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|NI FEATURE: THE EDITORIAL DEBATE IV-- PROS AND CONS
|Year : 2019 | Volume
| Issue : 3 | Page : 662-663
Chiari I malformation: Surgical considerations
Department of Neurosurgery, University of Texas Medical Branch at Galveston, Galveston, Texas, USA
|Date of Web Publication||23-Jul-2019|
Dr. Aaron Mohanty
Department of Neurosurgery, 301 University Boulevard, 9.204, Galveston, Texas 77555
Source of Support: None, Conflict of Interest: None
|How to cite this article:|
Mohanty A. Chiari I malformation: Surgical considerations. Neurol India 2019;67:662-3
Since the initial studies by Williams in the 1990s, crowding at the region of the foramen of magnum associated with cerebrospinal fluid (CSF) flow disturbances has been thought to be the pathophysiology of Chiari malformation More Details and associated syringomyelia. Considering this, foramen magnum decompression, removal of the posterior arch of Atlas More Details with various modes of posterior fossa dural expansion, has remained the mainstay of treatment of Chiari malformation for the past several decades. The overall results of bone decompression and duroplasty have been quite reproducible in various studies, making this procedure the first line of treatment in symptomatic patients. Syringomyelia often coexists with Chiari malformation, and the incidence has been reported to be as high as 22% in some studies. Associated syringomyelia in the cervical and often in the thoracic spine has been thought to be a progression of the pathophysiological abnormalities in the region of the craniovertebral junction associated with Chiari malformation.
Several diagnostic and management controversies are associated in the management of Chiari malformation. With a poorly documented natural history, the patients with asymptomatic Chiari malformation often present diagnostic and therapeutic dilemmas. With a magnetic resonance imaging (MRI) incidence of around 1%, it is not uncommon to have mild or moderate tonsillar descent in patients who are asymptomatic or are vaguely symptomatic. In addition, the extent of tonsillar herniation often poorly correlates with the type and severity of clinical manifestations. The degree of tonsillar descent has been found to change with time in nonoperated children; a recent study reported a reduction of a few millimeters in 38%, worsening in 12%, while it remained status quo in the rest of 50% patients over a 7-year follow-up period.
The not completely understood pathogenesis for Chiari and associated syringomyelia leads to controversy in the nature and extent of surgical management. The predominant discussion has been the extent of decompression and performance of duroplasty. It is well agreed that the three major complications of Chiari surgery – CSF leak, rescarring causing recurrence of the symptoms, and posterior fossa aseptic meningitis – are associated with dural opening and can be avoided with extradural decompression or partial-thickness dural opening. Decompression without duroplasty has also been found to have a shorter operative time, a shorter length of stay, and a lower total cost of initial hospitalization. A meta-analysis of 582 patients found equivalent outcome with respect to the resolution of symptoms and syringomyelia after Chiari decompression with and without expansile duroplasty. The duroplasty group, however, required a lower reoperation rate but had a higher incidence of CSF leak. In another study of 156 children, about 90% had improvement or resolution of symptoms following posterior fossa decompression without dural opening. No patients with less than 8 mm descent required reoperation, while patients with tonsillar descent till the mid or inferior margin of C2 required re-surgery. This observation of failure of limited posterior fossa expansion in patients without duroplasty with a significant tonsillar herniation has also been acknowledged by others. Some have reserved it only for patients without syringomyelia. In a survey of pediatric neurosurgeons pertaining to symptomatic Chiari 1 malformation, most considered duroplasty as the primary option, although 6% mentioned that they would avoid a duroplasty and another 6% indicated using ultrasound guidance to decide on duroplasty. An associated syrinx reduced the incidence of the two later groups from 6% to 4%.
In the recent years, attention has been focused towards the associated bony abnormalities and craniovertebral junction abnormality in Chiari I malformation. Association of craniovertebral junction abnormality predisposes to instability in the region which may require a fusion. Basilar invagination, which may be associated in up to 38% of cases, would require craniovertebral junction stabilization. Ventral brain stem compression by a retroflexed odontoid is often observed in about 20% of cases. The amount of retroflexion of the odontoid is measured by a pB-C2 line and is used to predict the necessity of a fusion. A line is drawn extending from the basion to the posterior and inferior point of the body of C2 (B-C2 line); the pB-C2 is the perpendicular distance between the ventral dura and the B-C2 line. A pB-C2 distance of more than 9 mm has been found to be associated with a symptomatic ventral brainstem compression and craniovertebral junction abnormality requiring fusion. Another measurement, the clivoaxial angle, is the angle formed by Wackenheim's clivus base line and the line drawn along the posterior surface of C2 and odontoid process. A clivoaxial angle of 145–160° in a neutral position is considered to be normal. The clivoaxial angle reduces by about 10° in flexion and increases by 10° in extension. An angle of less than 125° has been associated with the requirement for a stabilization procedure. In addition, patients who have associated brain stem herniation through the foramen magnum (Chiari 1.5, where the obex is located below the level of foramen magnum associated with tonsillar herniation, in the preoperative MRI) may require a a fusion. Moreover, association of genetic syndromes such as Ehlers–Danlos syndrome, Marfans syndrome, and Down's syndrome has also been associated with craniovertebral junction abnormality.
In Chiari malformation, the tonsillar descent with crowding in the region of the foramen magnum obstructs the normally observed rapid bidirectional CSF flow occurring during the cardiac cycle. This obstruction has been postulated to result in greater cervical subarachnoid pressure waves which compress the spinal cord and force the CSF to enter into the spinal cord through the perivascular spaces, thus initiating and propagating the syrinx. Considering this, several studies have described the CSF flow characteristics in patients with Chiari I malformation – either by radionuclide cisternogram  or with cine-flow MRI., Some of them have attempted to measure the CSF flow velocities in the region of foramen magnum across the obstruction in the pre- and postoperative period. A heterogeneous pattern of CSF flow with regions of increased CSF velocity (CSF velocity jets) and regions of reduced velocity, has been described in the preoperative imaging. With decompression, the flow pattern returns to the normal sinusoidal pattern with reduction in the velocity of the flow. The results have revealed improvement of the varied, heterogeneous CSF flow with some authors finding its correlation with the outcome., However, studies have failed to find a CSF flow characteristic which may consistently differentiate between symptomatic, and asymptomatic or mildly symptomatic patients.
The authors of the study reported in this issue found a decrease in the peak CSF velocity at the foramen magnum following foramen magnum decompression without duroplasty, at 6 months of follow-up. However, on analyzing the CSF flow velocity, they did not find any correlation between the changes in maximal CSF velocities and the degree of clinical improvement. The authors suggest that the degree of CSF flow obstruction should be considered as a criterion for the preoperative selection of patients for posterior fossa decompression. In the future, it would be interesting to see whether the criteria based on the preoperative cine MRI findings would suggest the nature and extent of surgical intervention for a given patient.
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