RECENT ADVANCES IN NEUROSURGERY Current Concepts in the Pathogenesis, Diagnosis, and Management of Type I Chiari Malformations CODY A. DOBERSTEIN, BS; RADMEHR TORABI, MD; PETRA M. KLINGE, MD ABSTRACT incidentally in approximately 1% to 4% of patients under- Type 1 Chiari malformations (CMs) are a group of con- going brain or cervical spine magnetic MRI studies.4 genital or acquired disorders which include the abnormal Most cases of type 1 CM are sporadic. Type 1 CMs can be presence of the cerebellar tonsils in the upper spinal ca- found in association with other conditions such as neurofibro- nal, rather than the posterior fossa. The resulting ana- matosis, idiopathic intracranial hypertension (IIH), tethered tomic abnormality causes crowding of the structures at spinal cord, connective tissue disorders, craniosynostosis the craniocervical junction and can impair the normal and skull base abnormalities, intracranial hypotension and flow of cerebral spinal fluid (CSF) in this region. This cerebellar hypertrophy in polymicrogyria.5 It is still not fully impairment in CSF flow dynamics can led to the devel- understood whether these co-existing conditions are mere opment of syringomyelia or hydrocephalus. Type 1 CMs coincidences or true co-morbidities. The precise natural his- have been associated with a wide array of symptoms re- tory of this disorder remains unclear although patients gen- sulting from either cerebellar and brainstem compression erally have symptomatic progression. There have been a few and distortion or disturbances in CSF dynamics, and can published reports of spontaneous resolution of type 1 CMs affect both children and adults. The clinical diagnosis but most symptomatic cases require surgical intervention.5.6 may be difficult. Age usually matters in the clinical pre- sentation, and in symptomatic patients, surgical inter- vention is usually required. PATHOGENESIS KEYWORDS: Chiari I Malformation, cerebrospinal fluid, Most cases of type 1 CM are congenital. Skull base abnor- hydrocephalus, syringomyelia malities are seen in approximately 50% of type 1 CM cases, (i.e., basilar invagination, retroflect odontoid, platybasia etc.).7 Although the exact etiology is unknown, this con- dition is thought to be secondary to insufficiency of the paraxial mesoderm after neural tube closure with underde- INTRODUCTION velopment of the occipital somites.7,8 Milorat and cowork- Chiari malformations are a group of disorders defined by ers examined reconstructed CT and MRI images in 388 structural defects of the cerebellum, pons, fourth ventricle, patients with classic type 1 CMs, and morphometric anal- and upper spinal cord in relation to the foramen magnum ysis revealed reductions in the posterior cranial size and and the skull base. In 1891, Chiari was the first to describe volume.9 In severe cases, downward herniation of the brain- and define hindbrain herniation, representing downward stem may occur and is sometimes referred to as a type 1.5 displacement of the cerebellum, fourth ventricle, and brain- CM.7 Despite evidence supporting a genetic contribution to stem.1 Type 1 CMs are characterized by herniation of the type 1 CMs (i.e., twins, familial clusters, and co-segregation cerebellar tonsils through the foramen magnum into the with known genetic syndromes), limited research has been upper spinal canal. The resulting compaction and crowding conducted to identify the specific genetic factors involved.8 at the craniocervical junction can disrupt normal cerebro- Acquired type 1 CMs can occur when there is a signifi- spinal fluid flow, produce the so-called “Valsalva-induced” cant cerebral spinal fluid (CSF) pressure gradient across the headaches, and may lead to the formation of a spinal cord craniocervical junction, i.e., CSF leakage or lumboperitoneal syrinx or hydrocephalus.2 shunts can produce negative downward pressure gradients Chiari malformations are still listed as a rare disease by leading to the development of a type 1 CM. In addition, con- the Office of Rare Diseases of the National Institutes of ditions associated with raised intracranial pressure, such as Health. The estimated prevalence in the United States of hydrocephalus and IIH, can promote downward pressure gra- type 1 CMs is less than one percent with a slight female pre- dient. The association of CM1 with tethered cord has led to dominance.2 Speer et al. have estimated that 215,000 Amer- the “caudal traction theory.” 6 icans may harbor a type 1 CM.3 However, the routine use of Syringomyelia is identified in 30-85% of patients.5,10,11 magnetic resonance imaging (MRI) has led to more frequent There are many hydrodynamic theories to explain the for- identification of this disorder and type 1 CMs can be seen mation of syringomyelia11. Abnormal and increased pulsatile WWW.RIMED.ORG | ARCHIVES | JUNE WEBPAGE JUNE 2017 RHODE ISLAND MEDICAL JOURNAL 47 RECENT ADVANCES IN NEUROSURGERY motion of the cerebellar tonsils (“tonsillar pistoning”) can failure to synchronize transmission of systolic CSF pres- produce selective obstruction of CSF flow during systole. sures between the cranial and cervical subarachnoid space.12 The increased systolic CSF waves are then transmitted to the spinal subarachnoid space and drive the CSF into the central canal of the spinal cord through engorged perivascu- SURGICAL MANAGEMENT lar and interstitial spaces and lead to syrinx formation.12,13 The management of acquired forms of type 1 CM is directed at correcting the primary causative condition. For example, ventricular shunting for the treatment of hydrocephalus, DIAGNOSIS repairing spinal CSF leakage, or correcting a tethered spinal The clinical findings vary dependent on the age at pre- cord usually results in anatomic and physiologic correc- sentation. Occipital headache and neck pain are the most tion of the acquired CM. Intervention to directly treat the common symptoms in adults.10 In infants, oropharyngeal acquired CM is typically not necessary. dysfunction or sleep apnea and other cranial nerve findings, Asymptomatic patients who have an incidental finding on i.e., strabismus, are the most common presenting symp- imaging are usually observed and monitored with follow-up toms, while older children often present with headaches MRI studies. Most patients with symptoms, or those who aggravated by “ Valsalva maneuvers” during coughing and harbor a large associated spinal cord syrinx, should be rec- sneezing or strain, and scoliosis.14,15 Symptoms are based on ommended surgical intervention. Close follow-up and serial the structural and functional (impaired “CSF-dynamics”) MRI imaging is required in patients who undergo observa- pathology associated with CM, which often leads to a wide tion alone in the presence of a syrinx. Appropriate man- spectrum of focal and non-focal findings in the clinical and agement of an asymptomatic patient with a small syrinx is neurological presentation, making it difficult to diagnose. controversial.17,18 Even more challenging is the often reported “brain fog” that Many different surgical techniques are utilized to treat has been largely attributed to chronic pain, depression and type 1 CMs, and there is no consensus. Surgical correction anxiety associated with the unknowns and physical chal- of type 1 CMs may include bony decompression of the pos- lenges of this disorder. In traditional thinking, a disorder terior fossa with or without duraplasty, arachnoid dissec- like Chiari affecting the craniocervical junction and the cer- tion, or shrinking of the cerebellar tonsils. The goal of any ebellum, has not been thought to affect cognitive function: of these operations is to restore adequate CSF flow at the Altered MRI diffusion tensor imaging (DTI) metrics in the level of the foramen magnum and establishment, basically genu of the corpus callosum, splenium, fornix have been cor- an “anatomical reconstruction,” of the Cisterna magna related with cognitive neurocognitive function in Chiari.16 (Figure 1A, B). Bony decompression alone has been asso- Magnetic resonance imaging is the widely accepted diag- ciated with a decreased risk of CSF related complications nostic tool for type 1 CMs. The McRae line is a radiographic such as pseudomeningocele, meningitis, and hydrocephalus. line drawn on a lateral midsagittal sec- tion of CT or MRI, joining the basion Figure 1. (A) Sagittal T2 STIR magnetic resonance imaging showing Chiari I with significant cer- and opisthion representing the level of vical syringomyelia (black asterisk) and the classical “crowding” of the cerebellum and the brain the foramen magnum. The traditional stem at the level of the foramen magnum (dashed white line equals the McRae line, which indi- definition of type 1 CM as greater than cates the level of the foramen magnum on a midsagittal section of CT or MRI joining the basion 5 mm displacement of the cerebellar ton- and opisthion). (B) At 3 months follow-up, there is evidence of restored CSF signal anterior to the sils below the foramen magnum is chal- brain stem, decompression of the obex and restoration of the cisterna magna associated with an lenged.15 Even a “mild” displacement of almost complete resolution of the syrinx. 3-5 mm may be considered significant in the presence of neurological signs or A B symptoms or in the presence of syringo- myelia. Also, the level of tonsillar ecto- pia evidenced in the sagittal MRI varies based on head position, and whether the measurement of the tonsillar posi- tion is based on a brain or spinal MRI. Recently, upright MRIs have challenged this view also, as gravity might reveal tonsillar displacement that was not seen in the traditional supine MRI versions. The future lies in computation of the CSF space at the craniocervical junction and the resulting altered compliance and WWW.RIMED.ORG | ARCHIVES | JUNE WEBPAGE JUNE 2017 RHODE ISLAND MEDICAL JOURNAL 48 RECENT ADVANCES IN NEUROSURGERY However, multiple studies have shown that reoperation rates tions as guide to clinical management. Acta Neurochir (2010) are higher for patients who have undergone bony decompres- 152: 1117. 19,20,21 7. Sgouros S, Kountouri M, Natarajan K (2007) Skull base growth sion alone.