Hemangiopericytoma in the Olfactory Groove: A Rare and Unusual Presentation

Intracranial hemangiopericytomas (HPCs) are solitary fibrous tumors of the smooth muscle and the mesenchymal origin. While meningiomas located within the olfactory groove are common, an HPC in this location has never been reported previously. Here we describe the rare presentation of a differentiated HPC masquerading as an olfactory groove meningioma in a 33-year-old female presenting with the progressive headaches, anosmia, and visual field disturbances. Following resection, the histopathological analysis confirmed a grade II HPC. A preoperative understanding of the radiographic differences between the meningiomas and HPCs may confirm the treatment planning. An HPC must be considered in the differential diagnosis of the tumors located within the olfactory groove.

Endovascular Mechanical Thrombectomy in Large-Vessel Occlusion Ischemic Stroke Presenting with Low National Institutes of Health Stroke Scale: Systematic Review and Meta-Analysis

INTRODUCTION

Mechanical thrombectomy has become the standard of care for management of most large vessel occlusion (LVO) strokes. When patients with LVO present with minor stroke symptomatology, no consensus on the role of mechanical thrombectomy exists.

METHODS

A systematic review and meta-analysis were performed to identify studies that focused on mechanical thrombectomy, either as a standalone treatment or with intravenous tissue plasminogen activator (IV tPA), in patients with mild strokes with LVO, defined as a baseline National Institutes of Health Stroke Scale score ≤5 at presentation. Data on methodology, quality criteria, and outcome measures were extracted, and outcomes were compared using odds ratio as a summary statistic.

RESULTS

Five studies met the selection criteria and were included. When compared with medical therapy without IV tPA, mechanical thrombectomy and medical therapy with IV tPA were associated with improved 90-day modified Rankin Scale (mRS) score. Among medical patients who were not eligible for IV tPA, those who underwent mechanical thrombectomy were more likely to experience good 90-day mRS than those who were not. There was no significant difference in functional outcome between mechanical thrombectomy and medical therapy with IV tPA, and no treatment subgroup was associated with intracranial hemorrhage or death.

CONCLUSIONS

In patients with mild strokes due to LVO, mechanical thrombectomy and medical therapy with IV tPA led to better 90-day functional outcome. Mechanical thrombectomy plays an important role in the management of these patients, particularly in those not eligible for IV tPA.

Endovascular Thrombectomy Alone versus Combined with Intravenous Thrombolysis

BACKGROUND

To date, no randomized trial has directly addressed the question of whether intravenous (IV) tissue plasminogen activator (tPA) improves outcomes in IV tPA-eligible patients who will eventually undergo endovascular therapy (EVT), or whether a direct EVT strategy is equally effective. We performed a systematic review and meta-analysis to compare the efficacy and safety of direct EVT versus endovascular treatment with IV tPA (EVT+IV tPA) in adults with acute ischemic stroke.

METHODS

We performed electronic searches of 6 databases from their inception to January 2017. Data were extracted and analyzed according to predefined clinical endpoints.

RESULTS

Twelve comparative studies, comprising 1275 patients in the EVT-only arm and 1340 patients in the combined EVT+IV tPA arm, were included. The rates of good functional outcomes (modified Rankin Scale score ≤2) and 90-day mortality were not statistically significantly different between the EVT and EVT+IV tPA arms (44% vs. 48%; odds ratio [OR], 0.80; 95% confidence interval [CI], 0.64-1.002; P = 0.052 and 20.4% vs. 19.4%, OR 1.19; 95% CI, 0.83-1.71; P = 0.34, respectively). The rate of symptomatic intracranial hemorrhage also was not significantly different between the EVT and EVT+IV tPA arms (3.7% vs. 3.8%; OR, 0.98; 95% CI, 0.65-1.48; P = 0.91). There were no between-group differences in the rates of other complications.

CONCLUSIONS

No significant differences between the 2 groups were found in terms of favorable functional outcome, mortality rate, or complications based on contemporary endovascular therapies.

Treatment of Tandem Internal Carotid Artery Aneurysms Using a Single Pipeline Embolization Device: Evaluation of Safety and Efficacy

BACKGROUND AND PURPOSE

Tandem aneurysms are defined as multiple aneurysms located in close proximity on the same parent vessel. Endovascular treatment of these aneurysms has rarely been reported. Our aim was to evaluate the safety and efficacy of a single Pipeline Embolization Device for the treatment of tandem aneurysms of the internal carotid artery.

MATERIALS AND METHODS

A retrospective analysis of consecutive aneurysms treated with the Pipeline Embolization Device between 2009 and 2016 at 3 institutions in the United States was performed. Cases included aneurysms of the ICA treated with a single Pipeline Embolization Device, and they were divided into tandem versus solitary. Angiographic and clinical outcomes were compared.

RESULTS

The solitary group (median age, 58 years) underwent 184 Pipeline Embolization Device procedures for 184 aneurysms. The tandem group (median age, 50.5 years) underwent 34 procedures for 78 aneurysms. Aneurysms were primarily located along the paraophthalmic segment of the ICA in both the single and tandem groups (72.3% versus 78.2%, respectively, P = .53). The median maximal diameters in the solitary and tandem groups were 6.2 and 6.7 mm, respectively. Complete occlusion on the last angiographic follow-up was achieved in 75.1% of aneurysms in the single compared with 88.6%% in the tandem group (P = .06). Symptomatic thromboembolic complications were encountered in 2.7% and 8.8% of procedures in the single and tandem groups, respectively (P = .08).

CONCLUSIONS

Tandem aneurysms of the ICA can be treated with a single Pipeline Embolization Device with high rates of complete occlusion. While there appeared to be a trend toward higher thromboembolic complication rates, this did not reach statistical significance.

Evaluating the complexity of online patient education materials about brain aneurysms published by major academic institutions

OBJECTIVE

Health care education resources are increasingly available on the Internet. A majority of people reference these resources at one point or another. A threshold literacy level is needed to comprehend the information presented within these materials. A key component of health literacy is the readability of educational resources. The National Institutes of Health (NIH) and the American Medical Association have recommended that patient education materials be written between a 4th- and a 6th-grade education level. The authors assessed the readability of online patient education materials about brain aneurysms that have been published by several academic institutions across the US.

METHODS

Online patient education materials about brain aneurysms were downloaded from the websites of 20 academic institutions. The materials were assessed via 8 readability scales using Readability Studio software (Oleander Software Solutions), and then were statistically analyzed.

RESULTS

None of the patient education materials were written at or below the NIH's recommended 6th-grade reading level. The average educational level required to comprehend the texts across all institutions, as assessed by 7 of the readability scales, was 12.4 ± 2.5 (mean ± SD). The Flesch Reading Ease Scale classified the materials as “difficult” to understand, correlating with a college-level education or higher. An ANOVA test found that there were no significant differences in readability among the materials from the institutions (p = 0.215).

CONCLUSIONS

Brain aneurysms affect 3.2% of adults 50 years or older across the world and can cause significant patient anxiety and uncertainty. Current patient education materials are not written at or below the NIH's recommended 4th- to 6th-grade education level.

The Use of Single Stent-Assisted Coiling in Treatment of Bifurcation Aneurysms: A Multicenter Cohort Study With Proposal of a Scoring System to Predict Complete Occlusion

BACKGROUND

The development of stent-assisted coiling has allowed for the endovascular treatment of wide-necked bifurcation aneurysms. A variety of options exist, and little is known about the optimal stent configuration in this setting. We report a large multicenter experience of stent-assisted coiling of bifurcations aneurysms using a single stent, with attention to factors predisposing to aneurysm recanalization.

OBJECTIVE

To assess the safety and efficacy of single stent-assisted coiling, in addition to analyzing the factors associated with recanalization, and proposal of a predictive scoring scale.

METHODS

A multicenter retrospective analysis of bifurcation aneurysms treated with a single stent-assisted coiling technique between 2007 and 2015 was performed. Clinical and radiographic data were collected and used to develop a scoring system to predict aneurysm occlusion.

RESULTS

A total of 74 bifurcation aneurysms were treated with single stent-assisted coiling. At a median follow-up of 15.2 mo, complete occlusion or remnant neck was achieved in 90.6% of aneurysms. Aneurysm location, maximal diameter, neck size, and alpha angle were predictive of aneurysm occlusion at last follow-up. A scoring system to predict complete occlusion based on these factors was developed. An increasing score correlated with a higher rate of complete occlusion.

CONCLUSION

The treatment of bifurcation aneurysm using single stent technique for stent-assisted coiling is safe and effective. Complete occlusion or remnant neck occlusion was achieved in 90.6% of cases. Class III aneurysms can be effectively treated using a single stent, while class I may require Y-stent technique.

Pipeline Embolization Device in Treatment of 50 Unruptured Large and Giant Aneurysms

INTRODUCTION

Treatment of large (≥20 mm) and giant (≥25 mm) intracranial aneurysms is challenging and can be associated with a high rate of morbidity and mortality. The Pipeline Embolization Device (PED) has been used effectively for the treatment of intracranial aneurysms achieving a high rate of complete occlusion. However, its safety and efficacy in treatment of large and giant aneurysms has not been evaluated fully.

METHODS

A retrospective analysis of consecutive aneurysms treated with PED between 2009 and 2016 at 3 academic institutions within the United States was performed. Large (≥20 mm) and giant aneurysms (≥25 mm) were selected for evaluation of occlusion and complication rates following treatment with PED.

RESULTS

A total of 50 large and giant aneurysms were individually treated using PED. Aneurysms were fusiform (74%) or saccular (26%) in morphology. PED alone was used for treating 78% of the aneurysms, whereas PED with adjunctive coiling was used for treating 22%. The median length of angiographic follow-up was 13 months (mean follow up 20.4 months). At last follow-up, complete or near-complete occlusion (90-100%) was achieved in 76.9% of aneurysms. Symptomatic thromboembolic complications were encountered in 12% of procedures and symptomatic hemorrhagic complications in 8%.

CONCLUSIONS

The use of PED for the treatment of large and giant intracranial aneurysms is associated with good occlusion rates, but also a greater complication rate compared to aneurysms of smaller size. There was no significant difference in occlusion rate based on aneurysm shape or size, number of PEDs placed, or adjunctive coiling.

Comparison of Stent-Assisted Coil Embolization and the Pipeline Embolization Device for Endovascular Treatment of Ophthalmic Segment Aneurysms: A Multicenter Cohort Study

BACKGROUND

Stent-assisted coil embolization and flow diversion with the Pipeline embolization device (PED) are both effective endovascular treatment options for ophthalmic segment aneurysms (OSAs) of the internal carotid artery. Here we present a large comparative cohort study.

METHODS

A multicenter, retrospective cohort comparison study of consecutively treated OSAs was conducted at 2 academic institutions in the United States comparing stent-coiling (between 2007 and 2015) and PED (between 2011 and 2016).

RESULTS

A total of 62 of OSAs were treated with stent-coiling and 106 were treated with the PED. The stent-coiling-treated aneurysms were larger, although the maximum diameter was not significantly different between the 2 groups (P = 0.05). The median duration of follow-up was 22.5 months for the stent-coiling group and 8.7 months for the PED group (P = 0.0002). Complete occlusion at last follow-up was achieved in 75.9% of aneurysms in the stent-coiling group and in 81.1% of aneurysms in the PED group (P = 0.516). The retreatment rate was higher with stent-coiling, but the difference did not reach statistical significance (P = 0.062). A good functional outcome was achieved in 96.6% of patients in the stent-coiling group and in 94.7% of those in the PED group (P = 0.707). The rate of neurologic complications was 4.8% in the stent-coiling group and 9.4% in the PED group (P = 0.376).

CONCLUSION

Stent-coiling and the PED were equally effective for treating OSAs. There were no significant differences in terms of procedural complications, angiographic, functional, and visual outcomes. PED may be more favorable for multiple adjacent OSAs.

Use of Platelet Function Testing Before Pipeline Embolization Device Placement

Background and Purpose—

Thromboembolic complications constitute a significant source of morbidity after neurointerventional procedures. Flow diversion using the pipeline embolization device for the treatment of intracranial aneurysms necessitates the use of dual antiplatelet therapy to reduce this risk. The use of platelet function testing before pipeline embolization device placement remains controversial.

Methods—

A retrospective review of prospectively maintained databases at 3 academic institutions was performed from the years 2009 to 2016 to identify patients with intracranial aneurysms treated with pipeline embolization device placement. Clinical and radiographic data were analyzed with emphasis on thromboembolic complications and clopidogrel responsiveness.

Results—

A total of 402 patients underwent 414 pipeline embolization device procedures for the treatment of 465 intracranial aneurysms. Thromboembolic complications were encountered in 9.2% of procedures and were symptomatic in 5.6%. Clopidogrel nonresponders experienced a significantly higher rate of thromboembolic complications compared with clopidogrel responders (17.4% versus 5.6%). This risk was significantly lower in nonresponders who were switched to ticagrelor when compared with patients who remained on clopidogrel (2.7% versus 24.4%). In patients who remained on clopidogrel, the rate of thromboembolic complications was significantly lower in those who received a clopidogrel boost within 24 hours pre-procedure when compared with those who did not (9.8% versus 51.9%). There was no significant difference in the rate of hemorrhagic complications between groups.

Conclusions—

Clopidogrel nonresponders experienced a significantly higher rate of thromboembolic complications when compared with clopidogrel responders. However, this risk seems to be mitigated in nonresponders who were switched to ticagrelor or received a clopidogrel boost within 24 hours pre-procedure.

Aspirin and Risk of Subarachnoid Hemorrhage

Background and Purpose—

Recent studies have suggested that the use of low-dose aspirin may reduce the risk of aneurysmal subarachnoid hemorrhage (aSAH). We aimed to evaluate any association between aspirin use and risk of aSAH based on the literature, and whether this is influenced by duration or frequency of aspirin use.

Methods—

A search of electronic databases was done from inception to September 2016. For each study, data on risk of aSAH in aspirin versus nonaspirin users were used to generate odds ratios and 95% confidence intervals, and combined using inverse variance–weighted averages of logarithmic odds ratios in a random-effects models.

Results—

From 7 included studies, no significant difference was noted between aspirin use of any duration or frequency and nonaspirin users (odds ratio, 1.00; 95% confidence interval, 0.81–1.24; P =0.99). We found a significant association between short-term use of aspirin (<3 months) and the risk of aSAH (odds ratio, 1.61; 95% confidence interval, 1.20–2.18; P =0.002). No significant difference was found in terms of risk of aSAH for 3 to 12 months, 1 to 3 years, and >3 years of durations of use. No significant association was found between infrequent aspirin use (≤2× per week) or frequent use (≥3× per week) with risk of aSAH.

Conclusions—

Current evidence suggests that short-term (<3 months) use of aspirin is associated with increased risk of aSAH. Limitations include substantial heterogenity of the included studies. The role of long-term aspirin in reducing risk of aSAH remains unclear and ideally should be addressed by an appropriately designed randomized controlled trial.

Ultra-Early Angiographic Vasospasm After Aneurysmal Subarachnoid Hemorrhage: A Systematic Review and Meta-Analysis

OBJECTIVE

After aneurysmal subarachnoid hemorrhage (aSAH), prognosis is affected heavily by the presence of delayed cerebral ischemia (DCI). There is growing recognition of ultra-early angiographic vasospasm (UEAV) occurring within 48 hours of aSAH; however, its relationship with DCI and ultimately prognosis remains unclear.

METHODS

Various databases limited to the English language through September 2016 were searched systematically. Eligible studies were those comparing UEAV with control non-UEAV outcomes and follow-up. Two independent reviewers evaluated the quality of studies and abstracted the data, with discrepancies resolved by a third. We calculated odds ratios (ORs) and 95% confidence intervals for all outcomes by using random-effects meta-analyses and performed a heterogeneity analysis.

RESULTS

Four comparative studies were selected for analysis. Pooled analysis demonstrated that UEAV compared with no-UEAV was associated with greater proportion of rupture aneurysms sized greater than 12 mm (38.3% vs. 24.3%, P < 0.00001). A significantly greater number of patients with UEAV had ruptured MCA aneurysms compared with patients without UEAV (29.7% vs. 19.9%, P = 0.005). Compared with no-UEAV, patients with UEAV were significantly associated with symptomatic cerebral vasospasm (OR 2.07, P = 0.05) and DCI/infarction (OR 2.52, P = 0.02). A significant association also was found between UEAV and an unfavorable outcome at follow-up (OR 1.64, P = 0.03) and greater mortality (OR 2.65, P < 0.00001).

CONCLUSIONS

UEAV was significantly associated with symptomatic cerebral vasospasm, DCI/infarction, unfavorable outcome at follow-up, and greater mortality. Patients with intracerebral hematoma, intraventricular hemorrhage (Fisher Grade IV), larger ruptured aneurysms >12 mm, and an MCA location were more likely to have UEAV.

Ischemic Stroke After Treatment of Intraprocedural Thrombosis During Stent-Assisted Coiling and Flow Diversion

BACKGROUND AND PURPOSE

Intraprocedural thrombosis poses a formidable challenge during neuroendovascular procedures because the risks of aggressive thromboembolic treatment must be balanced against the risk of postprocedural hemorrhage. The aim of this study was to identify predictors of ischemic stroke after intraprocedural thrombosis after stent-assisted coiling and pipeline embolization device placement.

METHODS

A retrospective analysis of intracranial aneurysms treated with stent-assisted coiling or pipeline embolization device placement between 2007 and 2016 at 4 major academic institutions was performed to identify procedures that were complicated by intraprocedural thrombosis.

RESULTS

Intraprocedural thrombosis occurred in 34 (4.6%) procedures. Postprocedural ischemic stroke and hemorrhage occurred in 20.6% (7/34) and 11.8% (4/34) of procedures complicated by intraprocedural thrombosis, respectively. Current smoking was an independent predictor of ischemic stroke. There was no statistically significant difference in the rate of ischemic stroke or postprocedural hemorrhage with the use of abciximab compared with the use of eptifibatide in treatment of intraprocedural thrombosis.

CONCLUSIONS

Current protocols for treatment of intraprocedural thrombosis associated with placement of intra-arterial devices were effective in preventing ischemic stroke in ≈80% of cases. Current smoking was the only independent predictor of ischemic stroke.

De novo AVM formation following venous sinus thrombosis and prior AVM resection in adults: report of 2 cases

Brain arteriovenous malformations (AVMs) are traditionally considered congenital lesions, arising from aberrant vascular development during the intrauterine period. Rarely, however, AVMs develop in the postnatal period. Individual case reports of de novo AVM formation in both pediatric and adult patients have challenged the traditional dogma of a congenital origin. Instead, for these cases, a dynamic picture is emerging of AVM growth and development, initially triggered by ischemic and/or traumatic events, coupled with genetic predispositions. A number of pathophysiological descriptions involving aberrant angiogenic responses following trauma, hemorrhage, or inflammation have been proposed, although the exact etiology of these lesions remains to be elucidated. Here, the authors present 2 cases of de novo AVM formation in adult patients. The first case involves the development of an AVM following a venous sinus thrombosis and to the authors' knowledge is the first of its kind to be reported in the literature. They also present a case in which an elderly patient with a previously ruptured AVM developed a second AVM in the contralateral hemisphere 11 years later. In addition to presenting these cases, the authors propose a possible mechanism for de novo AVM development in adult patients following ischemic injury.

Utility of plain radiographs and MRI in cervical spine clearance in symptomatic non-obtunded pediatric patients without high-impact trauma

PURPOSE

The optimal imaging modality for evaluating cervical spine trauma and optimizing management in the pediatric population is controversial. In pediatric populations, there are no well-established guidelines for cervical spine trauma evaluation and treatment. Currently, there is virtually no literature regarding imaging and management of symptomatic pediatric patients who present with cervical spine trauma without high-impact mechanism. This study aims to establish an optimal imaging strategy for this subgroup of trauma patients.

METHODS

We performed a retrospective review of pediatric patients (aged below 18 years) who were admitted to Monash Medical Centre, Melbourne, Australia between July 2011 and June 2015, who did not suffer a high-impact trauma but were symptomatic for cervical spine injury following cervical trauma. Imaging and management strategies were reviewed and results compared.

RESULTS

Forty-seven pediatric patients were identified who met the inclusion criteria. Of these patients, 46 underwent cervical spine series (CSS) plain radiograph imaging. Thirty-four cases underwent magnetic resonance imaging (MRI) and 9 patients underwent CT. MRI was able to detect 4 cases of ligamentous injury, which were not seen in CSS imaging and was able to facilitate cervical spine clearance in a further two patients whose CSS radiographs were abnormal.

CONCLUSION

In this study, MRI has a greater sensitivity and specificity when compared to CSS radiography in a symptomatic pediatric low-impact trauma population. Our data call in to question the routine use of CSS radiographs in children.

An analysis of malpractice litigation related to the management of brain aneurysms

OBJECTIVE Given the highly complex and demanding clinical environment in which neurosurgeons operate, the probability of facing a medical malpractice claim is high. Recent emphasis on tort reform within the political sphere has brought this issue to the forefront of medical literature. Despite the widespread fear of litigation in the medical community, few studies have provided an analysis of malpractice litigation in the field. Here, the authors attempt to delineate the medicolegal factors that impel plaintiffs to file medical malpractice claims related to the management of brain aneurysms, and to better characterize the nature of these lawsuits. METHODS The online legal database WestLawNext was searched to find all medical malpractice cases related to brain aneurysms across a 30-year period. All state and federal jury verdicts and settlements relevant to the search criterion were considered. RESULTS Sixty-six cases were obtained. The average age of the patient was 46.7 years. Seventy-one percent were female. The cases were distributed across 16 states. The jury found in favor of the plaintiff in 40.9% of cases, with a mean payout of $8,765,405, and in favor of the defendant in 28.8% of the cases. A failure to diagnose and/or a failure to treat in a timely manner were the 2 most commonly alleged causes of malpractice. Settlements, which were reached in 25.8% of the cases, had a mean payout of $1,818,250. Neurosurgeons accounted for 6.7% of all defendants. CONCLUSIONS Unlike other medical specialties, a majority of the verdicts were not in the defendant's favor. The mean payouts were nearly 5-fold less in cases in which a settlement was reached, as opposed to a summary judgment. Neurosurgeons accounted for a small percentage of all codefendants.

Moore J, Adeeb N, Griessenauer CJ, Patel AS, Chua MH, Thomas AJ, Ogilvy CS. Clinical presentation, progression, and treatment outcomes of moyamoya disease in the elderly

BACKGROUND

Moyamoya disease is a vascular disorder characterized by progressive stenosis of the internal carotid artery. The presentation, progression, treatment options, and post-operative clinical outcomes for elderly (60 and older) Moyamoya patients have never been reported.

METHODS

A retrospective analysis of all patients who were diagnosed with Moyamoya disease by the senior authors between 1991 and 2016 was performed. Patients who were 60 years or older at the time of surgery or last follow-up were further evaluated.

RESULTS

Seventy patients were diagnosed with probable or definite Moyamoya disease during the study period (1991-2016). Eight patients (11.4 %; six females: two males; median age 63; range, 60-71 years) were found to be 60 years or older at the time of surgery or last follow-up and were included in the study. All patients had a modified Rankin scale (mRS) of either one or two (median 1) pre-operatively. Six patients (75 %) underwent surgical treatment on a total on seven hemispheres. Post-surgery, one patient had an improved mRS score, three had no changes, and two had worsening in their mRS scores. Both patients who did not undergo surgical interventions suffered from intra-parenchymal hemorrhages post-diagnosis.

CONCLUSIONS

Moyamoya disease is most commonly seen in young and middle-aged patients. Presentation in the elderly (defined as 60 years and older in this study) is rare, and has never been reported in the literature. In this study, both direct and indirect revascularization procedures demonstrated potential benefit in some of these patients, with stabilization of progressive symptoms.

The rapid formation of Sputnik Planitia early in Pluto's history

Pluto's Sputnik Planitia is a bright, roughly circular feature that resembles a polar ice cap. It is approximately 1,000 kilometres across and is centred on a latitude of 25 degrees north and a longitude of 175 degrees, almost directly opposite the side of Pluto that always faces Charon as a result of tidal locking. One explanation for its location includes the formation of a basin in a giant impact, with subsequent upwelling of a dense interior ocean. Once the basin was established, ice would naturally have accumulated there. Then, provided that the basin was a positive gravity anomaly (with or without the ocean), true polar wander could have moved the feature towards the Pluto-Charon tidal axis, on the far side of Pluto from Charon. Here we report modelling that shows that ice quickly accumulates on Pluto near latitudes of 30 degrees north and south, even in the absence of a basin, because, averaged over its orbital period, those are Pluto's coldest regions. Within a million years of Charon's formation, ice deposits on Pluto concentrate into a single cap centred near a latitude of 30 degrees, owing to the runaway albedo effect. This accumulation of ice causes a positive gravity signature that locks, as Pluto's rotation slows, to a longitude directly opposite Charon. Once locked, Charon raises a permanent tidal bulge on Pluto, which greatly enhances the gravity signature of the ice cap. Meanwhile, the weight of the ice in Sputnik Planitia causes the crust under it to slump, creating its own basin (as has happened on Earth in Greenland). Even if the feature is now a modest negative gravity anomaly, it remains locked in place because of the permanent tidal bulge raised by Charon. Any movement of the feature away from 30 degrees latitude is countered by the preferential recondensation of ices near the coldest extremities of the cap. Therefore, our modelling suggests that Sputnik Planitia formed shortly after Charon did and has been stable, albeit gradually losing volume, over the age of the Solar System.