syringomyelia is seen in many of these patients.

FIGURE 179-2 Sagittal magnetic resonance image demonstrating the Chiari II malformation.

Chiari III Malformation

The Chiari III malformation is the rarest of the Chiari malformations, with herniation of cerebellum and brainstem into a posterior encephalocele. This is the most severe form of hindbrain herniation, and its management is often problematic from both a technical and an ethical point of view. Lesions that prominently involve the posterior fossa contents must be distinguished from high cervical myelomeningoceles, which may look the same superficially but carry a more favorable prognosis.³^,⁴ Patients with a Chiari III malformation generally have a poor prognosis. Severe neurological, developmental, and cranial nerve defects, in conjunction with seizures and respiratory insufficiency, are common. Surgical planning follows the same basic principles of encephalocele closure.

Chiari IV Malformation

Chiari expanded his initial classification in a follow-up study published in 1896, in which he added a fourth subgroup consisting of cerebellar hypoplasia or aplasia. In these patients, the posterior fossa is relatively normal in size, and there is an absence of any hindbrain herniation. Although this was included in Chiari’s classification of rhombencephalic malformations, it is more appropriate to include this in the category of posterior fossa cysts.

Chiari 0 Malformation

The Chiari 0 malformation is defined as syringomyelia without tonsillar herniation that responds to posterior fossa decompression. Iskandar and associates ⁵ identified five patients with syringomyelia and no evidence of tonsillar herniation. MRI of the entire neuraxis ruled out other causes of a syrinx. Ultimately, abnormal CSF flow at the posterior fossa or foramen magnum was the suspected cause. All five patients underwent a posterior fossa decompression and duraplasty without direct fenestration or management of the syrinx. Significant syrinx and symptom resolution was observed in all patients. Their response to surgery suggests that “Chiari-like” pathophysiology may be present in the absence of tonsillar herniation, which may possibly be intermittent. This contention is supported by the identification of a crowded foramen magnum in two patients, multiple arachnoid adhesions in two others, and a fourth ventricular arachnoid veil in one. Each of these findings can alter CSF flow, although at present the pathophysiology resulting in syrinx formation in this group of patients is poorly understood. As such, a thorough evaluation to exclude other causes of syrinx is necessary before consideration of posterior fossa decompression.

Chiari 1.5 Malformation

Implying that only cerebellar tonsillar tissue is herniated through the foramen magnum, the Chiari I malformation (CIM) does not adequately define varieties of hindbrain herniation that also include descent of the brainstem. From our series of 130 surgical pediatric patients with CIM, we found that 17% had an additional component of brainstem descent.⁶^,⁷

Because most cases of hindbrain herniation that come to medical attention are Chiari types I and II, the remainder of this chapter focuses on the presentation and management of these patients.

Signs and Symptoms

Chiari I Malformation

Patients with CIM may present with a variety of symptoms and signs ranging from headache to severe myelopathy and brainstem compromise (Table 179-2). The most common presenting symptom is pain (60% to 70%),⁸^–¹⁰ usually occipital and upper cervical in location, and often induced by Valsalva maneuvers such as laughing, sneezing, and coughing. In infants and children who are unable to communicate verbally, headaches may manifest simply as crying and irritability. Other common symptoms include weakness or numbness, loss of temperature sensation, and unsteadiness. Careful investigation reveals that more than 70% to 80% of patients have some type of ophthalmologic or otologic disturbance at diagnosis. Ophthalmologic symptoms include blurry vision, nystagmus, extraocular muscle palsies, diplopia, and visual field deficits.¹¹^–¹⁴ Otologic complaints consist of tinnitus, fluctuating hearing loss, vertigo, and nausea. Signs at presentation have included weakness, atrophy, hyperreflexia, cape-like sensory loss, ataxia, and lower cranial nerve dysfunction. By stretching the centrally located ventral white commissure through a syrinx, pain and temperature signals cannot cross to the contralateral spinothalamic tract. The loss of pain and temperature sensation occurs only at the levels that are served by stretched spinothalamic fibers. Down-beat nystagmus is reported to be specific for lesions involving the cervicomedullary junction. Abnormal abdominal reflexes are seen in patients with associated syringomyelia. Children younger than 3 years are more likely to present with lower cranial nerve dysfunction.¹⁵ This can manifest as poor feeding, failure to thrive, recurrent aspiration pneumonia, dysphagia, choking, or stridor. A diminished gag reflex is common. Vocal cord paralysis with stridor or hoarseness may be present. An inability to maintain airway patency with lower cranial nerve dysfunction may promote sleep apnea, which may be a cause for sudden death in this group.

TABLE 179-2 Clinical Presentation of the Chiari I Malformation

SYMPTOMS

Occipitocervical headaches, dysesthesia

Nonradicular pain in the back, shoulders, and limbs

Motor and sensory symptoms

Clumsiness

Dysphagia

Dysarthria

CEREBELLAR SYNDROME Truncal and appendicular ataxia BRAINSTEM SYNDROME

Respiratory irregularities

Nystagmus

Lower cranial nerve dysfunction, including otologic disturbances

Trigeminal or glossopharyngeal neuralgia

SPINAL CORD SYNDROME

Motor and sensory losses, especially in the hands

OTHER SIGNS

Spinal cord dysfunction is the result of direct cord compression or syringomyelia. The incidence of syringomyelia varies between 30% and 70% in CIM. Syrinx location is typically cervical, followed by cervicothoracic. Left untreated, permanent spinal cord damage can result. Progressive scoliosis is a relatively common manifestation (30%) of CIMs when there is coexistent syringomyelia.¹⁰^,¹⁶ Clinical and radiographic signs that raise the suspicion of an underlying neurological defect in a patient with scoliosis include a convexity to the left, leg or foot asymmetry, male gender or prepubertal female, and obviously, any neurological deficit.^3,^4,¹⁶^–¹⁹ The mechanism by which a syrinx causes scoliosis is not fully understood.

Chiari II Malformations

The CIIM occurs in most (>95%) patients with myelomeningocele and is the leading cause of death in treated myelodysplastic patients today. About one third of these patients develop brainstem symptoms by 5 years of age, and in excess of one third of those die, usually of respiratory failure (Table 179-3). In fact, as many as 20% of patients with symptomatic CIIMs may present as a neurological emergency. When presenting acutely, there is dysfunction of the 9th and 10th cranial nerves, affecting respiration, swallowing, and vocal cord functions; this is often accompanied by stridor, opisthotonos, and nystagmus. Symptomatic deterioration with progressive brainstem dysfunction may be irreversible and lead to death, irrespective of the type of therapy given or the speed with which treatment is instituted.²⁰^–²³ This potentially catastrophic syndrome occurs most frequently in infants younger than 2 years, particularly younger than 3 months, and has been shown to be unrelated to intracranial pressure and the size or extent of symptoms of the neural tube defect.²¹ Patients with symptoms who survive the high-risk period (2 to 3 months of age) may improve and become clinically stable. Especially difficult to treat are neonates who fail to initiate adequate ventilation from birth. Although suspected of having inadequate respiratory drive centers and therefore little or no potential of sustained independent ventilation, their management is associated with a poor outcome and difficult ethical decisions.

TABLE 179-3 Clinical Presentation of the Chiari II Malformation

NEWBORNS

(Usually asymptomatic)

INFANTS

(Signs of brainstem compression)

Stridor secondary to vocal cord paralysis

Central and obstructive apnea

Aspiration secondary to dysphagia with potential pneumonia

Failure to thrive secondary to dysphagia

Breath-holding spells with possible loss of consciousness

Hypotonia and quadriparesis

Irritability

OLDER CHILDREN AND YOUNG ADULTS

(Spinal, cerebellar, and ophthalmologic signs)

Occipitocervical pain

Hand weakness and loss of muscle bulk

Defects of pursuit movements and convergence

Defects of optokinetic movements

Scoliosis

Dysarthria

NEUROLOGICAL EMERGENCY

(Usually <2 years of age, commonly by 3 months of age)

Progressive neck pain

Progressive brainstem dysfunction

Unlike patients with CIM, there is a strong relationship between the type of symptoms and the age of onset. Newborns usually have symptoms. Older children and young adults most commonly display symptoms and signs of spinal cord and cerebellar dysfunction.²^–⁴ ²⁴ A multitude of other symptoms and signs may occur in older patients. Common among these are ophthalmologic findings, which include strabismus, horizontal nystagmus (especially when looking upward), abnormalities of pursuit movements and convergence, and defects of optokinetic movements.²⁵

Diagnostic Studies

Computed Tomography and Magnetic Resonance Imaging

Chiari I Malformations

The diagnosis of CIM should include the absence of an intracranial mass lesion, Dandy-Walker malformation, or hydrocephalus, all of which may cause tonsillar displacement secondary to raised intracranial pressure.²⁶ The true incidence of CIM is not known. However, Meadows and colleagues²⁷ found that of 22,591 patients who underwent MRI of the head, 175 (0.775%) were found to have tonsillar herniation extending more than 5 mm below the foramen magnum.

The position of the cerebellar tonsils in the normal population was studied by Aboulezz and colleagues.²⁸ In a review of 800 MRI examinations, the authors noted that normal or asymptomatic patients may have tonsils that extend 3 mm below the foramen magnum. The tonsillar herniation was noted to be clearly pathologic when it exceeded 5 mm and borderline between 3 and 5 mm. Similarly, Barkovich and colleagues²⁹ studied 200 normal patients and 25 patients with a “firm” diagnosis of CIM. The authors concluded that, in the absence of syringomyelia, 2 mm of tonsillar ectopia was of minimal clinical significance. Recently, Mikulis and associates³⁰ determined that age affects the normal position of the cerebellar tonsils, with ascent of the tonsils with increasing age.

Other associated radiologic anomalies occur infrequently and include most commonly atlanto-occipital assimilation, platybasia, basilar invagination, and fused cervical vertebrae.²⁶ Despite the less frequent occurrence, these changes should be sought because they can lead to cervical instability.

In addition to the absolute position of the tonsillar tips, the configuration of the tonsil is important. The tonsillar tip may be pointed and drawn out and may carry more pathologic significance, or it may be blunt and rounded and be less concerning. Syringomyelia occurs commonly with CIMs and can be seen in 50% to 70% of patients (Fig. 179-3).

FIGURE 179-3 A, Child with Chiari I malformation and preoperative syrinx. B, Postoperative magnetic resonance image noting improvement of this patient’s syrinx.

Chiari II Malformations

CIIMs are characterized by the elongation and caudal displacement of the cerebellar vermis and brainstem, the presence of a myelomeningocele in virtually all cases and hydrocephalus in most, and the common (40% to 95%) existence of syringomyelia, especially in the lower cervical cord. However, the changes of the CIIM constitute a set of cranial and spinal malformations ranging from the posterior fossa, upper cervical canal, ventricular system, and neural tissues of the brain.³¹^–³⁷

Associated neurological anomalies include tectal beaking, secondary to partial or complete fusion of the colliculi into a single backward pointed peak, and kinking at the level of the cervicomedullary junction (Fig. 179-4). The latter anomaly is caused by caudal displacement of a portion of the medulla in conjunction with a spinal cord that is held in relative immobility by the dentate ligament. The cerebellum is smaller than usual, and upward herniation of the cerebellum may be evident. Finally, callosal agenesis or dysgenesis, as well as abnormalities of the cerebral cortical pattern termed polygyria (not to be confused with the abnormal four-layered cortex seen in polymicrogyria) have frequently been described in Chiari II patients.³⁴

FIGURE 179-4 Autopsied midsagittal specimen with multiple features of the Chiari II malformation. Note the extensive kink at the cervicomedullary junction (arrow).

In addition to hydrocephalus, the ventricular system displays multiple abnormalities. The third ventricle may be only mildly dilated and contains a large massa intermedia. The fourth ventricle is typically small or nonvisualized and often flattened and elongated, and it extends into the cervical canal. The lateral ventricles may be asymmetrically dilated, with prominence of the atria and occipital horns (colpocephaly), and the septum pellucidum is frequently absent. This sharpness of the frontal horn (lemon sign) and the caudal displacement of the fourth ventricle (banana sign) are relatively easily seen on in utero ultrasound examinations.³⁸^,³⁹

The upper cervical canal also displays several bony and spinal cord anomalies in association with the CIIM. The posterior C1 arch is often missing. Klippel-Feil fusion anomalies of the cervical spine are present in a small group of patients. Basilar impression and C1 assimilation are uncommon in CIIM. Significant shortening and scalloping of the clivus can be seen.⁴⁰ Other radiographic signs of CIIM include Lückenschädel, scalloping of the posterior surface of the petrous pyramid, falx hypoplasia, falx fenestration, tentorial hypoplasia with a wide incisura and tiny posterior fossa, and enlargement of the foramen.³⁵^,³⁶

Cerebrospinal Fluid Flow Studies and Cine-Mode Magnetic Resonance Imaging

One technique to assist in the surgical decision making of patients with Chiari malformations is motion-sensitive MRI, or cine-mode MRI. As opposed to static MRI scanning, cine MRI ⁴¹^–⁴⁴ may demonstrate lack of CSF flow patterns that can occur in the setting of CIM. The presence of a CIM and a syrinx resulting from disturbed CSF flow, which can be demonstrated by this technique, adds helpful information to the clinical assessment.⁴⁵ Repeating the study postoperatively may help in evaluating the adequacy of the decompression. Having said this, we find cine MRI of minimal utility in clinical practice and especially in patient selection for operation.

Electrophysiologic Studies

The role of brainstem auditory evoked potentials (BAEPs) in Chiari patients has been investigated, especially in the Chiari II population. Worley and coworkers ²⁴ studied 37 newborn infants with myelomeningoceles at a median age of 8 days and followed them for 30 months. None of the infants clinically had brainstem dysfunction at initial testing. Of 12 infants who subsequently developed brainstem dysfunction at a median age of 3 months, 11 had abnormal BAEPs on the first evaluation. Conversely, of the 25 patients who did not develop clinical signs of brainstem dysfunction, 10 had abnormal BAEPs early on in life. Multiple case reports have noted clinical and electrophysiologic improvement after posterior fossa decompression in symptomatic Chiari II patients, demonstrating objective confirmation of the observed clinical improvement.⁴⁵^–⁴⁷ The potential uses of BAEPs to evaluate neonatal myelomeningocele patients is still evolving, and this test alone is not a reliable indication for surgical intervention but should only be used as added criteria to a detailed history and physical examination.⁴⁸

Pathophysiology

The theories of pathogenesis of the Chiari malformations may be grouped into the hindbrain dysgenesis and developmental arrest theory, the caudal traction theory, the hydrocephalus and hydrodynamic theory of Gardner, the small posterior fossa/hindbrain overgrowth theory, and the lack of embryologic ventricular distention theory.⁴⁹

It is unlikely that Chiari types I and II malformations share a common pathoembryologic origin. Nonetheless, the result of cerebellar descent, crowded foramen magnum, and impaired CSF flow across the craniocervical junction may share a common etiology in the development of syringomyelia.

Chiari I Malformation

CIM may be congenital or acquired. Chiari’s belief that hydrocephalus causes CIM has been abandoned because it is present in only a minority of cases. Idiopathic CIM may be the result of mesodermal defects that create a congenitally small posterior fossa leading to compression of the neural elements and herniation through the foramen magnum. Neural dysfunction and deleterious alterations of CSF flow may ensue, resulting in the symptoms associated with CIM.

Nishikawa and coworkers ⁵⁰ suggested that underdevelopment of occipital somites within the paraxial mesoderm creates a small posterior fossa and CIM. This contention is supported by the association of CIM with other spine, skull, somatic, and craniofacial abnormalities, which are the result of mesodermal maldevelopment. The association of craniosynostosis and CIM is known and appears strongest in cases of syndromic, multisuture, and lambdoid synostosis. Lambdoid suture closure, typical in Crouzon’s syndrome, can directly reduce posterior fossa volume. Cephalocranial disproportion in multisuture synostosis can elevate intracranial pressure and promote herniation of posterior fossa elements.

Other medical conditions may promote formation of an abnormally small posterior fossa. Familial vitamin D–resistant rickets causes bony overgrowth of the posterior fossa, thus reducing its volume. Up to 20% of patients with growth hormone deficiency have CIM. Development of a craniospinal pressure gradient across the foramen magnum may cause or hasten the development of CIM. The gradient results from impaired CSF flow across the foramen magnum. Negative CSF pressure in the spinal compartment relative to the intracranial compartment creates a “sump effect” that forces the tonsils down through the foramen magnum. Once CSF flow is blocked at the foramen magnum, low intraspinal pressures can be accentuated and perpetuated by continuous absorption of CSF through spinal pathways, further worsening the clinical situation. Lumboperitoneal shunting, repetitive lumbar punctures, lumbar drainage, and chronic spinal CSF leaks of an iatrogenic nature are all familiar causes of an acquired CIM.⁵¹^,⁵² Small posterior fossae have not been identified in all patients with CIM.⁵³^–⁵⁵

Although CIM is thought to occur sporadically, inheritable genetic factors may be responsible for a minority of cases. This conclusion is based on studies in which disease incidence is abnormally higher in some families than in the general population.

Chiari II Malformation

Because the cerebellar vermis develops before the tonsils, an abnormal pressure differential that develops in utero would cause abnormal displacement of the vermis and brainstem structures, without any tonsillar involvement. Such a pressure differential could occur with fluid leakage from the myelomeningocele.⁴^,⁵⁶

McLone and Knepper ⁴⁹ have advanced a “unified theory,” which would explain the cause of the CIIM along with most, if not all, of the associated anomalies. This theory is based on the above-mentioned assumption that the neural tube defect occurs first, and all the other manifestations, including the Chiari malformation and hydrocephalus, follow secondarily. Leakage of CSF through the spinal defect causes a lack of distention of the primitive cranial ventricular system. In experimental animals, venting fluid from the embryonic ventricular system causes, for example, disorganization of the developing cerebral cortex and abnormal development of the pontine flexure.

Syringohydromyelia

In the 1960s, Gardner ⁵⁶ presented the hydrodynamic theory. Gardner’s theory stated that in normal embryology, CSF pulsations from the choroid plexus play a significant role in the expansion of the neural tube.⁵⁷ According to Gardner, these pulsations help with the development of the arachnoid pathways as well as with modeling of the expanding brain. He believed that the balance between the pulsatile flow in the supratentorial and fourth ventricular choroid plexus directed brain growth differentially; therefore, if the fourth ventricular pulsations were overactive, the tentorium would be pushed upward, and a Dandy-Walker malformation could develop. Conversely, if the supratentorial pulsations were overactive, tentorial migration becomes such that the posterior fossa is small, allowing the development of a Chiari malformation; in addition, the CSF outlets of the fourth ventricle would remain closed, directing the CSF into the patent opening at the obex, thus causing syringomyelia.⁵⁷^,⁵⁸

Based on experimental evidence using manometric measurements in normal and Chiari patients, Williams ⁵⁹ expanded on Gardner’s theory by suggesting that Valsalva maneuvers resulted in epidural venous congestion and intracranial as well as intraspinal pressure elevation, causing fluid to flow both cranially and caudally. Although flow into the cranial compartment meets no resistance, caudal flow is delayed by hindbrain adhesions and outlet obstruction, thus creating a pressure differential between the cranial and spinal compartments. This pressure differential may last a few seconds and cause worsening hindbrain impaction and syringomyelia. Repeat measurements were made after surgical decompression, showing equilibration of the pressures in the two compartments, which, in turn, correlated with clinical improvement. However, spinal cord cavitation is often acquired (as in posttraumatic syringomyelia), and a connection between the cyst and the fourth ventricle is not always present, which raises doubts about the adequacy of this theory.

Oldfield and associates ⁶⁰ investigated the anatomy and dynamics of movement of the cerebellar tonsils and CSF during the respiratory and cardiac cycles to explore the mechanism of syringomyelia progression in patients with CIM. During systole, there is normally movement of CSF in a caudal direction across the foramen magnum to counter the increased intracranial volume of blood and maintain physiologic intracranial pressure. This flow reverses in diastole. Dynamic movement of subarachnoid fluid is mirrored by caudal and cranial pulsations of fluid within the central canal during systole and diastole, respectively. In patients with CIM, the cerebellar tonsils are forced down and obstruct CSF flow across the foramen magnum during systole. This piston-like movement of the cerebellar tonsils imparts a systolic pressure wave in the spinal CSF that acts on the surface of the spinal cord, forcing fluid into the cord through perivascular and interstitial spaces. Oldfield and collaborators⁶⁰ demonstrated the dynamic CSF flow into the syrinx in patients with CIM preoperatively by dynamic MRI and intraoperatively by ultrasound and further demonstrated the resolution of pathologic flow following bony and dural decompression. Postoperatively, adequate decompression of the foramen magnum allows resolution of the syringomyelia.

Treatment

Chiari I Malformation

There is no effective nonsurgical alternative to operative decompression for patients with symptomatic CIM. However, a detailed understanding of the natural history is not objectively known because the true incidence and prevalence are not well recognized. The challenge is identifying which patients will benefit most from posterior fossa decompression. Appropriate selection of patients likely to respond to surgical therapy can be challenging. Surgical indications vary among surgeons, especially for subjective symptoms such as headache. A 2004 opinion survey of neurosurgeons concluded there was widespread agreement to treat patients with a syrinx and progressive scoliosis or symptoms.⁶¹ Opinions were mixed on how to manage individuals with asymptomatic syringes as well as on the most appropriate surgical technique.

Acquiring adequate imaging and clinical data is the first step in managing this diverse group of patients, with MRI clearly the study of choice. Patients suspected of having CIM should undergo MRI of the posterior fossa and entire cervical spine to assess tonsillar ectopia and screen for a syrinx. Further, patients with long-tract signs and no evidence of a cervical syrinx should also have their thoracic and lumbar spine screened. Also necessary is imaging to rule out hydrocephalus. Flexion-extension views of the cervical spine are suggested for individuals in whom there is a concern for instability.

Once patients are determined to have idiopathic CIM, they are divided into two broad categories based on the presence or absence of a syrinx. All patients with a syrinx regardless of the size, location, or other associated symptoms are offered surgical intervention. This strategy is based on the belief that a syrinx is indicative of pathologic forces acting on the spinal cord that should be corrected to prevent permanent cord damage. However, a recently published study by Nishizawa and coworkers ⁵⁰ challenges this notion. They followed nine asymptomatic patients who had incidentally discovered syringes for 11 years with serial MRIs and clinical examinations. In this period, only one patient showed any deterioration that required surgery. The syringes in the others did not change. They concluded that small, asymptomatic syringes can be safely followed with serial examinations and imaging.

The decision about whether to operate is a bit more complicated in patients without a syrinx. Individuals with minimal tonsillar ectopia, no objective neurological findings, and mild headaches that are not lifestyle limiting are observed. Surgery is deferred until symptoms worsen and become unacceptable to the patient and headaches are refractory to multidisciplinary management. Patients with lifestyle-limiting headaches or objective neurological abnormalities, especially respiratory or cranial nerve dysfunction, receive earlier surgical intervention.

Some basic principals are generally agreed on. If hydrocephalus coexists or any question of raised intracranial pressure is present, this situation should be resolved before consideration is given to a decompression. Only a minority of patients (<10%) fall into this category. After shunting or third ventriculostomy, if symptoms persist or an associated syrinx is large and unchanged for months, then consideration should be given to a Chiari decompression (Figs. 179-5 and 179-6).

FIGURE 179-5 Flow diagram for the treatment of asymptomatic patients with Chiari I malformation.

FIGURE 179-6 Flow diagram for the treatment of symptomatic patients with Chiari I malformation.

After a decision has been made to do a Chiari decompression, the patient is positioned prone with the head held flexed in a pin fixation device (Fig. 179-7). The midline incision begins in the mid-occiput, just below the inion, and extends to the second vertebra. The soft tissues and occipital musculature are separated in the midline through a relatively avascular plane. The foramen magnum and posterior arch of C1 are exposed the entire width of the dura. There is no reason for exposure more lateral than this because exposure of the vascular structures on each side of C1 carries risk without benefit. The bone of the occiput is removed, followed by the dorsal arch of C1. By leaving the muscle attachments and laminae of C2 intact, the postoperative pain and potential spinal instability are minimized. Rarely, it may be necessary to remove the superior aspect of the C2 laminae. By minimizing the opening and maximizing the decompression, we believe we have enhanced our surgical results and have not seen any cervical kyphosis or postoperative instability.

FIGURE 179-7 Surgical positioning for the decompression of a Chiari malformation.

The purpose of the operation is to enlarge the bony area of the craniocervical junction and expand the dura surrounding the brainstem. By doing this, the surgeon wishes to relieve any direct compression and facilitate adequate outflow of CSF from the fourth ventricle. Some have championed the idea of bony decompression alone, and this is being revisited at a number of institutions currently. Alternatively, some surgeons split the dura, opening only the outer layer. Although these methods may be sufficient in many patients, in the more severe forms of tonsillar herniation, especially when a syrinx is also present or intradural arachnoid adhesions are identified preoperatively, an intradural approach is needed. Of help in determining the need for dural grafting and intradural exploration is visualization of the movement of the tonsils seen through the dura or by intraoperative ultrasound. Once the dura is opened in the midline, the tonsils are gently separated to inspect for veils covering the outlets of the fourth ventricle (Fig. 179-8). This re-establishes free flow of CSF from the foramen of Magendie. We consider visualization of the choroid plexus of the fourth ventricle and free flow of CSF into the subarachnoid space as evidence of adequate decompression. Occasionally, in cases with severe tonsillar ectopia, the foramen of Magendie remains occluded, and CSF egress is limited. Extrapial coagulation of one or both tonsillar tips shrinks the tonsils sufficiently to restore CSF flow.

FIGURE 179-8 Operative exposure of the Chiari I malformation. The cerebellar tonsils are shown being spread apart so that the floor of the fourth ventricle is visualized.

Finally, the need for a dural graft has been debated.⁶² We believe that such a graft allows for protection of the neural tissues from chemical contamination while providing a capacious dural sac, which probably optimizes the decompression and minimizes postoperative adhesions. We prefer pericranium as the graft. After decompression, the fasciae and skin are closed in a routine manner.

The likelihood of a craniocervical decompression resolving appropriate symptoms is quite high with minimal operative risk. In a similar vein, significantly sized syringes should clearly show evidence of decreasing in size or totally resolving within weeks to months of surgery. It is so likely that Chiari decompression will resolve the situation that an inadequate clinical outcome almost always is due to an inadequate decompression. This has been true in the cases referred to us operated on elsewhere as well as our own initial failures. In patients who initially improved clinically and radiographically with decompression, and then worsened, the most likely explanation is a reclosure of the outlet foramen, which will respond to repeat decompression and possibly resection of a portion of the tonsil rather than any other surgical approach. In the rare patient who fails adequate decompression, consideration should be given to insertion of a syringopleural or syringoperitoneal shunt. Care should be given not to resort to this option except to resolve a syrinx that is large or is growing despite adequate decompression.

A challenging group of patients includes those in whom there is a significant component of ventral compression. We have defined significant ventral compression as greater than 9 mm of reclination of the odontoid process from a line connecting the basion to the posterior aspect of the body of the axis (Fig. 179-9).⁶³ In these patients, spinal stability should be ensured by performing dynamic cervical radiographs in flexion and extension before decompression. These patients often present with a greater degree of brainstem symptoms and may not benefit from posterior decompression alone. In our hands, dorsal decompression is addressed first with close observation in an intensive care setting postoperatively. Symptoms and signs of respiratory compromise, swallowing difficulty, and hemodynamic instability herald ongoing brainstem compression and warrant occipitocervical stabilization and possibly ventral decompression.

FIGURE 179-9 Patient with Chiari I malformation and retroversion of the odontoid process with resultant ventral compression.

Chiari II Malformation

Early surgical intervention may prove life sustaining in Chiari II patients whose symptoms are referable to the medullary dysfunction.¹⁸^,¹⁹ Timing of decompression before bilateral vocal cord paralysis may predict a better outcome.⁶³ Pollack and colleagues⁶⁴ have described favorable outcomes for patients undergoing early surgical intervention, that is, occipital craniectomy and cervical laminectomy for neurogenic dysphagia.

Indications for surgical intervention are based on clinical symptoms. The presence of a significant syrinx or one that has progressively enlarged should be considered for therapy. Symptoms include inspiratory stridor at rest or progressive by history, aspiration pneumonia due to palatal dysfunction or gastroesophageal reflux, central apnea with or without cyanosis, especially during sleep, opisthotonus, functionally significant or progressive spasticity of the upper extremities, and functionally significant or progressive truncal or limb ataxia.²³^,²⁴

Before considering surgical decompression, patients with symptomatic Chiari II malformation must have physiologic intracranial pressure. This assurance requires a functioning shunt. It has been our experience and that of others, that a properly functioning ventricular shunt can often obviate the need for decompression of hindbrain herniation. Many times, this can only be determined by surgical inspection of the shunt. Caldarelli and colleagues ²⁰ reported that of 11 symptomatic CIIM patients, 5 had resolution of their symptoms following shunt revision alone. Milhorat and associates⁶⁵ found in a retrospective study of a small number of patients that improvement in the size of their syrinx was observed after only ventriculoperitoneal shunting or revision. Tomita and McLone⁶⁶ concluded that shunt revision can reverse acute respiratory arrest. In contrast, lower cranial nerve findings may not improve after the shunt revision but rather only after posterior fossa decompression.

One important caveat is that stable ventricular size is not a reliable indicator of a functioning ventricular shunt. Iskandar and colleagues ⁶⁷ found that in 20% of myelodysplastic children with shunted hydrocephalus, CT studies were unchanged in the presence of shunt malfunction as demonstrated by surgical exploration following continued clinical decline. After shunt revision, all patients in this myelodysplasia group had resolution of preoperative symptoms (Figs. 179-10 and 179-11).

FIGURE 179-10 Flow diagram for the treatment of asymptomatic patients with the Chiari II malformation.

FIGURE 179-11 Flow diagram for the treatment of symptomatic patients with the Chiari II malformation.

The Chiari II anomaly is a challenging surgical entity with unusual and highly variable anatomy. Unlike the Chiari I operation, which can be performed by any experienced neurosurgeon with a good knowledge of the anatomy of the region, a Chiari II decompression requires significant experience with this particular entity for the procedure to be done safely and efficaciously. The main challenge in this procedure is the variable anatomy. For instance, the cerebellar tissue usually extends into the lower cervical spine; it may be very adherent to the medulla, and occasionally the two tissues may even seem indistinguishable or fused. The confluence of sinuses can be as low as the rim of the foramen magnum, and the dura may contain large venous sinuses.

Once adequate shunt function is ensured and the symptoms are progressive or serious and due to the CIIM, consideration should be given to surgical decompression. Helpful preoperative diagnostic studies include BAEPs, swallow study, direct vocal cord visualization by an otolaryngologist, and assessment of pulmonary function including obstructive and central apnea (sleep study) as well as hypercapnic ventilatory drive by a pulmonary specialist. Thorough study of the preoperative MRI scan is important in planning the procedural details. The position of the confluence of sinuses, cerebellar vermis, cervicomedullary kink, and choroid plexus should be specifically identified. The extent of the bony opening should encompass the cerebellar hindbrain hernia but need not include the medullary kink or the occipital bone, especially if the confluence of sinuses is low lying. The foramen magnum is generally enlarged in this condition, and the patient is unlikely to benefit if it is made even larger. Minimizing the bony decompression is important to reduce the concern for delayed cervical instability and postlaminectomy kyphosis.

The salient aspects of the procedure are that the patient is first placed prone in a pin fixation device with the neck slightly flexed. The skin, fascia, and muscle opening are similar to the Chiari I procedure. The dura is opened in the midline, and the neural structures are exposed. Dense arachnoidal adhesions are common, as is striking superficial hypervascularity. The choroid plexus is identified by its yellow-orange color and granular appearance. It maintains its early embryologic extraventricular location and marks the entrance into the fourth ventricle. The interface between vermis and medulla is usually densely adherent and difficult to separate. The procedure is not thought to be finished until the avascular floor of the fourth ventricle is well visualized. Use of intraoperative ultrasound may be helpful in some cases. The tip of vermis is coagulated to maintain an opening out of the fourth ventricle to the subarachnoid space. The dura is grafted and the wound closed in routine fashion.

Complications

Posterior fossa decompression is relatively safe but not without complications. Direct vascular or neural injury, pseudomeningocele, CSF leaks, and meningitis are well recognized. Bleeding from dural venous lakes can be profuse at times. Less common complications include occipital-cervical instability, acute postoperative hydrocephalus secondary to infratentorial hygromas, and anterior brainstem compression from a retroflexed odontoid.⁶³^,⁶⁸ A complication unique to posterior fossa craniectomies is cerebellar slump or ptosis, which results from extending a craniectomy so far laterally that the cerebellum herniates through the craniectomy defect. This can cause headaches (different from typical Chiari I headaches), obstruction of CSF flow with syrinx formation, and a variety of motor, sensory, and cranial nerve deficits. Cranioplasty to buttress the cerebellum into place is the most definitive treatment. Simple shunting is not adequate. We have not seen this complication in more than 400 patients by limiting the bony removal to the width of the spinal dura. Complications in our early series of 130 patients were minimal and included acute postoperative hydrocephalus in 2 patients, requiring temporary external ventricular drainage. Severe anterior brainstem compression from a retroflexed odontoid required a transoral odontoidectomy in 1 patient. Neither pseudomeningocele, meningitis, nor cerebellar slump was observed.⁶ In an attempt to more specifically categorize these complications in Chiari I patients and syringomyelia, Menezes⁶⁹ reviewed a series of 35 children and identified a set of complications associated with surgery. These included excessive bleeding from venous lakes, failure to get into the fourth ventricle secondary to adhesions, persistent variation of blood pressure and heart rate, failure to awaken, respiratory compromise, and weakness. Although any of these can occur, many can be avoided by meticulous preparation and surgical execution and a thorough understanding of the pathology.

Results and Prognosis

Chiari I Malformation

With the natural history known incompletely, surgical data accumulated from a series of retrospective studies claiming to improve on the natural history should be looked at cautiously. Saez and associates ⁷⁰ attempted to classify patients into preoperative prognostic categories. The poorest prognosis was seen in patients with central cord signs; the best prognosis was found in patients with paroxysmal intracranial hypertension.

Chiari II Malformation

The natural history of these lesions can be disastrous. Therefore, the idea of surgical intervention early in a child with symptoms is widely accepted, although only a few studies exploring the role of decompression in Chiari II patients are available in the literature. Pollack and colleagues ⁶⁴ have prospectively treated neonates and young infants displaying symptomatic CIIMs with urgent laminectomies and limited suboccipital craniectomies. Ten of the 13 children recovered normal or almost normal neurological function postoperatively, whereas the other 3 exhibited bilateral vocal cord paralysis and severe central hypoventilation. The authors concluded that early recognition and treatment of brainstem compromise in Chiari II patients produces prompt and long-term clinical recovery.

Follow-Up

Chiari I Malformation

The follow-up regimen is determined by preoperative pathology and the postoperative clinical course. Our clinical paradigm includes seeing patients without a syrinx and symptomatic improvement at 1, 6, and 12 months, then every 12 to 24 months thereafter without repeat imaging. Patients with a preoperative syrinx receive a follow-up MRI in 6 to 12 months. No further imaging is obtained if symptoms improve or the syrinx decreases in size significantly. If the syrinx improves minimally, additional imaging is obtained at the surgeon’s discretion. As long as the syrinx progressively shrinks and no additional symptoms or signs occur, no matter how slowly, we continue to follow the patient conservatively with imaging. When the syrinx fails to improve or symptoms referable to a persistent syrinx are present, a second surgery is performed.

From our initial series of CIM, eight patients required a second posterior fossa exploration for syrinx persistence.⁶ Again, no attempt was made to manipulate the syrinx. The second surgery is more aggressive than the first. Unilateral tonsillar coagulation was performed for each patient. Stents were placed in four patients when free egress of CSF from the fourth ventricle was not achieved after lysis of subarachnoid adhesions and tonsil coagulation, or when no obvious pathology explaining syrinx persistence was identified. Syringes improved in four of six patients, and imaging studies are pending in the others. Interestingly, stents were not placed in the two patients who failed to improve. We stress that re-exploration of the posterior fossa is the best strategy for dealing with a recalcitrant syrinx.