Summary of Key Points
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Chiari malformations are a spectrum of conditions involving bone and parenchymal abnormalities of the hindbrain and craniocervical junction (CVJ). The most common variant, Chiari I, is characterized by an anatomic and physiologic aberration at the CVJ. The anatomic abnormality includes tonsillar descent of greater than 5 mm below the foramen magnum.
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The physiologic abnormality observed is an aberration in the normal cranial to caudal cerebrospinal fluid (CSF) flow patterns or obstruction of CSF flow through the foramen magnum on cine-mode magnetic resonance imaging (cine-MRI) CSF studies.
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The underlying pathophysiology of these malformations remains genetically and embryologically unclear based on genomic screening.
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Chiari II is often associated with myelomeningocele/spina bifida and hydrocephalus. Chiari I is not associated with spina bifida. In one of the largest large series of Chiari I malformations in pediatric patients, approximately 9.6% demonstrated hydrocephalus and 57% had syringomyelia and 18% scoliosis.
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Presentation is variable and depends on the specific type of malformation. Common symptoms in Chiari I malformation include occipital or cervical pain induced with Valsalva or “tussive” maneuvers, and neurologic deficits include dysesthetic pain, numbness, weakness related to syringomyelia, or cranial nerve paresis from brain stem compression. Patients with Chiari II malformations present with brain stem compression symptoms such as stridor or cranial nerve paresis at a young age or with similar brain stem findings at an older age during a shunt malfunction.
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Surgical management of Chiari I malformation is accomplished via posterior fossa decompression. The presence of syringomyelia or the progression of scoliosis may be indications for surgical management.
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The utility of duraplasty during surgery remains an area of ongoing study, with some studies suggesting that bone-only decompression in children has similar rates of symptomatic relief and fewer operative complications but an increased reoperation rate. Outcomes in appropriately selected patients undergoing decompression for Chiari type 1 are good, with the majority reporting improvement or stabilization in symptoms and signs such as tussive headaches and syringomyelia. Surgical outcomes with other symptoms and signs and conditions are less clear, and patient selection becomes critical in determining which patients are likely to benefit from surgical intervention.
Chiari malformations are commonly encountered in both pediatric and adult neurosurgical practices. Multiple variations in the anatomy of the rhombencephalon have been described, leading to the familiar Chiari I through IV designations. Although many patients are characterized as having a Chiari malformation, and symptoms can overlap among the various Chiari types, it is unlikely that Chiari malformation represents a unified disease with a single causation. Treatment has been based on both symptomatology and imaging findings, with variations on the Chiari decompression procedure widely described in the literature. Symptom arrest or regression is the expected surgical outcome, with resolution or arrest of the spinal cord syrinx, if present. Outcomes after decompression usually are good, making Chiari decompression an effective surgical option for appropriately chosen patients.
History
John Cleland (1835–1925) was among the first to describe hindbrain herniation in a patient with myelodysplasia. 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. In 1891 and 1896, Hans Chiari (1851–1916) provided descriptions of hindbrain herniation in postmortem specimens in which variations I through IV were described, work that led to his name being associated with the condition.
The surgical treatment of Chiari malformations began with the technique described in 1938 by Penfield and Coburn, in which they presented a patient who underwent posterior fossa decompression and who subsequently died in the postoperative period. Early experience with this procedure was marked by a high rate of operative complications and death. Gardner and associates noted five deaths among 74 patients undergoing posterior fossa decompression. Williams 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 rate of death or reoperation less than 2% in a large, multi-institutional study.
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. Although 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.
Although causation remains unclear, mechanistic theories abound to describe Chiari I and II malformations and the common finding of syringomyelia. Gardner 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) preferentially transits the potential space of the central canal in the spinal cord, causing slow progressive dilation. Oldfield suggested that the downward pistoning of the cerebellar tonsils seen on cine-mode magnetic resonance imaging (cine-MRI) drives CSF flow into the spinal cord via perivascular and interstitial spaces, resulting in a syrinx. A link between anatomic variance in the posterior fossa and development of syringomyelia has also been supported by analysis of patients with Chiari 0 malformation, with imaging studies demonstrating syringomyelia associated with caudal displacement of the obex, an increased angle between the fourth ventricle and clivus, and increased anterior-posterior midsagittal distance of the foramen magnum and cervicomedullary junction. Regarding Chiari II, McLone and colleagues 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.
One of the more controversial avenues of study in Chiari malformation is the association and management of ventral brain stem compression and occipitocervical instability in pediatric and adult patients. The incidence of connective tissue diseases in this population as a potential cause for cranial and cervical pain syndromes or instability is the source of contention. Ehlers-Danlos syndrome (EDS) is a broad term used for grouping a growing number of hereditary disorders of the connective tissue characterized by joint pain and hypermobility. The patients enjoy a normal life span but suffer debilitating pain symptoms ranging from headache to fatigue. The incidence of occipitoatlantoaxial hypermobility or instability associated with EDS in Chiari I malformation is both hotly debated and poorly understood. Although occipitocervical fusion for frank instability in the craniocervical junction (CVJ) from traumatic etiology or ventral brain stem compression and cranial settling is better understood, the use of this surgical repertoire for patients with Chiari I malformation and suspected EDS is far less understood or uniformly embraced.
Signs, Symptoms, and Imaging: Pathologic Features
Chiari I
There are two pathologic findings in patients with Chiari I malformation: one is anatomic and the other is physiologic. Chiari I malformation involves caudal herniation of the cerebellar tonsils more than 5 mm below the level of the foramen magnum. In addition, there is an associated obstruction of normal cranial to caudal CSF flow at the CVJ. Chiari I is rarely associated with hydrocephalus ( Table 164-1 ) but has been associated with numerous conditions ( Box 164-1 ). Patients typically present with nondermatomal “tussive” pain in the occipital or cervical region that is exacerbated by the Valsalva maneuver. Pain in younger children may manifest as irritability, crying, or failure to thrive. Sleep apnea is another common finding in younger patients. Neurologic symptoms described in association with Chiari I malformation include numbness, weakness, clumsiness, dysphagia, dysarthria, ataxia, and incontinence ( Box 164-2 ). Signs on examination can include nystagmus, lower extremity hyperreflexia, upper extremity hyporeflexia, cerebellar signs, and lower cranial nerve dysfunction, including dysarthria, palatal weakness, and decreased gag reflex. Scoliosis can also 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, obstruction of CSF flow at the craniovertebral junction |
| Syringomyelia and scoliosis possible | |
| Chiari II | Herniation of brain stem, 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 brain stem and fourth ventricle |
| Caudal displacement of cerebellar tonsils | |
| No associated myelomeningocele | |
| Chiari 0 | Crowded posterior fossa without hindbrain herniation; obstruction of CSF flow at CVJ |
| Syringomyelia |
Klippel-Feil syndrome
Neurofibromatosis
Apert syndrome
Crouzon syndrome
Metopic and multisuture synostosis
Odontoid retroflexion
Pierre-Robin syndrome
Caudal regression syndrome
Costello syndrome
Paget disease
Craniometaphyseal dysplasia
Growth hormone deficiency
Cloacal exstrophy
Hemihypertrophy
Rickets
Acromegaly
Lipomyelomeningocele
Multiple findings are demonstrated on imaging and at autopsy in patients with Chiari I malformation ( Fig. 164-1 ). 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 may show decreased flow posteriorly at the craniocervical junction. 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 typically involves the lower cervical and upper thoracic cord, but this may vary and holocord syrinx is not uncommon. Volumetric analysis has demonstrated reduced posterior fossa volumes, increased CSF volumes, and normal brain volume in cohorts of patients with Chiari malformation.
The role of skull base and craniocervical junction abnormalities in the development of Chiari I malformation and cranial settling has been increasingly recognized. The degree of bone compression of the foramen magnum can be evaluated by assessing the clival-basal angle (CXA), with significant kyphosis characterized by an angle less than 150 degrees. To objectively understand what amount of encroachment from ventral compression by the odontoid process into the cervical canal is tolerated, a line can be drawn between the basion and posterior inferior aspect of C2 (pB-C2). This line, described by Grabb and colleagues, is used to measure the distance from that line to the ventral cervical canal. Patients with more than 9 mm of encroachment by the odontoid at that point may benefit from an occipitocervical fusion prior to or during a cervical decompression for their Chiari I malformation. We tend to use 10 mm of ventral compression as the criterion but remain cautious when we observe any significant ventral compression prior to performing a decompression for Chiari I malformation. Other abnormalities, such as basilar invagination and clival hypoplasia, may be seen with Chiari I malformation. Interestingly, increased bony compression was negatively correlated with syrinx formation. Assessment of bony ventral canal narrowing demonstrated higher rates of symptomatic relief and syrinx resolution in patients with greater than 3 mm of encroachment.
Bollo and coworkers demonstrated that underlying atlanto-occipital instability contributes to the pathophysiology and symptom progression of Chiari malformation, with CXA less than 125 degrees, Chiari 1.5, and basilar invagination suggesting that patients may benefit from occipitocervical fusion as an adjunct to standard posterior fossa decompression.
We agree with those findings and meticulously analyze the radiologic studies for frank instability, CXA, pB-C2, and Wackenheim line prior to proceeding with a posterior fossa and C1 decompression.
Chiari II
Chiari II malformation is characterized by caudal herniation of the cerebellar vermis, the brain stem, and the fourth ventricle in the setting of myelomeningocele. Hydrocephalus, along with multiple skeletal and intracranial abnormalities ( Box 164-3 ), is common in patients with this condition. After closure of the myelomeningocele and treatment of any associated hydrocephalus, 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 and the attendant risk of 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 164-2 ). Symptom onset in older children usually indicates spinal cord tethering, shunt malfunction, or the development of syringomyelia.
Skeletal
Craniolacunia
Small posterior fossa
Frontal bone scalloping
Petrous bone scalloping
Clival concavity
Low-lying inion
Large occipital keel
Atlas assimilation
Klippel-Feil deformity
Basilar invagination
Intracranial
Hydrocephalus
Colpocephaly
Asymmetry of the lateral ventricles
Vertical straight sinus
Low-lying torcula
Complete or partial agenesis of corpus callosum
Polygyria
Cortical interdigitation
Enlarged massa intermedia
Tectal beaking
Spinal Cord
Split cord malformation
Syringomyelia
Imaging and autopsy findings in patients with Chiari II malformations include complete or partial agenesis of the corpus callosum and septum pellucidum, prominence of the 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. Bony abnormalities of the skull can include scalloping of the petrous bone and lückenschädel, which is a beaten copper or fenestrated appearance of the calvaria on imaging. 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 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. 164-2 ).
Chiari III
Chiari III malformation is the presence of an occipital or cervical encephalocele in association with many of the intracranial abnormalities seen with Chiari II malformations. The tissue in the encephalocele is variable in its extent and is dysplastic.
Chiari IV
Chiari IV malformation is characterized by cerebellar aplasia or hypoplasia with concomitant aplasia of the tentorium. Hindbrain herniation is absent.
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 brain stem and fourth ventricle below the basion-opisthion line.
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. Structural abnormalities in the posterior fossa may also play a role in syrinx formation in these patients.
Patient Selection and Surgical Management
Diagnostic Imaging
High-resolution MRI and computed tomography (CT) imaging are critical for appropriate evaluation of the patient referred for consideration of Chiari decompression. In all patients who present for initial evaluation, it is our practice to obtain high-resolution brain MRI, including cine-MRI and constructive interference steady-state (CISS) sequences, as well as sagittal MRI of the complete spine to screen for syringomyelia or tethered cord. Cine-MRI is used to evaluate CSF pulsatility at the craniocervical junction, whereas use of a highly T2-weighted MRI sequence (CISS) in the sagittal plane provides improved visualization of CSF around the cerebellum and tonsils. MRI makes it possible to assess the degree of tonsillar descent as measured from the basion-opisthion line and provides some indication of CSF dynamics (on cine-MRI). It also provides information regarding the presence or absence of hydrocephalus, spinal cord syrinx, low-lying spinal cord, or tethering and can detect concurrent brain/spinal cord abnormalities (e.g., manifestations of Chiari II malformation, split cord formation, or associated scoliosis) and other associated or incidental findings. Operative candidates also undergo preoperative CT imaging to evaluate for potential associated skull-base and cervical spine abnormalities such as atlanto-occipital assimilation, basilar invagination, and Klippel-Feil abnormalities and to confirm that skull thickness is sufficient for intraoperative pin fixation. Any concerns of cervical spinal instability are further evaluated with flexion-extension plain radiographs before any surgical intervention. We routinely measure the CXA, pB-C2, and Wackenheim line to assess the concern for cranial settling and ventral brain stem compression.
Surgical Indications
Chiari I
Controversy remains regarding an exact algorithm for patient selection in regard to successful and appropriate Chiari I decompression. The initial step should be to ensure that symptoms are not caused by hydrocephalus or spinal cord tethering. Evidence of hydrocephalus should lead to implementation of CSF diversion, followed by reassessment to determine whether the symptoms and imaging findings have resolved after normalization of intracranial pressure. Spinal cord tethering, though uncommon, is assessed on preoperative screening and spinal MRI and is defined as termination of the conus below the L2-3 disc space. The filum terminale is considered abnormal if the diameter is greater than 1 mm at L5-S1.
Once hydrocephalus and spinal cord tethering have been excluded or addressed, our evaluation begins with an assessment of the degree of tonsillar descent and presence or absence of a spinal cord syrinx. Like many of our colleagues, we consider a spinal cord syrinx an indicator of abnormal CSF dynamics consequent to the Chiari malformation.
Patients with a spinal cord syrinx are offered surgical decompression unless they are completely asymptomatic and have only a small (< 2 mm) syrinx. Patients in this small subset are monitored closely with serial imaging every 6 months and offered surgery if symptoms occur or the syrinx increases in size. In several of our asymptomatic patients who were followed conservatively, the syrinx resolved over time.
Symptomatic patients without a syrinx pose the larger diagnostic and therapeutic challenge. Patients with more than 5 mm of tonsillar descent and clear Valsalva-induced headache or neck pain often benefit from decompression. Associated neurologic symptoms, as well as progressive scoliosis in a patient with Chiari malformation, support the decision for operative management.
Patients with headaches unrelated to Valsalva maneuvers who have a minimal Chiari malformation (< 10-mm tonsillar descent) are the most challenging to treat and often are the least likely to benefit from operative intervention. A conservative approach often is warranted in these patients, with referral to a headache management specialist and close neurosurgical follow-up with serial imaging on a yearly basis. Symptomatic progression or the development of a syrinx favors surgical intervention. Patients who present with more obscure neurologic findings (e.g., fussiness, head banging, nystagmus, essential hypertension, recurrent aspiration) pose a similar challenge, and correlation of symptoms with radiographic findings is especially important. Surgical decompression may have a role in management of these patients, but a conservative approach often is warranted and care should be taken to rule out alternative etiologies.
Chiari II
In our practice, surgical decompression in the setting of Chiari II malformation is most commonly performed in children under the age of 2 years who have symptomatic brain stem dysfunction or progressive spinal cord syrinx. Common indications for operative management include respiratory failure, recurrent aspiration, dysphagia, and other symptoms suggestive of lower cranial nerve dysfunction. It is critical to interrogate for shunt malfunction as the first step in management of these patients. Any concern for shunt malfunction should prompt shunt exploration before any other neurosurgical procedures are performed; in our experience, a number of patients have had resolution of symptoms after revision of a poorly or nonfunctioning shunt. After shunt malfunction has been excluded, we typically perform a bone-only posterior fossa decompression and avoid an open duraplasty. Outcomes for Chiari II decompression can be poor, especially if bilateral vocal cord paralysis is present preoperatively or if other signs of profound and long-standing brain stem dysfunction are present. It is important to convey realistic expectations to the family before proceeding with Chiari II decompression.
Surgical Technique
Anesthetic and Positioning Considerations
General endotracheal anesthesia is required for all patients. A percutaneous arterial catheter and an indwelling Foley catheter are placed, but central venous catheters are not usually required. Intubation is done either via fiber-optic guidance or with minimal neck extension. Total intravenous (IV) anesthesia typically is used to facilitate neurophysiologic monitoring. Once anesthesia is induced, the patient is connected to neuromonitoring devices to assess brain stem auditory-evoked potentials, motor-evoked potentials, spontaneous electromyography, and somatosensory-evoked potentials. The use of electrophysiologic monitoring in this operation is not a standard of care in all regions. Intravenous antibiotics (cephalosporin) are administered and redosed every 3 hours. If the dura and arachnoid are opened, dexamethasone is also given every 4 hours and continued for 48 to 72 hours postoperatively (authors’ preference). A lumbar drain or a ventriculostomy are not used unless there is concomitant hydrocephalus with suspected increased intracranial pressure. That is a rare occurrence in Chiari I. Patients older than 5 years of age and with appropriate calvarial thickness as determined on preoperative CT scan are put into three-point Mayfield pin fixation and positioned prone on gel rolls that run parallel to the long axis of the body. If the patient is younger than 3 years of age or has inadequate bone thickness, we use a horseshoe headholder. Considerable attention to avoiding pressure or contact with the eyes is necessary when using the horseshoe headholder, and we utilize additional foam padding to bolster and protect the orbits. All contact points are meticulously padded, with special attention to the iliac crest in thin patients. We ensure that the axilla is free and that the shoulders are neutrally positioned. The neck is carefully positioned in military tuck (in-line flexed) position, ensuring at least two finger-breadths separate the neck and chin. After positioning, we confirm there is an appropriate amount of neck flexion by verifying that there has been no significant increase in airway pressures. It also is important to ensure that the chin is not in contact with the bed, as the patient can move slightly during intraoperative positioning, and this may result in a postoperative pressure sore. The arms are padded and placed at the patient’s side, and a draw sheet (placed prior to positioning) is then brought over the patient’s torso and secured with towel clips. Strong cloth tape and a security strap are then used to secure the patient to the operating table. A small strip of hair is trimmed and the field is prepared with Betadine/alcohol or 4% chlorhexidine gluconate antiseptic (ChloraPrep, CareFusion, San Diego, CA). A standard suboccipital incision is marked, starting at the inion and running caudal along the midline to terminate at approximately the spinous process of C2 ( Fig. 164-3 ). The senior author (RGE) also prefers to use autologous pericranium for duraplasty, and a separate incision is marked from the inion superiorly for approximately 3 cm to harvest this tissue, and it is used to prepare the operative field and the edge of the surrounding hair. During final preoperative positioning, the patient’s head is placed at a 90-degree angle from the anesthesiologist, with surgeons situated on either side of the patient’s head and the scrub technician standing across from the anesthesiologist; a Mayo stand is positioned over the patient’s torso. The operative microscope is positioned for use during the intradural portion of the procedure in a head-to-head configuration with the base behind the assistant ( Fig. 164-4 ).
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