Chiari Malformation – Miles to go Before we Sleep!!

More than a hundred years after it was first described Chiari malformation remains wrapped in layers of uncertainty and intrigue. Hans Chiari, an Austrian pathologist, was the first to describe a group of hindbrain anomalies, which he initially classified into three types: Chiari I, II, and III. The type II variant is also named Arnold Chiari in recognition of the contribution of Julius Arnold a German pathologist. Chiari later introduced a fourth type, and over time, additional subtypes have been identified. Today, this broad spectrum of eight recognized variants is collectively referred to as Chiari malformation.[1] Despite the presence of multiple variants, the unifying theme of Chiari malformation is tonsillar ectopia and displacement of the hindbrain through the foramen magnum. Chiari I is the least severe and often found incidentally characterized by descent of one or both cerebellar tonsils 5 mm or more below the foramen magnum. Chiari II consists of brainstem herniation in addition to the herniated cerebellar tonsils and vermis due to an open distal spinal dysraphism. Chiari III involves herniation of the hindbrain (cerebellum with or without the brainstem) into a low occipital or high cervical meningoencephalocele. Chiari IV additionally involves hypoplasia of the cerebellum but is now considered to be redundant by a few. The newly added variants include Chiari 0, which is essentially syringomyelia without hindbrain herniation. Chiari 1.5 is an interim variant between I and II without spinal dysraphism and Chiari 3.5 is an interim version of Chiari 3 and 4. Chiari V, the most severe variant, represents cerebellar agenesis with occipital lobe descent and herniation through the foramen magnum. Chiari 0 does not involve tonsillar herniation and including it as a Chiari variant is debatable. Likewise, CM3, CM3.5, CM4, and CM5 seem to represent separate conditions, with tonsillar ectopia playing a relatively minor role. Interestingly, while tonsillar herniation is a defining feature of Chiari malformations, not all individuals with tonsillar herniation can be included under Chiari malformation. The tonsillar herniation seen in conditions like spontaneous intracranial hypotension, idiopathic intracranial hypertension or in patients with hydrocephalus appear to have a aetiopathogenesis rather distinct from other Chiari types. This is evident by the fact that several authors have reported reversal of tonsillar herniation following successful treatment of the primary condition. Chiari postulated that underdevelopment of the bony posterior fossa—resulting in a reduced posterior fossa volume—was the primary cause of the condition. This theory is supported by the observation that women, who generally have smaller posterior fossae, exhibit a higher incidence of Chiari malformation. Furthermore, posterior fossa decompression often leads to the repositioning of the cerebellar tonsils and resolution of the syrinx in many, though not all, cases, adding credibility to the postulate that limited posterior fossa volume is a key etiological factor. However, this correlation has been challenged by reported cases of individuals with small posterior fossae but no Chiari malformation, as well as patients with Chiari malformations despite having normal posterior fossa volumes.[1] The association of Chiari malformation with syringomyelia is another unsolved mystery. Theories abound regarding the underlying pathogenesis, the oldest being the Gardeners who attributed it to incomplete perforation of the rhombencephalic roof during embryogenesis. Physiologic flow studies using phase contrast magnetic resonance imaging and ultrasound propose that occlusion of the subarachnoid spaces at foramen magnum leads to formation of the syrinx cavity. This obstruction may not necessarily be due to associated tonsillar descent and syringomyelia may exist in the absence of tonsillar ectopia as seen in Chiari 0. It needs to be noted that not all patients with Chiari develop an associated syringomyelia, and it is reasonable to conclude that syringomyelia results from cerebrospinal fluid (CSF) flow obstruction in the foramen magnum region, where tonsillar herniation may or may not be one of the contributing factors. The latest and rather controversial etiopathogenesis proposed by Goel et al. focuses on atlantoaxial joint instability as the prime culprit.[2] Tonsillar herniation, he believes, is not the primary cause but rather a compensatory mechanism, acting as an airbag that protects the bulbomedullary junction from dynamic compression. He advocates stabilizing the joint to resolve the alteration in CSF flow, alleviating symptoms, and preventing the progression of the condition. This varied aetiopathogenesis of Chiari malformation makes one wonder whether tonsillar herniation is a primary pathology or a secondary one – whether it is the Victim or the culprit !! Regardless of the nomenclature and the underlying pathophysiology what is of importance clinically is decision making in terms of intervention. It has now been increasingly recognized that in its simplest form it may not be a pathological variant at all requiring intervention and just a simple anatomical aberration best left alone. Current evidence thereby suggests that there is a significant “simple” group of Chiari that are best left alone and only patients with progressive clinical signs and symptoms need to be surgically managed. Commonly employed options include bony foramen magnum decompression alone, bony decompression with duroplasty, arachanoidolysis, and tonsillar resection. None of these techniques can claim superiority over the other. The dural ligamentous complex at the posterior fossa rim is unusually thick in almost all cases of craniovertebral junction anomalies.[3] We, therefore, believe in adding a lax duroplasty to the complement the bony decompression. The extent of bony foramen magnum decompression is again debatable. Guilherme et al. after analyzing the efficacy of C2 laminectomy in the treatment of Chiari malformation failed to provide a strong recommendation in view of the heterogeneity of techniques and the lack of standardized evaluation of results.[4] The optimal surgical management of Chiari malformation thus remains a holy grail, the search for which so far has been disappointing. Long-term outcomes following surgery remain unpredictable and surgery more often assists in arresting the progression. Goel et al., however, claim significant radiological and clinical resolution with their advocated technique of atlantoaxial stabilization.[2] As appealing as this theory may be, it still requires validation through larger studies and broader acceptance. A key criticism lies in the need to stabilize a joint that appears radiologically stable. Chiari malformation and syringomyelia remain enigmatic conditions, shrouded in mystery. When it comes to understanding their etiopathogenesis and determining the optimal management strategies, we are still far from our destination—with many miles to go before we sleep. Financial support and sponsorship Nil. Conflicts of interest There are no conflicts of interest.