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Neurosurgery:
doi: 10.1227/NEU.0000000000000032
Research-Human-Clinical Studies

Comprehensive Assessment of 1-Year Outcomes and Determination of Minimum Clinically Important Difference in Pain, Disability, and Quality of Life After Suboccipital Decompression for Chiari Malformation I in Adults

Parker, Scott L. MD; Godil, Saniya S. MD; Zuckerman, Scott L. BS; Mendenhall, Stephen K. BS; Wells, John A. BS; Shau, David N. BS; McGirt, Matthew J. MD

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Author Information

Department of Neurosurgery, Vanderbilt University Medical Center, and Vanderbilt Spinal Column Surgical Outcomes and Quality Research Laboratory, Nashville, Tennessee

Correspondence: Scott L. Parker, MD, 4347 Village at Vanderbilt, Nashville, TN 37232-8618. E-mail: scott.parker@vanderbilt.edu

Received April 22, 2013

Accepted June 4, 2013

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Abstract

BACKGROUND: To date, there has been no study to comprehensively assess the effectiveness of suboccipital craniectomy (SOC) for Chiari malformation I (CMI) using validated patient-reported outcome measures.

OBJECTIVE: To determine the effectiveness and minimum clinically important difference thresholds of SOC for the treatment of adult patients with CMI using patient-reported outcome metrics.

METHODS: Fifty patients undergoing first-time SOC and C1 laminectomy for CMI at a single institution were followed up for 1 year. Baseline and 1-year postoperative pain, disability, quality of life, patient satisfaction, and return to work were assessed. Minimum clinically important difference thresholds were calculated with 2 anchors: the Health Transition Index and North American Spine Society satisfaction questionnaire.

RESULTS: The severity of headaches improved in 37 patients (74%). Improvement in syrinx size was seen in 12 patients (63%) and myelopathy in 12 patients (60%). All patient-reported outcomes showed significant improvement 1 year postoperatively (P < .05). Of the 38 patients (76%) employed preoperatively, 29 (76%) returned to work postoperatively at a median time of 6 weeks (interquartile range, 4-12 weeks). Minimum clinically important difference thresholds after SOC for CMI were 4.4 points for numeric rating scale for headache, 0.7 points for numeric rating scale for neck pain, 13.8 percentage points for Headache Disability Index, 14.2 percentage points for Neck Disability Index, 7.0 points for Short Form-12 Physical Component Summary, 6.1 points for Short Form-12 Mental Component Summary, 4.5 points for Zung depression, 1.7 points for modified Japanese Orthopaedic Association, and 0.34 quality-adjusted life-years for Euro-Qol-5D.

CONCLUSION: Surgical management of CMI in adults via SOC provides significant and sustained improvement in pain, disability, general health, and quality of life as assessed by patient-reported outcomes. This patient-centered assessment suggests that suboccipital decompression for CMI in adults is an effective treatment strategy.

ABBREVIATIONS: CI, confidence interval

CMI, Chiari malformation type I

EQ-5D, Euro-Qol-5D; HDI, Head Disability Index

HTI, Health Transition Index

MCID, minimum clinically important difference

MCS, Mental Component Summary

mJOA, modified Japanese Orthopaedic Association

NASS, North American Spine Society

NDI, Neck Disability Index

NRS, numeric rating scale

PCS, Physical Component Summary

PRO, patient-reported outcome

ROC, receiver-operating characteristic

SF-12, Short Form-12-Item Survey

Chiari malformation type I (CMI) is a fairly common anomaly of the hindbrain that was first classified over a century ago by Hans Chiari during postmortem examination.1 Although the true frequency of CMI is unknown, Speer et al2 estimated a prevalence of 0.1% to 0.5%. CMI represents a heterogeneous group of conditions characterized by a ≥5-mm downward displacement of the cerebellar tonsils into the foramen magnum, which may lead to compression, central cord, and cerebellar symptoms.3,4 Approximately 60% to 90% of patients also have associated syringomyelia caused by obstruction of cerebrospinal fluid (CSF) at the level of the foramen magnum.4

CMI is commonly considered a condition of the pediatric population, but the majority of patients become symptomatic in the second or third decade of life. The pathogenesis and progression of CMI remain uncertain. A broad spectrum of clinical manifestations is present in patients with CMI, with the onset of symptoms often occurring insidiously once the patient reaches adolescence or adulthood.4-6 The most dominant symptom in CMI is tussive suboccipital headache, which ranges from intermittent to persistent headache and is aggravated by coughing or physical exertion.4 Symptoms such as cranial nerve dysfunction, sensory loss, dysphagia, snoring, pain, and weakness are often associated with syringomyelia. Left untreated, these symptoms may hinder daily function and significantly decrease overall quality of life.4,5,7

Treatment for asymptomatic patients remains controversial8,9; however, surgical decompression is highly recommended in symptomatic patients with CSF flow obstruction.10,11 There is little consensus on the surgical technique used for decompression, although it is commonly achieved via a suboccipital craniectomy and C1/C2 laminectomy, with or without dural splitting, duraplasty, tonsillar coagulation/resection, arachnoid lysis, and posterior fossa reconstruction, with the goal being an increase in retrocerebellar space and improvement in CSF flow. A recent meta-analysis by Durham and Fjeld-Olenec12 concluded that surgical decompression without duraplasty is associated with a lower complication rate compared with surgical decompression with duraplasty, with no difference in the 2 surgical techniques with respect to clinical and radiological improvement.

Several studies suggest that the severity and duration of CMI-associated symptoms before surgical treatment affect postoperative outcomes.7,13-16 Although posterior decompression via suboccipital craniectomy is widely accepted as the surgical treatment of choice and has been carried out for several decades, outcomes after decompressive surgery have been widely variable and inconsistent.8,14-24 To date, no study has comprehensively assessed the effectiveness of suboccipital craniectomy with validated patient-reported outcome (PRO) measures. None of the PROs available have been validated in the CMI patient population, and there is no gold standard for assessing outcomes in this population. In light of this, we set out to determine the effectiveness of suboccipital craniectomy for the treatment of adult patients with CMI using PRO metrics. Because suboccipital headache and neck pain often represent the most disabling symptoms for these patients, it is important to assess improvement in headache and neck pain. Therefore, we used a numeric rating scale (NRS) for neck pain (NRS-Neck) and head pain (NRS-Head) to assess the severity of neck and head pain and the Neck Disability Index (NDI) and Headache Disability Index (HDI) to assess disability related to neck pain and headache. Because a recognized shortcoming of such questionnaires is that their numerical scores lack a direct, clinically significant meaning, the concept of a minimum clinically important difference (MCID) has been created to measure the critical threshold needed to achieve clinically meaningful treatment effectiveness. Treatment effects that are greater than or equal to the MCID threshold imply clinical significance and justification for implementation in clinical practice. Simply put, MCID is the smallest change in the outcome measure that is important to patients. In the present study, we also determined the most appropriate MCID threshold values for the PRO measures assessed.

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PATIENTS AND METHODS

Patient Selection

Fifty patients undergoing first-time suboccipital craniectomy and C1 laminectomy for CMI at a single medical center were followed up for 1 year. The primary inclusion criteria were radiological evidence of CMI (Figure 1); tussive suboccipital headache, cranial nerve dysfunction, and/or cerebellar symptoms; age of 18 to 70 years; and failure of medical management of symptoms. Patients were excluded if they had history of a previous cervical spinal surgery.

Figure 1
Figure 1
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Surgical Technique

The patient was positioned prone on chest rolls with the head placed into 3-point Mayfield fixation. The neck was flexed to open space between the occiput and posterior arch of C1 and secured, maintaining a neutral position. The occipitocervical spine region was shaved, prepped, and draped in a sterile fashion. A midline incision from the inion to the C2 spinous process was made, followed by subperiosteal dissection down the midline raphe to expose the inferior aspect of the occiput and the lamina of C2 superiorly.

The pneumatic drill was then brought into the field using a cutting tip. Suboccipital craniectomy was performed and taken up to the inion, as determined by preoperative magnetic resonance imaging scan. Removal of bone above the foramen magnum did not exceed 3 cm high by 3 cm wide. The drill was then used to perform the C1 laminectomies, and rongeurs were used to complete the laminectomies laterally. If a pericranial graft was harvested, it was done at this time. Depending on surgeon preference, the ultrasound was brought into the field to assess tonsillar movement and CSF pulsation across the cranial cervical junction. If constricting dural bands were visualized, they were lysed and ultrasound was again used to assess flow. If CSF pulsation was diminished, a durotomy was performed.

The dura was then opened with a Y-shaped incision, extending superiorly to the cerebellar hemispheres and vermis. Care was taken not to enter the transverse sinus, given its low-lying position in Chiari malformations. Venous bleeding was tamponaded and cauterized. If significant arachnoid thickening was noted, the arachnoid adhesions were lysed. If tonsillar coagulation was performed, it occurred at this time. Once the area was well decompressed, the ultrasound was brought into the field to assess tonsillar movement and CSF pulsation.

If a pericranial graft was not harvested, a synthetic dural graft was brought into the field and prepared per manufacturer recommendations. The Alloderm dural patch (LifeCell Corp) and Medtronic Durepair system were the 2 different kinds of synthetic grafts used in our patient population. The graft was cut to shape and sewed to the lateral edges of the exposed durotomy with interrupted 4-0 Nurolon sutures. Before final closure was completed, the intradural region was irrigated to check for bleeding or retained blood clots. On completion of closure, Valsalva was performed to assess for CSF leak along suture lines. A watertight closure was not achieved in all cases with interrupted stitches, and Duraseal sealant was used for augmentation in these cases. The wound was then copiously irrigated and closed in layers.

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Clinical Outcome Measures

Patient demographics, comorbidities, clinical presentation, indications for surgery, radiological studies, operative variables, and surgical morbidity were assessed for each case. Baseline and 1-year postoperative pain, disability, quality of life, patient satisfaction, narcotic use, and return to work were assessed via phone interview by an independent investigator not involved with clinical care. PRO instruments included NRS-Neck and NRS-Head,25 NDI,26,27 HDI,28 Zung Depression Scale, Euro-Qol-5D (EQ-5D),29 Short Form 12-Item Survey (SF-12; Physical Component Summary [PCS] and Mental Component Summary [MCS]),30 and modified Japanese Orthopaedic Association (mJOA).31,32

Patients were asked to rate their current health compared with their health state before surgery using the Health Transition Index (HTI) of the Short Form-36 health survey. The choices provided include “markedly better,” “slightly better,” “unchanged,” “slightly worse,” and “markedly worse.” In the MCID analysis, patients answering “slightly better” or “markedly better” were classified as responders; those answering “unchanged,” “slightly worse,” or “markedly worse” were classified as nonresponders.33-40 A 4-item North American Spine Society (NASS) Satisfaction Questionnaire was used to determine patients’ satisfaction with their surgery. The choices provided include the following: choice 1, the treatment met my expectations; choice 2, I did not improve as much as I had hoped, but I would undergo the same treatment for the same outcome; choice 3, I did not improve as much as I had hoped, and I would not undergo the same treatment for the same outcome; and choice 4, I am the same as or worse than before treatment. For the purposes of the MCID analysis, patients answering choice 1 were classified as responders, whereas those answering choices 2 through 4 were classified as nonresponders.33-40

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MCID Threshold Calculation

We defined the MCID threshold as the lower limit of the 95% confidence interval (CI) for the median change score of each outcome metric for the patients classified as responders based on each anchor (NASS satisfaction and HTI). Additionally, the probability that scores will correctly discriminate between responders and nonresponders (accuracy) was depicted by the area under the receiver-operating characteristic (ROC) curve. This value ranges from 0.5 (discrimination is no better than pure chance) to 1.0 (all patients are able to be correctly discriminated). An area of 0.7 to 0.8 is considered adequate; an area of 0.8 to 0.9 is considered excellent.38

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RESULTS

Patient Population

A total of 50 patients were enrolled in this study. Overall, the mean age at the time of surgery was 38.5 ± 13.5 years, with 14 men (28%) and 36 women (72%). Twelve patients (24%) had chronic obstructive pulmonary disease, 12 (24%) had depression, 10 (20%) had hypertension, 10 (20%) had a history of seizures, 7 (14%) had heart disease, and 17 (34%) were current smokers (Table 1).

Table 1
Table 1
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Clinical Presentation

All patients presented with tussive suboccipital headache and radiographic evidence of CMI. Twenty patients (40%) had myelopathy as assessed by mJOA, 20 (40%) had a syrinx, 13 (26%) had cranial nerve dysfunction, and 3 (6%) had baseline ventriculomegaly. Only 9 of 20 patients (45%) with syrinx presented with myelopathy. Mean tonsillar descent below foramen magnum was 9.5 ± 4.1 mm (Table 1). Of the 20 patients (40%) presenting with a syrinx, the syrinx extended into the cervical cord in 7 (35%), extended into the thoracic cord in 12 (60%), and was holocord in 1 (5%).

For all patients at presentation, mean NRS-Neck and NRS-Head were 4.8 ± 4.0 and 8.1 ± 2.4, respectively. Mean preoperative HDI and NDI scores were 51.6 ± 33.6% and 45.0 ± 25.9%, respectively. Mean preoperative SF-12 PCS and SF-12 MCS scores were 37.7 ± 10.7 and 43.4 ± 15.2, respectively. Mean preoperative Zung depression and mJOA scores were 36.6 ± 13.2 and 13.7 ± 3.0, respectively. Mean preoperative EQ-5D perceived health state was 0.49 ± 0.23 quality-adjusted life-years.

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Operative Technique

Suboccipital craniectomy with C1 laminectomy was performed in all patients. Intraoperative ultrasound was used in 18 patients (36%). Duraplasty was performed in 44 patients (88%), with synthetic dural graft used in 37 (84%) and pericranial graft in 7 (16%). The Alloderm dural patch and Medtronic Durepair system were the 2 different kinds of synthetic grafts used in our patient population. A partial-thickness durotomy was performed in 3 patients (6%), and the dura was not opened in the remaining 3 patients (6%). Arachnoid lysis was performed in 17 patients (34%) and tonsil coagulation in 4 patients (8%). Mean length of postoperative hospital stay was 3.0 ± 2.2 days.

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Postoperative Outcomes

The severity of headaches improved in 37 patients (74%), remained the same in 11 (22%), and worsened in 2 (4%; Figure 2). Of the 20 patients (40%) presenting with a syrinx, 19 (95%) had postoperative magnetic resonance imaging scans. Improvement in syrinx size was seen in 12 patients (63%; 4 cervical, 8 thoracic), whereas the remaining 7 patients (37%; 3 cervical, 4 thoracic) showed no change. Improvement in myelopathy was seen in 12 patients (60%). Baseline ventriculomegaly improved in 1 of 3 patients (33.3%).

Figure 2
Figure 2
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The most common complication was development of a postoperative pseudomeningocele, which occurred in 9 patients (18%). Definitive treatment of the pseudomeningocele included placement of a ventriculoperitoneal shunt (n = 6, 67%), primary dural closure (n = 2, 22%), and temporary lumbar drainage (n = 1, 11%). Four patients (8%) presented back with CSF leaks and concern about infection, requiring antibiotic treatment.

For all patients, at 1 year postoperatively, mean improvement in NRS-Head and NRS-Neck scores was 4.3 (95% CI, 3.3-5.3; P < .001) and 1.2 (95% CI, 0.1-2.6; P = .03), respectively. Mean improvement in HDI and NDI scores was 17.7% (95% CI, 8.0-27.4; P < .001) and 9.3% (95% CI, 0.9-17.8; P = .04), respectively. Mean improvement in SF-12 PCS and SF-12 MCS scores was 5.5 (95% CI, 1.6-9.2; P = .01) and 5.3 (95% CI, 1.4-9.1; P = .01), respectively. Mean improvement in Zung depression and mJOA scores was 3.7 (95% CI, −0.1 to 7.5; P = .06) and 2.0 (95% CI, 1.1-2.9; P = .02), respectively. Mean improvement in EQ-5D was 0.29 quality-adjusted life-year (95% CI, 0.21-0.38; P < .001; Figure 3).

Figure 3
Figure 3
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Using HTI, 22 (44%) patients classified themselves as significantly better, 11 patients (22%) classified themselves as slightly better, 6 patients (12%) classified themselves as the same, 8 patients (16%) classified themselves as slightly worse, and 3 patients (6%) classified themselves as markedly worse. Thus, 33 patients (66%) were classified as responders and 17 (34%) as nonresponders for the HTI anchor (Figure 4A). For the NASS satisfaction anchor, 28 (56%) answered that the surgery met their expectations (choice 1); 12 patients (24%) answered that they did not improve as much as hoped but would undergo same treatment (choice 2); 4 patients (8%) answered that they would not undergo same treatment (choice 3); and 6 patients (12%) answered that they were the same as or worse than before treatment (choice 4). Thus, 28 patients (56%) were classified as responders and 22 patients (44%) as nonresponders for the NASS satisfaction anchor (Figure 4B).

Figure 4
Figure 4
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Of the 38 patients (76%) employed preoperatively, 29 (76%) returned to work postoperatively at a median time of 6 weeks (interquartile range, 4-12 weeks; Figure 5).

Figure 5
Figure 5
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Subset Analyses

Baseline PROs were similar in patients with and without syrinx except for Zung depression scores (Table 2). NRS-Head, HDI, and EQ-5D scores were significantly improved at 1 year in both cohorts; however, significant improvement in 1-year SF-12 PCS, SF-12 MCS, and mJOA scores was seen only in patients without syrinx (Table 3).

Table 2
Table 2
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Table 3
Table 3
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The degree of tonsillar herniation significantly correlated with the severity of symptoms (preoperative PROs) and postoperative outcomes at 1 year (P < .05).

There was significant difference in rate of complications (25.0% vs 52.4%; P = .04) in patients who were satisfied vs those who were not satisfied with their outcome, but improvement in myelopathy (56.5% vs 29.4%; P = .09), return to work (85.7% vs 6.7%; P = .13), and improvement in syrinx (45.5% vs 87.5%; P = .06) were similar.

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MCID Thresholds

Improvements in each PRO instrument for patients classified as responders and nonresponders on the basis of the HTI and NASS anchors are provided in Figure 6 and Table 4. MCID thresholds (lower limit of 95% CI for the median change score of each outcome metric) and the area under the ROC curve for each of the PRO instruments assessed are provided in Table 5. All ROC curves were statistically significant (P < .05) except NRS-Neck and SF-12 MCS with the NASS anchor. The area under the ROC curve ranged from 0.68 to 0.93 for the HTI anchor and 0.59 to 0.87 for the NASS satisfaction anchor. The area under the ROC curve was consistently greater for the HTI anchor for the vast majority of PRO instruments. Additionally, the average area under the ROC curve for the 9 PRO measures assessed was greater for the HTI than for the NASS anchor (0.80 vs 0.76), suggesting that HTI is the more valid anchor in this patient population. With the use of the HTI anchor, the MCID thresholds for adult patients undergoing suboccipital decompression for MCI was 4.4 points for NRS-Head, 0.7 points for NRS-Neck, 13.8 percentage points for HDI, 14.2 percentage points for NDI, 7.0 points for SF-12 PCS, 6.1 points for SF-12 MCS, 4.5 points for Zung depression, 1.7 points for mJOA, and 0.34 quality-adjusted life-years for EQ-5D.

Figure 6
Figure 6
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Table 4
Table 4
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Table 5
Table 5
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DISCUSSION

In a 1-year longitudinal cohort study, we set out to determine the effectiveness and associated MCID thresholds of suboccipital craniectomy and C1 laminectomy for the treatment of adult CMI using validated PRO metrics. We observed a significant and sustained improvement in all PROs assessing pain, disability, general health, quality of life, depression, and myelopathy. Additionally, tussive headache, one of the main presenting symptoms of CMI, improved in 74% of the patients. Approximately 66% of the patients reported improvement in their general health 1 year after surgery; 80% would undergo the same surgery again given their outcomes; and 76% of the patients employed preoperatively returned to work after surgery at a median time of 6 weeks. These benefits are consistent with other elective neurosurgical procedures that have been studied previously with these or similar satisfaction metrics,33,34,36,37,41 and demonstrate that suboccipital craniectomy and C1 laminectomy are clinically effective treatments for patients with CMI. The MCID thresholds of this patient population were found to be 4.4 points for NRS-Head, 0.7 points for NRS-Neck, 13.8 percentage points for HDI, 14.2 percentage points for NDI, 7.0 points for SF-12 PCS, 6.1 points for SF-12 MCS, 4.5 points for Zung depression, 1.7 points for mJOA, and 0.34 quality-adjusted life-year for EQ-5D.

CMI affects up to a quarter million people in the United States.42 The majority of cases occur in early to middle adulthood. The mean age at the time of presentation is 35.9 ± 16.8 years, with a female preponderance.7 Although patients can have a variable clinical presentation, the most common and earliest presenting symptom is tussive suboccipital headache that is exacerbated by coughing or physical exertion.3 Caudal displacement of the cerebellar tonsils into the foramen magnum leads to compression, central cord, and cerebellar symptoms, and as a result, patients can present with pain, dizziness, numbness, sensory loss, weakness, and cranial nerve dysfunction.6,7,20 The clinical presentation of patients in our series was comparable to that in the literature. In our study, the mean age at presentation was 38.5 ± 13.5 years, and the majority of patients (72%) were female. All patients had tussive suboccipital headache, and many patients presented with cranial nerve dysfunction or myelopathic symptoms. Syrinx was present in 40% of the patients, which is similar to the reported rate of 50% to 90% in the literature.4,43

Treatment for asymptomatic CMI patients remains controversial8,9,21; however, surgical decompression is highly recommended in symptomatic patients with CSF flow obstruction.10,11 Several studies have suggested that the severity and duration of CMI-associated symptoms before surgical treatment affect postoperative outcomes.7,13-16 There is little consensus concerning the surgical technique used for CMI, and many surgical approaches have evolved over time. These include extensive posterior fossa craniectomy, cervical (C1/C2) laminectomy, intradural dissection of syrinx, duraplasty, plugging of the obex, tonsillar coagulation/resection, arachnoid lysis, and posterior fossa reconstruction with the goal of increasing retrocerebellar space and improving CSF flow. A recent meta-analysis was conducted by Durham and Fjeld-Olenec12 on 7 studies (5 retrospective and 2 prospective) including 582 patients with CMI that compared posterior fossa decompression with and without duraplasty. It concluded that surgical decompression with duraplasty is associated with a lower rate of reoperation but a higher rate of CSF leak and pseudomeningocele formation; however, there is no difference in the 2 surgical techniques with respect to clinical and radiological improvement. Again, the significant weakness of all the studies included in the meta-analysis was the lack of standardization of surgical technique.12 In our study, all patients underwent suboccipital craniectomy and C1 laminectomy. Duraplasty was performed in 88% of the patients, with arachnoid lysis and/or tonsil coagulation performed in only a minority of patients. The surgical technique used in our series showed some degree of variability and surgeon preference, as highlighted in the literature.

Although posterior decompression via suboccipital craniectomy is widely accepted as the surgical treatment of choice, outcomes for decompressive surgery have been widely variable and inconsistent.14-19,21,22 In a retrospective study, Levy et al44 found that only 46% of patients improved after decompressive surgery. Dones et al45 found that the majority of patients who underwent decompressive surgery actually did not experience improved symptoms; rather, they found that the main benefit of surgery was the arrestment of disease and symptom progression. However, in a more recent, long-term retrospective follow-up of 157 surgically treated CMI patients, Aghakhani et al14 reported that decompressive surgery was an effective and safe treatment method of Chiari-related syringomyelia with up to a 90% chance of long-term stabilization or improvement. Alzate et al46 reported excellent outcomes in 82% of the patients, with none showing worsening of symptoms after suboccipital decompression. Tisell et al47 assessed long-term outcomes in patients with CMI and demonstrated that 75% of patients showed improvement at long-term follow-up (median time, 3.2 years) after surgery. Finally, Mueller and Oro,48 in a prospective study using questionnaires on 112 patients undergoing posterior fossa craniotomy for CMI, demonstrated that self-perceived quality of life improved significantly postoperatively.

Using validated PROs, our study provides further evidence that suboccipital decompression is a clinically effective treatment method in patients with CMI, resulting in improved perception of pain, disability, and quality of life. Patients in our study showed significant improvement at 1 year in all PROs assessing pain (NRS-Head and Neck), disability (HDI and NDI), general health (SF-12 PCS and MCS), quality of life (EQ-5D), depression (Zung Depression Scale), and myelopathy (mJOA). Additionally, approximately 76% of patients employed preoperatively returned to work after surgery at a median time of 6 weeks, and 78% would undergo the same surgery again considering their outcomes at 1 year. This is the first comprehensive study that reports validated PROs, patient satisfaction, and return to work in this unique patient population.

PRO questionnaires assess pain, disability, and general health state; however, the numeric scores generated lack a direct, clinically significant meaning. Absolute PRO change scores may not correlate with a patient’s intrinsic view of a meaningful improvement because the definition of what is a meaningful extent of improvement may vary from patient to patient. Furthermore, it has often been observed that a statistically significant effect does not routinely correlate with a clinically significant or meaningful patient improvement.49,50 Thus, the use of MCID to determine clinical significance allows the detection of the smallest change in an outcome measure that is important to patients.

Several studies have used various MCID calculation methods to define thresholds for various neurosurgical procedures, and to date, no consensus has been reached as to what is the superior MCID calculation method. Calculation methods that have been previously supported in the literature include standard error of measurement,51-53 one-half standard deviation,54-56 effect size,57-60 minimum detectable change,61 and ROC curve.40 Our group has previously identified MCID thresholds and revealed the minimum detectable change approach as the most appropriate calculation technique after transforaminal lumbar interbody fusion in patients with low-grade degenerative lumbar spondylolisthesis, extension of lumbar fusion for patients with adjacent-segment disease, and neural decompression and fusion for patients with same-level recurrent lumbar stenosis.32,34,36 The minimum detectable change approach defines MCID as the smallest change that can be considered above the measurement error with a given level of confidence (often 95% confidence level); therefore, the MCID value is equal to the upper limit of the 95% CI for the average change score seen in the cohort defined to be nonresponders. In the present study, there was a dramatic difference in change scores between responders and nonresponders, as evidenced by Figure 6 and Table 4. Not only was there a lack of improvement in nonresponders, but some patients even worsened as a result of complications. Therefore, if we were to have used the upper 95% CI of the nonresponders, the calculated MCID threshold would have been considerably lower than the mean change scores of patients who reported any level of improvement after surgery. Thus, we used the lower limit of the 95% CI of the patients classified as responders to define the MCID threshold, which provided a better reflection of the minimum average change score perceived as clinically important by patients. On the basis of the assertion that a truly sound MCID value should be at least greater than the measurement error and should correspond to the patient perception of importance of change,61 this calculation technique provides a statistically and clinically appropriate MCID threshold.

Aliaga et al62 have recently introduced a novel scoring system for assessing outcomes in CMI patients. They compared their scoring system with the Gestalt outcome group classifying patients as improved, unchanged, or worse, which in essence is similar to the HTI anchor used in our study. Aliaga et al showed that the scores of their novel scoring system correlated with the Gestalt outcome grouping, supporting our notion that HTI is a strong discriminator of responders vs nonresponders.

Headache is the most debilitating symptom in Chiari patients and usually the sole reason for seeking medical attention. The HDI is a 25-item PRO instrument that has been reported in the medical headache and migraine literature as a standard measure of disability in patients with headache (Figure 7)28,63 It has a good internal consistency (0.90), strong test-retest reliability (0.83), and good construct validity.28 We also used HDI to assess disability resulting from headache in our patient cohort. Significant improvement (P < .001) was seen in the NRS-Head and HDI scores compared with other PROs, and 74% of patients reported improvement in headache severity. This clearly highlights that suboccipital craniectomy provides symptomatic headache relief in patients with CMI.

Figure 7
Figure 7
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Presence of syrinx is very common in CMI patients and can alter presentation and outcome. Patients with syrinx often present with more severe neurological and myelopathic symptoms.46,64 In our subset analysis comparing patients with and without syrinx, there was no significant difference at baseline between the 2 groups except Zung depression scores, which were worse in patients without syrinx. Improvement in syrinx was observed in 63% of patients postoperatively. Both cohorts showed improvement in 1-year outcome after surgery, but patients with syrinx did not show significant improvement in physical and general health (SF-12 PCS and MCS) or myelopathy compared with patients without syrinx. Thus, the symptoms of pain and headache are more likely to resolve than depression and myelopathic symptoms after surgery.

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Limitations

The limitations inherent in our study resulting from the small sample size have implications for its interpretation. Data on duration of preoperative symptoms and details of medical management were not collected for this study. Furthermore, the surgical technique used in our series showed some degree of variability and surgeon preference, as highlighted in the literature also. Extensive subset analysis was not feasible in the present study secondary to the small size of patient subgroups; as a result, further studies on larger patient populations are warranted to validate our results. Additionally, our patient cohort had a fair amount of comorbidities, which may not be representative of all patient populations frequently treated with CMI and raises the concern that our patient cohort may not be entirely representative of those with fewer comorbidities. It is unclear whether a patient population with more comorbidities such as ours changes the results of how they report their outcomes and perceive improvement after surgery, but this limitation should be considered when interpreting the results of this study. We assessed outcomes at 1 year postoperatively; longer postoperative follow-up may result in slightly altered values than those presented here. Nevertheless, the present study is the first to comprehensively assess validated PROs and MCID thresholds in a CMI population and clearly highlights that surgical treatment for CMI is effective and leads to improvement in pain, disability, general health, and quality of life.

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CONCLUSION

Surgical management of CMI in adults via suboccipital craniectomy provides significant and sustained improvement in pain, disability, general health, and quality of life as assessed by PROs 1 year postoperatively. This patient-centered assessment suggests that suboccipital craniectomy for CMI in adults is an effective treatment strategy.

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Disclosure

The authors have no personal financial or institutional interest in any of the drugs, materials, or devices described in this article.

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3. Arnett BC. Tonsillar ectopia and headaches. Neurol Clin. 2004;22(1):229–236.

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COMMENT

The authors of this study have demonstrated that patients undergoing decompression for Chiari malformation I can experience considerable improvement, particularly in terms of pain. It is worth emphasizing that the authors specify that all patients had tussive headaches, and they have demonstrated improvement in patients with this particular headache syndrome. It is not uncommon, however, to see patients with Chiari malformation I without syrinx and other headache symptoms, and this article does not address whether these patients would improve with decompression. Furthermore, it should be kept in mind that although the majority of the patients in this study improved, more than a quarter were no better or worse after surgery.

Perry A. Ball

Lebanon, New Hampshire

Keywords:

Chiari malformation I; Minimum clinically important difference; Patient-reported outcomes; Suboccipital craniectomy; Surgical outcomes

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