Surgical Treatment of Chiari 1 Malformation: A ‘one fits all’ approach?

doi:10.21203/rs.3.rs-1738309/v1

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Surgical Treatment of Chiari 1 Malformation: A ‘one fits all’ approach?

https://doi.org/10.21203/rs.3.rs-1738309/v1

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OBJECTIVES

To assesses the postoperative outcome and the applicability of current Chiari 1 malformation (CM1) scores in a tailored surgical treatment approach in patients with CM1.

METHODS

This retrospective single center study analyzed 81 surgically treated patients between 2000 and 2020. Based on our institutional policy, four types of approaches were performed in that period based on a case by case decision: 1) Foramen magnum decompression (FMD) with dura splitting (FMDds), 2) FMD with duraplasty (FMDdp), 3) FMD with duraplasty and tonsillar manipulation (FMDao) and 4) tonsillar resection/reduction (TR). Patient characteristics, the Chicago Chiari Outcome Scale (CCOS), Chiari Severity Index (CSI), and the fourth ventricular roof angle (FVRA) were compared among the four groups to establish a treatment algorithm.

RESULTS

FMDds, FMDdp, FMDao and TR was performed in 11 (14%), 24 (30%), 21 (26%) and 25 (31%) patients, respectively. After a mean follow-up time of 6.6 years, 8/11 (73%), 19/24 (79%), 19/21 (90%) and 24/24 (100%) patients had a CCOS of 13–16 points after FMDds, FMDdp, FMDao and TR, respectively. Overall, 6/11 (55%), 4/17 (24%), 3/18 (17%) and 7/24 (29%) patients had a CSI grade 1 before FMDds, FMDdp, FMDao and TR. A weak negative and statistically not significant correlation between CCOS and CSI was observed (p = 0.07, n = 69). The preoperative FVRA was < 65° in 9/11 (82%), 13/15 (87%), 7/14 (50%) and 13/22 (59%) patients after FMDds, FMDdp, FMDao and TR, respectively. Despite the proportion of patients with a FVRA of < 65° decreasing with a more invasive surgical technique, differences between the surgical groups were statistically not significant (p = 0.45).

CONCLUSIONS

An individualized surgical approach in patients with CM1 is safe and effective. Currently available scores, such as the CSI and FVRA, may be used to determine the appropriate surgical approach for patients with CM1.

Chiari 1 malformation

Foramen magnum decompression

Tonsillar resection

Dural splitting

Syrinx

Chicago Chiari Outcome Scale

Chiari Severity Index

Chiari malformation type 1 (CM1) was already described by Hans Chiari in 1896 [6]. Still, the developmental mechanism of its origin, pathophysiological consequences, and treatment approaches remain poorly understood and controversially discussed [9, 24, 27]. Patients often present with mild or unspecific symptoms and the indication and type of a surgical treatment varies widely among centers [14]. Crowding at the level of the foramen magnum that results in disturbances of cerebrospinal fluid (CSF) flow and pulsation is one of the key problems and numerous approaches have attempted to resolve this challenge. The putatively easy task of creating space where there is no space has caused controversy over decades. While many neurosurgeons support the concept that a bony decompression with duraplasty is necessary in all patients irrespective of the clinical and radiological findings, others argue that a more tailored approach might be more adequate [15, 18, 23].

In addition, the selection of patients who may benefit from surgery can be challenging especially when symptoms and MRI findings are mild [22]. Various CM1 scores, including the Chiari Severity Index (CSI), Chicago Chiari Outcome Scale (CCOS), and fourth ventricular roof angle (FVRA), have been utilized to resolve this clinical equipoise [2, 16, 26]. While these scores are definitely helpful in the management of these patients, they do not take into consideration the many various surgical techniques available [4, 5, 7, 10, 12, 14, 17–19, 28–30]. Moreover, the FVRA has not yet been evaluated clinically [26].

Over the last two decades, it has been the policy at our institution that the heterogenous presentation of CM1 patients warrants a patient specific treatment rather than a one fits all approach. We selected the approach based on various clinical and neuroradiological variables on a case-by-case decision [9]. To verify the effectiveness of our concept we aimed to test the following hypotheses: (1) the CSI largely reflects our treatment stratification and may be used as a guidance for approach selection, with patients harboring CSI grade 1 being good candidates for a less invasive approach, (2) the fourth ventricular roof angle may be used to identify patients who need an intradural approach, and (3) the CCOS does not differ between our treatment groups and no surgical approach is superior or inferior to another.

We retrospectively reviewed the medical records of all patients who were operated for a CM1 at our institution between 2000 and 2020. The following parameters were assessed: patient´s demographics, pre- and postoperative neurological symptoms and neuroradiological characteristics including the preoperative CSI [16] and postoperative CCOS [2], the applied surgical technique and postoperative complications. We used the definition of CM1 according to the international consensus document by Massimi et al. and Ciaramitaro et al. [8, 22]. The FVRA was measured according to Seaman et al., using the midline sagittal T1- or T2-weighted sequences [26].

Surgical techniques

The different surgical techniques are summarized in Table 1. The procedures were performed in the prone position with the head in a neutral position and the upper body elevated by 30–40 degrees. After prepping and draping of the surgical field, a midline straight incision from the external occipital protuberance to the C2 spinous process was performed. A self-retaining retractor was placed and, after identifying the suboccipital planum, foramen magnum, and the posterior arch of C1, the atlantooccipital ligament was removed and the foramen magnum extended by performing a craniotomy of a 3x3cm sized bone flap. In most cases, this procedure was accompanied by a C1 laminectomy.

Table 1

Surgical techniques of craniocervical decompression
Dura splitting		- Incision or removal of the outer dural layer
Duraplasty	w/o arachnoidal opening	- Opening of the dura - Arachnoid membrane remains closed
	w/ arachnoidal opening	- Opening of dura and arachnoid membrane
Tonsillar reduction		- Resection of one or both tonsils - Reduction of one or both tonsils by subpial suction
w with, w/o without

In cases of “dura-splitting” (FMDds), the dura remained closed, with only the outer dural layer being incised or removed. If a “duraplasty” (FMDdp) was performed, the dura was opened in a modified Y-shaped fashion, always paying attention to keep the arachnoid membrane closed. If the surgeon had the impression of arachnoid scarring and adhesions, the arachnoid membrane was intentionally opened to loosen the tonsils (FMDao).

In cases of a distinct tonsillar descent without a pulsatile movement of the tonsils under direct inspection or with the help of ultrasound and prominent and/or gliotic changes of the cerebellar tonsils, a tonsillar resection or subpial reduction was performed. The decision between tonsillar resection and subpial reduction was up to the surgeon’s preference. In case of a subpial reduction technique, a small corticotomy at the mid-level of the tonsil was performed, followed by subpial suctioning until a sufficient reduction in the size of the tonsil was encountered. This approach theoretically prevents scarring of the tonsils at the level of the foramen magnum, which in turn may lead to a re-blocking of the CSF pulsation and circulation.

The dura was closed with an alloplastic dura graft (Neuro-patch®, B.Braun, Melsungen, Germany) and the muscular fascia was closed in a watertight fashion.

Patient´s follow-up

In general, patients were scheduled for the first follow-up visit 4–6 weeks after surgery in our outpatient clinic, usually followed by a second visit at 3–6 months’ time with a MRI and then on a yearly basis. Depending on the severity of the clinical and radiological findings this general schedule was adapted to meet the individual clinical needs.

Statistical Analysis

Quantitative variables are expressed as mean ± standard deviation (range) if normally distributed and as median (range) if not normally distributed. Qualitative variables are reported as absolute frequencies (percentage). Two-sided 95% confidence limits of percentages were computed using the Clopper-Pearson method. The Wilcoxon rank sum and Kruskal-Wallis tests were used for group comparisons of metric variables or ordinal variables when comparing two groups or three groups, respectively. Correlations between ordinal or metric variables were analyzed by Spearman correlation coefficients. The Fisher’s exact test was used to perform group comparisons of qualitative variables. Statistical analyses were performed using R software version 4.0.5. A p-value < 0.05 was considered as statistically significant. Due to the explorative character of the study, we did not adjust for multiple testing. The interpretation of the p-values is descriptive.

Patients’ characteristics

We identified 81 patients (male n = 29; female n = 52) with a mean age at operation of 29.7 years (range: 0.18–68.7) (Fig. 1). At presentation, the majority of the patients (76/81, 94%) were symptomatic. The mean time from onset of symptoms to the first outpatient visit was 27.8 months (range: 0-24.2 years). The mean time from diagnosis to surgery was 4.5 months (range: 0-5.4 years). The preoperative neurological symptoms and neuroradiological characteristics are summarized in Table 2 and Table 3.

Table 2

Preoperative neurological symptoms observed in patients with Chiari 1 malformation
Variable	Surgical technique (%)
Variable	all	FMDds	FMDdp/FMDao	TR	p-Value
Total number of patients	81	11 (14%)	45 (56%)	25 (31%)	-
Sex, male	29 (36%)	4 (36%)	17 (38%)	8 (32%)	0.95
Mean age – in years	29.7 ± 19.3	25.7 ± 23.8	33.5 ± 19.0	24.4 ± 16.9	0.15
Age < 18 years	32 (40%)	6 (55%)	13 (29%)	13 (52%)	0.10
Neurological symptoms	76 (94%)	10 (91%)	41 (91%)	25 (100%)	0.32
Pain	49 (60%)	8 (73%)	26 (58%)	15 (60%)	0.73
Suboccipital pain	30 (37%)	6 (55%)	14 (31%)	10 (40%)	0.32
Neck pain	31 (38%)	5 (45%)	16 (36%)	10 (40%)	0.85
Extremity pain	12 (15%)	0 (0%)	10 (22%)	2 (8%)	0.14
Headache	28 (35%)	5 (45%)	12 (27%)	11 (44%)	0.23
Holocranial	21	2	11	8	-
Bifrontal	7	3	1	3	-
Sensory symptoms	44 (54%)	3 (27%)	28 (62%)	13 (52%)	0.11
Motor deficits	19 (23%)	1 (9%)	15 (33%)	3 (12%)	0.08
Ataxia	29 (36%)	3 (27%)	19 (42%)	7 (28%)	0.47
Vertigo	21 (26%)	2 (18%)	13 (29%)	6 (24%)	0.82
Seizures	2 (2%)	1 (9%)	1 (2%)	0 (0%)	-
Vomiting	1 (1%)	0 (0%)	0 (0%)	1 (4%)	-
Dysphagia	3 (4%)	1 (9%)	1 (2%)	1 (4%)	-
Apnea	4 (5%)	0 (0%)	2 (4%)	2 (8%)	-
Syncope	3 (4%)	0 (0%)	1 (2%)	2 (8%)	-
Values are expressed as numbers (%) or as mean ± Standard Deviation (range). Fisher’s exact test was used to perform group comparisons of categorical variables. Metric variables were analyzed by Kruskal-Wallis tests. FMDds Foramen magnum decompression and dura-splitting, FMDdp Foramen magnum decompression and duraplasty, FMDao Foramen magnum decompression and arachnoid opening, TR tonsillar resection or subpial tonsillar reduction

Table 3

Preoperative neuroradiological characteristics of patients with Chiari 1 malformation
Variable	Surgical technique (%)
Variable	all (n = 81)	FMDds (n = 11)	FMDdp/FMDao (n = 45)	TR (n = 25)	p-Value
Tonsillar descent in mm – mean ± range NA	13.4 ± 5.8 (3–29) 16	13.7 ± 3.9 (7.9–21) 0	10.4 ± 4.4 (3–21) 13	17.6 ± 6.1 (6.5–29) 3	0.0001
Syrinx	48 (59%)	4 (36%)	29 (64%)	15 (60%)	0.25
width in mm – mean ± SD range NA	7.7 ± 3.9 (1.6–19) 13	5.6 ± 3.7 (2-9.9) 0	6.7 ± 2.9 (1.6–11.2) 12	9.5 ± 4.4 (2.7–19) 1	-
Syrinx location
cervical	8	0	3	5	-
cervicothoracic	31	4	22	5	-
thoracic	2	0	0	2	-
cervicothoracolumbal	7	0	4	3	-
Fourth ventricular roof angle in ° - mean ± SD range NA	59.3 ± 16.9 (28.7-103.6) 19	57.7 ± 12.0 (37.9–82.3) 0	58.2 ± 19.6 (28.7-103.6) 16	61.6 ± 15.6 (36.7–88.4) 3	0.69
Hydrocephalus	10 (12%)	2 (18%)	6 (13%)	2 (8%)	0.64
Scoliosis yes NA	13 (16%) 5	2 (18%) 0	5 (11%) 5	6 (24%) 0	0.44
Basilar invagination yes NA	24 (36%) 15	4 (36%) 0	9 (27%) 12	11 (50%) 3	0.22
Values are expressed as numbers (%) or as mean ± SD (range). Fisher’s exact test was used to perform group comparisons of categorical variables. Metric variables were analyzed by Kruskal-Wallis tests. FMDds Foramen magnum decompression and dura-splitting, FMDdp Foramen magnum decompression and duraplasty, FMDao Foramen magnum decompression and arachnoid opening, NA not available (i.e., number of missing observations), SD standard deviation, TR tonsillar resection or subpial tonsillar reduction

Table 4 Delineation of the CCOS, CSI and preoperative FVRA
Variable	CCOS
Variable	Improved (13–16)	Unchanged (9–12)	Worse (4–8)	NA
Surgical technique (n = 81)
FMDds (n = 11)	8 (73%)	2 (18%)	1 (9%)	0
FMDdp/FMDao (n = 45)	38 (84%)	7 (16%)	0 (0%)	0
TR (n = 25)	24 (100%)	0 (0%)	0 (0%)	1
	CSI
	1	2	3	NA
Surgical technique (n = 81)
FMDds (n = 11)	6 (55%)	4 (36%)	1 (9%)	0
FMDdp/FMDao (n = 45)	7 (20%)	17 (49%)	11 (31%)	10
TR (n = 25)	7 (29%)	9 (38%)	8 (33%)	1
	FVRA
	Mean (range, °)	< 65°	≥ 65°	NA
Surgical technique (n = 81)
FMDds (n = 11)	57.7 ± 12.0 (37.9–82.3)	9 (82%)	2 (18%)	0
FMDdp/FMDdao (n = 45)	58.2 ± 19.6 (28.7-103.6)	20 (69%)	9 (31%)	16
TR (n = 25)	61.6 ± 15.6 (36.7–88.4)	13 (59%)	9 (41%)	3
Values are expressed as numbers (%) or as mean ± Standard Deviation (range). CCOS Chicago Chiari Outcome Scale, CSI Chiari Severity Index, FMDds Foramen magnum decompression and dura-splitting, FMDdp Foramen magnum decompression and duraplasty, FMDao Foramen magnum decompression and arachnoid opening, FVRA fourth ventricular roof angle, NA number of missing observations, TR tonsillar resection or subpial tonsillar reduction

The mean follow-up time was 6.6 years (range: 0.49–265.1 months). Only one patient was lost to follow-up. Preoperative and postoperative MRI were available for analysis in 64/81 and 62/81 patients, respectively.

Surgical techniques

The decision for the surgical technique was dependent on the following factors: (1) neurological symptoms, (2) the existence and extent of a syrinx and (3) the degree of the tonsillar descent. For data analysis, patients were allocated to their respective surgical technique group based on their first operation and did not change treatment group after revision surgery. The following surgical techniques were performed based on a case-by-case decision:

Foramen magnum decompression and “dura splitting” (FMDds)

This approach was performed in 11/81 (14%) patients (Fig. 2). Preoperatively, 10/11 (91%) were symptomatic, 4/11 (36%) had a syrinx, the mean tonsillar descent was 13.7 ± 3.9 mm (range: 7.9–21 mm), and the mean FVRA was 57.7 ± 12° (range: 37.9–82.3°). A clinical improvement was observed in 6/11 (55%) patients. The tonsillar descent improved in 5/11 (45.5%) patients and remained unchanged in 5/11 (45.5%) patient.

Foramen magnum decompression and duraplasty (FMDdp)

This approach was performed in 45/81 (56%) patients (Fig. 3). The intention to keep the arachnoid membrane closed was achieved in 24/45 (53%) patients. In the remaining 21/45 (46%) small openings with CSF leakage were observed intraoperatively. Preoperatively, 22/24 (92%) were symptomatic, 14/24 (58%) had a syrinx, the mean tonsillar descent was 11.2 ± 3.9 mm (range: 4.6–17 mm), and the mean FVRA was 52.3 ± 17.9° (range: 29.1–89.8°). A postoperative clinical improvement was observed in 16/24 (67%) patients. Improved tonsillar descent was observed in 12/24 (50%) patients.

Foramen magnum decompression and arachnoid opening (FMDao)

The arachnoid membranes were opened intentionally in cases of arachnoid scarring and adhesions, judged intraoperatively by the surgeon. This approach was performed in 21 out of 45 (47%) patients, in which a duraplasty without tonsillar resection was performed. Before surgery, 19/21 (90%) were symptomatic, 15/21 (71%) had a syrinx, the mean tonsillar descent was 9.8 ± 4.8 mm (range: 3–21 mm), and the mean FVRA was 64.6 ± 20° (range: 28.7-103.6°). A postoperative neurological improvement was noted in 15/21 (71%) patients. Improved tonsillar descent occurred in 14/21 (67%) patients.

Foramen magnum decompression and tonsillar resection or subpial reduction (TR)

This approach was performed in 25/81 (31%) patients (Fig. 4). Before surgery, all patients (100%) were symptomatic, 15/25 (60%) had a syrinx, the mean tonsillar descent was 17.6 ± 6.1 mm (range: 6.5–29 mm), and the mean FVRA was 61.6 ± 15.6° (range: 36.7–88.4°). A postoperative clinical improvement was encountered in 23/25 (92%) patients, with 22/25 (88%) patients exhibiting improved tonsillar descent.

Postoperative complications

We experienced an overall complication rate of 13.6% (11/81) in this series. Seven of these eleven complications (64%) occurred in the FMDao group including 6/21 patients (29%) experiencing a CSF leakage and 1/21 patient (5%) suffering from intradural bleeding. Revision surgery was mandatory in five of these seven patients including subpial reduction of one tonsil in two and re-suturing of the dura allograft and tight closure of the muscular fascia in three of them. In the remaining two patients some secondary skin sutures were sufficient to stop the CSF leakage.

In the group of patients undergoing an upfront tonsillar resection, 2/25 patients (8%) had a CSF leakage, which required a revision in one patient and could be resolved by a secondary suture in the remaining. One additional patient suffered from an infarction of one posterior inferior cerebellar artery, which required urgent additional decompression resulting in an overall complication rate in this group of 12%.

In the FMDdp group only 1/24 patient (4%) had a CSF leakage, which could be managed by the insertion of a lumbar drain for one week.

We encountered no complications in the FMDds group. In 3/11 (27%) patients, however, surgery had to be repeated after a median time of 14 months (range: 10–39 months) due to unchanged symptoms postoperatively in two and recurrence in the remaining. A duraplasty was performed in two of them, while a tonsillar subpial reduction was performed in the remaining patient. The preoperative tonsillar descent was 14–21 mm. None had a syrinx.

Clinical Outcome

At last follow-up, 60/80 (75%) patients reported a clinical improvement, 19/80 (24%) patients reported an unchanged symptomatology and the remaining patient reported to have worsened symptoms. According to the CCOS, 70/80 (88%) patients had a CCOS between 13–16 points (improved) and 9/80 (11%) patients received 9–12 points (unchanged).

To evaluate if one surgical technique was superior compared with the others, we calculated the CCOS of our patients according to their surgical technique group. The CCOS was between 13–16 points in 8/11 (73%) patients after FMDds, 38/45 (84%) patients after FMDdp, and 24/24 (100%, one patient lost to follow-up) patients after TR (Table 4). Median CCOS (range) was 16 (6–16), 15 (10–16) and 16 (13–16) points in FMDds, FMDdp, and TR, respectively. Pairwise comparisons revealed a statistically significant difference between TR and FMDdp (Wilcoxon rank sum test: adjusted p-value [Bonferroni correction] = 0.021), but not between TR and FMDds or FMDds and FMDdp (adjusted p-value = 0.911 and 1, respectively).

Chiari Severity Index (CSI)

Among the 70 patients with sufficient preoperative clinical and imaging data available for the categorization of the CSI with one patient being lost to follow-up, the CSI was correlated with the postoperative outcome in 69 patients. Of these 69 patients 61 experienced a CCOS between 13–16 points (improved); 19/20 (95%) patients with CSI 1, 24/29 (83%) patients with CSI 2, and 18/20 (90%) patients with CSI 3. A weak negative and statistically not significant correlation between CCOS and CSI was observed (Spearman’s rho [95% CI]: -0.22 [-0.44; 0.02], p = 0.07, n = 69).

We then compared the CSI between the surgical technique groups in order to analyze if the CSI reflected our treatment stratification (i.e., more invasive approach with increasing CSI). A total of 6/11 (55% [23%;83%]) patients after FMDds, 7/35 (20% [8%; 37%]) patients after FMDdp/FMDao, and 7/24 (29% [13%; 51%]) patients after TR had a CSI 1 (Table 4). Comparison of the group of CSI 1 patients versus CSI 2/3 patients between the surgical technique groups revealed no significant difference. (p = 0.11, Fisher´s exact test)

Fourth ventricular roof angle (FVRA)

There were no statistically significant differences of the preoperative FVRA between the surgical technique groups (Kruskal-Wallis test: p = 0.69). Furthermore, no correlation was found between preoperative FVRA and CCOS or FVRA and CSI (Spearman’s rho [95% CI]: CCOS, -0.09 [-0.33; 0.17], p = 0.50, n = 62; CSI, 0.09 [-0.16; 0.34], p = 0.47, n = 62). We therefore dichotomized our patient cohort into a “<65°” and “≥65°” group, according to Seaman et al.[26]. A total of 9/11 (82%), 20/29 (69%), and 13/22 (59%) patients after FMDds, FMDdp and TR, respectively, were in the < 65° group. Despite the proportion of patients with a FVRA of < 65° decreasing with a more invasive surgical technique, differences between the surgical groups were statistically not significant (Fisher’s exact test: p = 0.45) (Table 4).

Impact of age on the surgical technique

A total of 32/81 (40%) patients were < 18 years of age at the time of surgery. The allocation of our patients according to their age group and the corresponding surgical technique are summarized in Table 2. The proportion of patients ≥ 18 years of age was statistically not significant among the different surgical technique groups (Fisher’s exact test: p = 0.098, n = 81). Furthermore, no statistically significant differences were observed between the age groups for the CSI (Wilcoxon rank sum test: p = 0.76, n = 70) and FVRA (Wilcoxon rank sum test: p = 0.384, n = 62). However, median CCOS was significantly higher in the < 18 years age group (median (range) < 18 years vs > = 18 years: 16 (12–16) vs 15 (6–16); Wilcoxon rank sum test: p = 0.005, n = 80).

Our concept of a patient specific surgical treatment resulted in a postoperative clinical improvement rate of 88% (95% Clopper-Pearson Confidence interval: [78%, 94%]), which is similar to previous reports [3, 7, 11, 19, 21, 29, 30]. This concept pursues the strategy to use the least invasive surgical technique necessary to achieve a satisfactory postoperative outcome in order to reduce the risk of complications from surgery for the individual patient. In this study we aimed to translate our institutional experience into a more widely applicable treatment algorithm by showing that the currently available clinical scores may be used in tailoring the best approach and act as surgical decision-making additives [1].

CSI as a stratification tool for the extent of surgery needed

We first tested if the CSI can be used as a stratification tool. This score was originally conceptualized as a grading system to predict which CM1 patients are most likely to benefit from surgery irrespective of the surgical approach. Our results indicate that the lower the CSI score the less invasive the approach needs to be to achieve a clinical improvement. Accordingly, patients harboring a CSI grade 1 may benefit from a dural splitting procedure only instead of a full tonsillar resection with duraplasty which is considered as the standard by many centers [18]. One may argue, however, that a less invasive approach harbors a significant risk for the need of a second surgery due to a high chance of recurring symptoms. However, in our series an escalation to a more invasive approach was only necessary in 5 out of 56 patients (9%), who did not receive upfront tonsillar resection. This need for an escalation should be discussed with the patients in the context of an increasing complication rate in response to more invasive approaches as seen in our series (0% FMDds; 4% FMDdp; 12% TR) and reported in the literature (23.7% after TR) [21].

In patients with a pronounced syrinx and severe neurological symptoms (CSI 2 and 3) a more invasive approach is necessary to achieve a satisfactory improvement [1, 16, 25]. Various techniques of tonsillar reduction/resection have been proposed [4, 12, 13, 21]. We have preferred a subpial reduction technique at our institution. We believe that this technique reduces the risk of damaging the surrounding neurovascular structures and risk of re-scaring at the level of the foramen magnum which in turn potentially leads to re-impairments of CSF circulation and pulsation. In addition, although we did not use intraoperative ultrasound routinely, we propose that its application further supports in the decision for the right surgical techniques, which is in line with previous notions [23].

In this context we have also learned from our analysis that the concept of pure arachnoid manipulation as performed in our FMDao group should be abandoned. In this group of patients we experienced the by far highest complication rate of 33%. Based on this experience, we believe that a pure manipulation of the arachnoid with the intend to just loosen the tonsils is insufficient to resolve the CSF pulsation and flow at the level of the foramen magnum. This may result in a persistently high CSF pressure and a high chance for a CSF leakage. Consequently, if the decision is made to dissect the arachnoid, at least one of the tonsils should be reduced in our opinion. While lower CSF leakage rates of 5.8–18.5% after arachnoid opening have been described in the literature [7, 10, 19, 20, 31, 33], a review by Arnautovic et al. corroborates with our notion reporting this complication rate in some series to be up to 47% [3].

FVRA to estimate if an intradural approach is recommended

Second, we tested if the FVRA may assist in selecting the appropriate surgical technique. The FVRA degree has been suggested to assess fourth ventricular bowing in CM1 and to be more predictive of brainstem dysfunction than tonsillar herniation [26]. Our analysis show that a cut off value of 65° may help in stratifying between the need for a more extensive surgical approach, i.e. intradural approach. However, this warrants further investigations as the number of patients was too low to draw definite conclusions.

It is a matter of the appropriate selection of the surgical technique rather than the technique itself

Lastly, we wanted to show that none of our surgical approaches per se is superior or inferior to another by evaluating our cohort according to the CCOS. While our analysis revealed a trend towards higher postoperative CCOS values in the more extensive approaches, the interpretation of these results needs to take into account that patients with mild and unspecific symptoms may have an unchanged score even with a successful surgical treatment [1, 32]. According to our concept, the FMDds approach was typically performed in patients with mild neurological symptoms and neuroradiological signs of CM1, which unsurprisingly showed less neurological or neuroradiological improvement compared with patients after more invasive approaches such as TR. Therefore, we think that none of our approaches is superior to another and suggest that FMDds should be sufficient in patients with mild neurological symptoms and mild neuroradiological signs of CM1, especially given that 73% in our cohort showed a CCOS between 13–16 points after this approach. Considering that the decision for a conservative or surgical treatment in this patient group may be particularly challenging, further supports the notion that the least invasive approach seems the most plausible one.

One of our limitations is the retrospective design of our study with all the inherent drawbacks. Furthermore, a statistical analysis seeking to evaluate a group stratification posthoc is humbled from a methodological standpoint. Sufficient pre- and postoperative imaging data was available in 64/81 and 62/81 patients only, which further reduces the power to draw any definite conclusions.

Our study suggests that an individualized surgical approach with escalating invasiveness in cases of a more severe clinical and neuroradiological presentation is safe and effective in patients with CM1. The least invasive approach necessary to achieve clinical improvement should be selected. To follow this concept, currently available scores, such as the CSI and FVRA, may be used to help in allocating patients to the right surgical approach.

Funding Information Open Access funding provided by Medical University of Vienna.

Conflict of Interest The authors declare that they have no conflict of interest.

Ethical Approval All procedures performed in studies involving human participants were in accordance with the ethical standards of the institutional and/or national research committee and with the 1964 Helsinki declaration and its later amendments or comparable ethical standards. The study was approved by the Ethics Committee of the Medical University of Vienna (EK 1549/2021).

Formal consent For this type of study formal consent is not required

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Surgical Treatment of Chiari 1 Malformation: A ‘one fits all’ approach?

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Abstract

OBJECTIVES

METHODS

RESULTS

CONCLUSIONS

Figures

Introduction

Methods And Material

Surgical techniques

Patient´s follow-up

Statistical Analysis

Results

Patients’ characteristics

Surgical techniques

Foramen magnum decompression and “dura splitting” (FMDds)

Foramen magnum decompression and duraplasty (FMDdp)

Foramen magnum decompression and arachnoid opening (FMDao)

Foramen magnum decompression and tonsillar resection or subpial reduction (TR)

Postoperative complications

Clinical Outcome

Chiari Severity Index (CSI)

Fourth ventricular roof angle (FVRA)

Impact of age on the surgical technique

Discussion

CSI as a stratification tool for the extent of surgery needed

FVRA to estimate if an intradural approach is recommended

Limitations

Conclusions

Declarations

References

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