Long-term outcome of operated Chiari I patients between 2005 and 2020 in Eastern Finland

Chiari malformation type 1 (CM1) is generally defined radiologically as cerebellar tonsillar herniation of more than 5 mm below the level of the foramen magnum (FM), but this definition has been criticized for its lack of clinical relevance and alternative definitions such as the functional obstruction of the FM by the inferior cerebellum have been proposed [23]. CM1 is associated with a large variety of symptoms—ranging from asymptomatic to severe. The primary treatment for symptomatic CM1 is the surgical decompression of the posterior fossa, which aims to re-establish cerebrospinal fluid flow and relief pressure on the hindbrain. Indications for surgical treatment of CM1 include persistent symptoms with a confirmed tonsillar herniation and/or the presence of a syrinx [15, 25]. Observation and follow-up are recommended for asymptomatic patients without a syrinx [4].

Surgical strategies for CM1 include posterior fossa decompression without dural opening (PFD) or posterior fossa decompression with duraplasty (PFDD). PFD and PFDD are found to differ regarding the probability of symptom improvement, complication rate, and syrinx resolution [1, 4]. PFDD is slightly favored compared to PFD due to the likelihood of clinical improvement (> 85% versus 80%) and improvement of syringomyelia [17]. PFDD also has a decreased likelihood of recurrence and reoperation rate in the presence of syringomyelia [4, 17, 26]. However, PFDD is associated with a higher rate of overall complications when compared to PFD [26]. PFD is considered an effective approach in pediatric patients as it is thought they have a more distensible dura, allowing for optimal decompression [27]. While PFD is also supported due to its low complication rate, it entails a higher risk of symptom recurrence requiring new surgery [1, 18]. The complication rate has been reported as 2.3–29% in the literature [1, 3, 6, 13, 16, 20, 21, 24, 25]. In a review of 145 operative series, the most common complications include pseudomeningocele, CSF leak, and aseptic meningitis, while surgical mortality was reported as 2–3% [3].

Successful management of CM1 heavily depends on both the correct indication to surgery and surgical technique, which should ultimately be tailored in accordance with the specific anatomical and clinical variations of the individual patient. This includes factors such as bony anatomy of FM, presence of syrinx, and extent of tonsillar herniation [4, 17].

Due to a lack of long-term outcome data regarding CM1 patients in Finland, our aim was to clarify the surgical outcomes of CM1 patients treated in Kuopio University Hospital between 2005 and 2020. Finland has a well-organized healthcare system with accessible, detailed long-term follow-up data, minimizing the likelihood of patients being lost during follow-up. Additionally, we aimed to examine how different PFDD techniques influenced these outcomes. We hypothesized that PFDD fixed by sutures would result in more favorable outcomes and less CSF-related complications compared to PFDD without sutures.

There is a paucity in the literature on which preoperative and intra-operative prognostic factors are associated with postoperative outcome in CM1 population [4, 15, 17]. In addition, the long-term outcome of operated CM1 patients in Finland has not been comprehensively reported. In this study, we aimed to clarify the long-term outcome of CM1 patients treated at KUH neurosurgery clinic over a 16-year period, and further analyze whether the different PFDD techniques affected overall outcome.

In CM1 treatment, the goal of the surgery is to relief the pressure within the posterior fossa and improve the CSF circulation around obliterated FM. This could be done by either performing bony decompression only (PFD) or further by duraplasty (PFDD) [1, 6, 26]. Previous studies have shown that PFDD might have a higher rate of clinical improvement but on the other hand, a higher rate of complications as compared with PFD only [1, 6, 19].

In our cohort, which is, the largest reported series of surgically treated CM1 patients in Finland [15], most of the patients underwent PFDD and no statistically significant factors between the variable extent of dural decompression, or type of duraplasty performed were identified to affect any postoperative outcomes, i.e., complication rate either. This may be due to the relatively small number of patients in the different subgroups. In this study, the majority (84%) of CM1 patients demonstrated overall improvement of preoperative symptoms, which is in line with previous literature, reporting improvement rates of 72–100% following FM decompression [1‐3, 6, 13, 15, 16, 20, 21, 24, 25].

Postoperative complication rates were 36.4% in the PFDD group and 0% in the PFD group; however, the PFD group consisted of only 6 patients, which may influence this result. Furthermore, there were no statistical differences in complication rates between different PFDD technique groups. Considering that the patients who underwent PFDD had more complications, the adjustment of different surgical techniques with the characteristics of the individual patient is essential [4, 17] and the use of PFD for well-selected patients should be kept in mind in the future. Furthermore, all patients in the less aggressive PFD group experienced improvement of symptoms, indicating successful patient selection for this subgroup.

In this study, the most common complication after CM1 surgery was aseptic meningitis in 25.8%, which is in line with previous literature [10]. Aseptic meningitis could be caused by exposure of blood and cellular debris to CSF during the operation [10], and a previous meta-analysis found an increased incidence of aseptic meningitis and CSF leak with PFDD compared to PFD [5]. In this study, most of the patients underwent PFDD procedure with opening of the arachnoid, which is reflected in the rate of aseptic meningitis, whereas in the PFD group there were no cases of aseptic meningitis. A previous study of 112 CM1 patients found that the rate of aseptic meningitis is lower in groups with autologous duraplasty, while the use of xenograft predisposed patients to postoperative aseptic meningitis after which they had a 9.8 times increased risk for requiring permanent CSF diversion [10]. They hypothesized that the use of xenograft or synthetic material as duraplasty may increase the risk for aseptic meningitis through immunologic reactions, as they contain more antigenic material compared to autografts [10]. Combining tissue sealants with a dural substitute may also increase the risk for aseptic meningitis and CSF leak; however, the underlying inflammatory mechanisms remain unclear [22]. In our study, PFDD was performed with xenograft (Dura-Guard & Durepair) in 11% patients and synthetic non-biological material (Neuro-Patch) in 62% patients. Moreover, tissue sealant was used in combination with all PFDD techniques. All these factors may explain our rate of aseptic meningitis. No autologous duraplasty material (pericranium, fascia) was used in our patients, but to reduce the rate of aseptic meningitis this should be considered more carefully in the future.

In this cohort, there was no mortality noted and the rate of major complications was low, as there were six (6.5%) cases of postoperative hydrocephalus requiring further shunt or ETV surgery and three cases of bacterial meningitis. Previous studies report a 5–8.7% incidence of hydrocephalus and 6.5–7.4% requirement for CSF diversion after PFDD [12, 28]. Rates of postoperative meningitis after CM1 surgery have been reported between 1 and 8%, with autografts being associated with the lowest and xenografts with the highest rates [14].

No patient in this study required additional FM decompression surgery. However, revision surgery was required due to pseudomeningocele, CSF leak, and hydrocephalus. Treatment of this patient group is challenging, as these CSF-related complications typically occur simultaneously in the same patient requiring for example shunt placement and wound revision. Postoperative aseptic or bacterial meningitis increases the risk for disturbances of CSF circulation and revision surgeries [10]. Moreover, revision surgeries further predispose patients to develop bacterial or aseptic meningitis, as noted in this study.

The presence of a syrinx due to CM1, even in asymptomatic patients, is often an indication for surgical decompression [8, 15]. In our study, all syringomyelia CM1 patients underwent PFDD, as supported by the current literature [4, 17, 26]. We found an 89.2% decrease of syrinx size after surgery, and the duraplasty technique did not significantly affect the outcome. These findings are comparable to previous studies reporting radiological and clinical improvement of syringomyelia in 65–88% of cases following FM decompression [9, 11, 15, 19]. Surprisingly, we found that the CM1 patients with syringomyelia experienced statistically significantly less postoperative complications (22% vs 43%) as compared with CM1 patients without syringomyelia.

The patient selection for surgery may play a role in this result, as accurately identifying the CM1 patients without syringomyelia whose condition will improve with FM decompression remains difficult due to unambiguity and high degree of personal subjectivity with preoperative symptomology, in addition to lack of other objective imaging parameters [7]. Therefore, the selection of CM1 patients without syringomyelia for decompressive surgery who are not susceptible to improvement while predisposing them to surgical complications and additional procedures may occur. Furthermore, the condition of almost all patients with syringomyelia due to CM1 is expected to stabilize or improve following FM decompression and duraplasty [24]. However, in this study, the improvement rate of preoperative symptoms was 84%, indicating successful patient selection. Considering the relatively high rate of postoperative complications with or without syringomyelia, the correct indication to surgery and a patient-tailored choice of surgical techniques are crucial in treatment.

This is a retrospective cohort study from a single neurosurgical center, entailing all the limitations of such a study design. The relatively few numbers of patients in certain groups made it difficult for our analyses to reach statistical significance. A prospective multi-institutional study with long-term follow-up data, using a validated outcome score should be considered.

In the present study, we found variability in surgical techniques performed for CM1 patients at KUH. The majority of patients underwent PFDD, including all syringomyelia patients. While no significant effect on outcome or complication rates between PFDD techniques were identified, postoperative improvement and complication rates were comparable to those reported in the literature. This study has analyzed the long-term surgical outcome data of CM1 patients treated in Kuopio University Hospital between 2005 and 2020, adding support to the current notion of tailoring the surgical approach to each individual patient.