Failed Foramen Magnum Decompression in Chiari I Malformation Is Associated With Failure to Restore Normal Intracranial Compliance: An Observational Cohort Study

Preoperative estimation of intracranial compliance in symptomatic children with Chiari malformation type 1: impact on outcome and risk of complications

BACKGROUND

The role of reduced intracranial compliance (ICC) in the outcome after foramen magnum decompression (FMD) was demonstrated in adults with Chiari malformation Type 1 (CMI). However, similar observations from children treated for CMI are missing.

METHODS

We reviewed pediatric cases of CMI referred to FMD between 2006 and 2022. Children with clinical and/or radiological signs suggesting reduced ICC (Group A) underwent overnight measurements of the pulsatile intracranial pressure (ICP): mean ICP wave amplitude (MWA) served as a surrogate marker of ICC. Children with more typical symptoms of CMI (Group B) underwent FMD without preoperative ICC estimation. This study presents the clinical, radiological, and outcome differences between these groups.

RESULTS

Sixty-four children (mean age 11.1 ± 4.3 years) underwent FMD: In Group A (n = 30), the finding of reduced ICC as estimated from preoperative ICP measurement resulted in CSF diversion (ventriculoperitoneal shunt) before FMD in 11 children. Two patients required shunt due to complications after FMD (total shunt rate 43%). In Group B (n = 34) treated with FMD without preoperative ICC estimation, five children (15%) required shunting due to complications. In Group A, we found a significantly higher frequency of headache, nausea, fatigue, and dizziness. The outcome assessed by the modified Chicago Chiari Outcome Scale (mean follow-up 83 ± 57 months) was comparable between the groups, but the complication rate after FMD was significantly lower in Group A (7% vs. 32%; p = 0.011). The number of procedures (ICP measurement, FMD, shunt, re-do FMD, shunt revisions) was significantly higher in Group A (2.6 ± 0.9 vs. 1.5 ± 1.1 per patient; p < 0.001).

CONCLUSION

In symptomatic children with CMI, the preoperative estimation of ICC from the overnight measurement of pulsatile ICP was more reliable for identifying those with reduced ICC than clinical and radiological assessment alone. When children with abnormally reduced ICC were identified and treated with CSF diversion before FMD, the complication rate was significantly reduced.

Combined Cerebrospinal Fluid Hydrodynamics and Fourth Ventricle Outlet Morphology to Improve Predictive Efficiency of Prognosis for Chiari Malformation Type I Decompression

OBJECTIVE

To identify the morphological characteristics together with cerebrospinal fluid (CSF) hydrodynamics on preoperative magnetic resonance imaging that improve the prediction of foramen magnum decompression (FMD) treatment outcome for Chiari malformations type I (CM-I) patients compared with the CSF hydrodynamics-based model.

METHODS

This retrospective study included CM-I patients who underwent FMD, phase-contrast cine magnetic resonance, and static MR between January 2018 and March 2022. The relationships of the preoperative CSF hydrodynamic quantifications derived from phase-contrast cine magnetic resonance and morphological measurements from static magnetic resonance imaging, clinical indicators with different outcomes, were analyzed with logistic regression analysis. The outcomes were determined using the Chicago Chiari Outcome Scale. The predictive performance was evaluated with receiver operating characteristic, calibration, decision curves and area under the receiver operating characteristic curve, net reclassification index, and integrated discrimination improvement and was compared with CSF hydrodynamics-based model.

RESULTS

A total of 27 patients were included. 17 (63%) had improved outcomes and 10 (37%) had poor outcomes. The peak diastolic velocity of the aqueduct midportion (odd ratio, 5.17; 95% confidence interval: 1.08, 24.70; P = 0.039) and the fourth ventricle outlet diameter (odd ratio, 7.17; 95% confidence interval: 1.07, 48.16; P = 0.043) were predictors of different prognoses. The predictive performance improved significantly than the CSF hydrodynamics-based model.

CONCLUSIONS

Combined CSF hydrodynamic and static morphologic MR measurements can better predict the response to FMD. A higher peak diastolic velocity of the aqueduct midportion and broader fourth ventricle outlet were associated with satisfying outcomes after decompression in CM-I patients.

Low versus High Intracranial Compliance in Adult Patients with Chiari Malformation Type 1-Comparison of Long-Term Outcome After Tailored Treatment

BACKGROUND

In patients with Chiari malformation type 1 (CMI) presenting with low intracranial compliance (ICC), foramen magnum decompression (FMD) often fails and the complication rate may be higher. We therefore routinely perform a preoperative assessment of ICC from intracranial pressure measurement. Patients with low ICC are treated with ventriculoperitoneal shunt (VPS) before FMD. In this study, we assess the outcome of patients with low ICC, compared with patients with high ICC treated with FMD alone.

METHODS

We reviewed the clinical and radiologic data of all consecutive patients with CMI treated between April 2008 and June 2021. ICC was assessed by overnight measurement of the pulsatile intracranial pressure: mean wave amplitude (MWA) above a previously defined threshold for abnormality was considered a surrogate marker for low ICC. The outcome was scored by Chicago Chiari Outcome Scale.

RESULTS

Of 73 patients, 23 with low ICC (average MWA 6.8 ± 1.2 mm Hg) received VPS before FMD, whereas 50 with high ICC (average MWA 4.4 ± 1.0 mm Hg) received only FMD. After a mean follow-up of 78.7 ± 41.4 months, 96% of all patients subjectively improved. The mean Chicago Chiari Outcome Scale score was 13.1 ± 2.2. Enlargement of cerebrospinal fluid space in the foramen magnum was achieved in 95% and regression of syringomyelia in 74% of cases. We found no significant difference in outcome between the patients with low and high ICC.

CONCLUSIONS

By identifying patients with CMI associated with low ICC and tailoring their treatment by VPS before FMD, we achieved favorable clinical and radiologic outcomes comparable to those with high ICC.

A new hypothesis for the pathophysiology of symptomatic adult Chiari malformation Type I

Chiari malformation Type I (CMI) is characterized by herniation of the cerebellar tonsils through the foramen magnum. The pathophysiology of CMI is not well elucidated; however, the prevailing theory focuses on the underdevelopment of the posterior cranial fossa which results in tonsillar herniation. Symptoms are believed to be due to the herniation causing resistance to the natural flow of cerebrospinal fluid (CSF) and exerting a mass effect on nearby neural tissue. However, asymptomatic cases vastly outnumber symptomatic ones and it is not known why some people become symptomatic. Recently, it has been proposed that CMI symptoms are primarily due to instability of either the atlanto-axial (AA) or the atlanto-occipital (AO) joint and the cerebellar tonsils herniate to prevent mechanical pinching. However, only a small percentage of patients exhibit clinical instability and these theories do not account for asymptomatic herniations. We propose that the pathophysiology of adult CMI involves a combination of craniocervical abnormalities which leads to tonsillar herniation and reduced compliance of the cervical spinal canal. Specifically, abnormal AO and/or AA joint morphology leads to chronic cervical instability, often subclinical, in a large portion of CMI patients. This in turn causes overwork of the suboccipital muscles as they try to compensate for the instability. Over time, the repeated, involuntary activation of these muscles leads to mechanical overload of the myodural bridge complex, altering the mechanical properties of the dura it merges with. As a result, the dura becomes stiffer, reducing the overall compliance of the cervical region. This lower compliance, combined with CSF resistance at the same level, leads to intracranial pressure peaks during the cardiac cycle (pulse pressure) that are amplified during activities such as coughing, sneezing, and physical exertion. This increase in pulse pressure reduces the compliance of the cervical subarachnoid space which increases the CSF wave speed in the spinal canal, and further increases pulse pressure in a feedback loop. Finally, the abnormal pressure environment induces greater neural tissue motion and strain, causing microstructural damage to the cerebellum, brainstem, and cervical spinal cord, and leading to symptoms. This hypothesis explains how the combination of craniocervical bony abnormalities, anatomic CSF restriction, and reduced compliance leads to symptoms in adult CMI.