Basilar Trunk Research Articles

Comparative Analysis of Orbitozygomatic and Subtemporal Approaches to the Basilar Apex: A Cadaveric Study

The subtemporal and orbitozygomatic approaches are the most commonly used surgical approaches for the treatment of basilar artery apex (BAX) aneurysms. Relative advantages and disadvantages are generally reported based on surgeons' experience. This study was performed to provide a detailed comparison between the subtemporal and orbitozygomatic approaches based on cadaveric dissection analysis for the treatment of BAX aneurysms. Subtemporal and orbitozygomatic approaches were performed on 5 cadaveric heads (10 sides), and the following variables were assessed and compared between the 2 approaches: 1) number of exposed perforators on P1-posterior cerebral arteries (PCA); 2) lengths of exposure and clipping for bilateral PCA, superior cerebellar arteries (SCA), and basilar trunk; 3) surgical area of exposure; and 4) surgical freedom at the BAX. Number of perforators exposed on P1-PCA was not different between the subtemporal and orbitozygomatic approaches. Exposure and clipping of ipsilateral SCA and PCA were superior using the subtemporal approach, and better for contralateral SCA and PCA using the orbitozygomatic approach, all reaching statistical significance. The orbitozygomatic approach provided greater exposure and clipping length for the proximal basilar trunk. Although the surgical area of exposure was similar between the 2 approaches, the overall surgical freedom was greater in the orbitozygomatic approach. The orbitozygomatic approach provides a greater number of surgical corridors to the BAX and is superior regarding multiple surgically relevant anatomic parameters. Importantly, control over the basilar trunk and over the contralateral SCA and PCA (blind spots) is superior with the orbitozygomatic approach.

The oculomotor-tentorial triangle. Part 2: a microsurgical workspace for vascular lesions in the crural and ambient cisterns.

Pathology in the region of the basilar quadrifurcation, anterolateral midbrain, medial tentorium, and interpeduncular and ambient cisterns may be accessed anteriorly via an orbitozygomatic (OZ) craniotomy. In Part 1 of this series, the authors explored the anatomy of the oculomotor-tentorial triangle (OTT). In Part 2, the versatility of the OTT as a surgical workspace for treating vascular pathology is demonstrated. Sixty patients with 61 vascular pathologies treated within or via the OTT from 1998 to 2017 by the senior author were retrospectively reviewed. Patients were grouped together based on pathology/surgical procedure and included 1) aneurysms (n = 19); 2) posterior cerebral artery (PCA)/superior cerebellar artery (SCA) bypasses (n = 24); 3) brainstem cavernous malformations (CMs; n = 14); and 4) tentorial region dural arteriovenous fistulas (dAVFs; n = 4). The majority of patients were approached via an OZ craniotomy, wide sylvian fissure split, and temporal lobe mobilization to widen the OTT. Aneurysm locations included the P1-P2 junction (n = 7), P2A segment (n = 9), P2/3 (n = 2), and basilar quadrification (n = 1). Aneurysm treatments included clip reconstruction (n = 12), wrapping (n = 3), proximal occlusion (n = 2), and trapping with (n = 1) or without (n = 1) bypass. Pathologies in the bypass group included vertebrobasilar insufficiency (VBI; n = 3) and aneurysms of the basilar trunk (n = 13), basilar apex (n = 4), P1 PCA (n = 2), and s1 SCA (n = 2). Bypasses included M2 middle cerebral artery (MCA)-radial artery graft (RAG)-P2 PCA (n = 8), M2 MCA-saphenous vein graft (SVG)-P2 PCA (n = 3), superficial temporal artery (STA)-P2 PCA (n = 5) or STA-s1 SCA (n = 3), s1 SCA-P2 PCA (n = 1), V3 vertebral artery (VA)-RAG-s1 SCA (n = 1), V3 VA-SVG-P2 PCA (n = 1), anterior temporal artery-s1 SCA (n = 1), and external carotid artery (ECA)-SVG-s1 SCA (n = 1). CMs were located in the midbrain (n = 10) or pontomesencephalic junction (n = 4). dAVFs drained into the tentorial, superior petrosal, cavernous, and sphenobasal sinuses. High rates of aneurysm occlusion (79%), bypass patency (100%), complete CM resection (86%), and dAVF obliteration (100%) were obtained. The overall rate of permanent oculomotor nerve palsy was 8.3%. The majority of patients in the aneurysm (94%), CM (93%), and dAVF (100%) groups had stable or improved modified Rankin Scale scores. The OTT is an important anatomical triangle and surgical workspace for vascular lesions in and around the crural and ambient cisterns. The OTT can be used to approach a wide variety of vascular pathologies in the region of the basilar quadrifurcation and anterolateral midbrain.

Assessment of the endoscopic endonasal approach to the basilar apex region for aneurysm clipping.

The expanded endoscopic endonasal approach (EEA) has shown promising results in treatment of midline skull base lesions. Several case reports exist on the utilization of the EEA for treatment of aneurysms. However, a comparison of this approach with the classic transcranial orbitozygomatic approach to the basilar apex (BAX) region is missing.The present study summarizes the results of a series of cadaveric surgical simulations for assessment of the EEA to the BAX region for aneurysm clipping and its comparison with the transcranial orbitozygomatic approach as one of the most common approaches used to treat BAX aneurysms. Fifteen cadaveric specimens underwent bilateral orbitozygomatic craniotomies as well as an EEA (first without a pituitary transposition [PT] and then with a PT) to expose the BAX. The following variables were measured, recorded, and compared between the orbitozygomatic approach and the EEA: 1) number of perforating arteries counted on bilateral posterior cerebral arteries (PCAs); 2) exposure and clipping lengths of the PCAs, superior cerebellar arteries (SCAs), and proximal basilar artery; and 3) surgical area of exposure in the BAX region. Except for the proximal basilar artery exposure and clipping, the orbitozygomatic approach provided statistically significantly greater values for vascular exposure and control in the BAX region (i.e., exposure and clipping of ipsilateral and contralateral SCAs and PCAs). The EEA with PT was significantly better in exposing and clipping bilateral PCAs compared to EEA without a PT, but not in terms of other measured variables. The surgical area of exposure and PCA perforator counts were not significantly different between the 3 approaches. The EEA provided better exposure and control if the BAX was located ≥ 4 mm inferior to the dorsum sellae. For BAX aneurysms located in the retrosellar area, PT is usually required to obtain improved exposure and control for the bilateral PCAs. However, the transcranial approach is generally superior to both endoscopic approaches for accessing the BAX region. Considering the superior exposure of the proximal basilar artery obtained with the EEA, it could be a viable option when surgical treatment is considered for a low-lying BAX or mid-basilar trunk aneurysms (≥ 4 mm inferior to dorsum sellae).

Neuronal nitric oxide synthase inhibition reduces brain damage by promoting collateral recruitment in a cerebral hypoxia-ischemia mice model.

The collateral circulation development is considered as a compensatory inherent mechanism to restore damaged blood perfusion after ischemia. We aimed to detect the collateral flow and the mean blood-flow velocities (mBFVs) level in the basilar trunk during or after cerebral hypoxia-ischemia in the mice brain and explore the effect of neuronal nitric oxide synthase (nNOS) inhibition on the collateral flow. C57BL/6J mice and the nNOS knockout (KO) mice were randomly divided into a sham-operated group (control) and the hypoxia-ischemia (HI) groups that were treated with the phosphate buffered solution (PBS) control or 7-nitroindazole (7-NI). Cortexes were harvested after the HI treatment for analysis of nNOS expression using Western blot and reverse transcription-polymerase chain reaction (RT-PCR). Ultrasound imaging experiments were performed to detect the collateral flow and the mBFVs level in the basilar trunk. After cerebral HI, the cortical nNOS mRNA and protein levels increased markedly compared with the sham-operated control mice. Besides, 7-NI treatment had no effect on the blood flow in the sham-operated control mice. What's more, either the 7-NI pretreatment or the nNOS gene knockdown before the HI procedure could attenuate the brain injury by the increased collateral flow and the decreased mBFVs level in the basilar trunk. nNOS inhibition protected hypoxic-ischemic-induced mice brain damage by the increased collateral flow and the decreased mBFVs level in the basilar trunk. Therefore, the 7-NI administration may have potential utility for the treatment of HI injury in human beings.

Surgical Treatment of Posterior Circulation Aneurysms – Anatomical Study and Surgical Technique

AbstractPosterior circulation aneurysms represent 10–15% of intracranial aneurysms. The diagnosis is usually secondary to subarachnoid hemorrhage due to its initial asymptomatic presentation and higher risk of rupture compared with aneurysms in the anterior circulation. The surgical treatment of posterior circulation aneurysms is complex and challenging for neurosurgeons because of the particular anatomy of the posterior circulation with its close relation to the brainstem and cranial nerves and also because of the depth and narrowness of the surgical approach. Aneurysms from different locations have specific anatomical relationships and surgical approaches for better visualization and dissection. Therefore, a detailed anatomy knowledge of the posterior circulation is mandatory for an individualized preoperative planning and good neurological and angiographic outcomes. We selected the main aneurysm sites on the posterior circulation, such as: posterior inferior cerebellar artery, basilar trunk, basilar bifurcation, posterior cerebral artery (PCA) and superior cerebellar artery for a detailed description of the relevant anatomy related to aneurysm, and the main surgical approaches for its surgical treatment. Furthermore, we performed a literature review with the most recent outcomes regarding to the surgical treatment of posterior circulation aneurysms.

Intracranial aneurysms of the posterior circulation associated with a fenestration: a systematic review.

A fenestration is an anatomical variation that most commonly involves vessels of the posterior circulation, which can predispose the formation of aneurysms and represents a further technical challenge for treatment. There are no large series and the incidence of complications is unknown. This paper is a systematic review on this topic. The relevant English literature was reviewed and the data was extracted for each patient and collected in a pool. Demographics, localization of fenestration, morphology and aneurysm topography, admission and follow-up clinical status, treatments, complications and occlusion rate were recorded. We analyzed 62 articles published between 1992 and 2016 including 120/133 patients/aneurysms. The most commonly involved segment was the vertebro-basilar junction (82.5%), followed by the basilar trunk (10%). About 96.6% of reported aneurysms were saccular and 80.3% were ruptured. The preferred treatment was endovascular (86.67%) and a surgical clipping was reported in just over 13% of patients. A complete/almost complete occlusion was obtained in about 80% of cases, with a global rate of complications of 12.5%. A serious clinical condition at onset has been independently associated with poor outcome at multivariate analysis (P<0.001). This systematic review has shown that endovascular occlusion is the treatment of choice for posterior circulation aneurysms associated with fenestrations, being highly effective, with an occlusion rate of about 80%, unchanged at follow-up. Although the presence of a fenestration can make more complex the aneurysm occlusion, and the occurrence of complications is not negligible, it is not considered a limiting condition for treatment.

Surgical treatment of large and giant vertebrobasilar aneurysms manifested by brainstem compression symptoms

The study included 8 patients operated on in the period between 2014 and 2017. All patients underwent intravascular intervention; two patients had open surgery at the second stage. Intravascular intervention, both alone and in combination with open surgery, is effective treatment of VB aneurysms whose clinical picture is manifested by brainstem compression symptoms. The use of flow-diverting stents in most cases has provided good radicalness in the long-term period, without worsening the functional outcome. Treatment of fusiform aneurysms of the basilar artery trunk requires separate consideration and an individual approach due to a high risk of thrombosis of short branches feeding the brainstem.

Delayed ischemic stroke due to stent marker band occlusion after stent-assisted coiling

A middle-aged patient with an internal carotid-posterior communicating artery aneurysm and basilar artery tip aneurysm was treated by stent-assisted coiling. One ischemic infarction and two transient ischemic attacks occurred with...

Super-selective coil embolization of a basilar perforator artery aneurysm previously treated by the stent-in-stent technique, using an extremely soft bare coil delivered through a one-marker microcatheter

Background Aneurysms of the perforating arteries arising from the basilar artery trunk are rare and represent a therapeutic challenge. Methods A basilar perforator aneurysm was initially treated by the stent-in-stent technique. Enlargement of the aneurysm was seen on follow up and the sac was secondarily coiled using two extremely soft bare coils, delivered through a one-marker microcatheter. Results At 6 months, the patient had a modified Rankin Scale score of 0 and cerebral arteriography demonstrated complete occlusion of the lesion. Conclusions We describe, as far as we are aware, the first case of basilar perforator aneurysm occlusion using extremely soft bare coils, inserted through the mesh of two stents previously placed in the basilar trunk.

The Interlenticulostriate Approach to Very High-Riding Distal Basilar Trunk Aneurysms.

Most high-riding distal basilar trunk aneurysms can be surgically approached via the transsylvian route and its orbitozygomatic variant. However, on rare occasions, the basilar bifurcation may be unusually high and an approach above the carotid terminus may be required. In this cadaveric study, we sought to determine the feasibility and exposure limits of the interlenticulostriate approach (ILSA). A standard transsylvian approach was performed in 10 cerebral hemispheres of 5 formalin-fixed, silicone-injected cadaver heads. The interpeduncular cistern was exposed via the opticocarotid window, carotid-oculomotor window, and supracarotid ILSA window. The latter was measured and an aneurysm clip or ventriculostomy stylet was placed as high as possible through each corridor. Using noncontrast 3-D rotational angiography, clip/stylet positions were measured relative to the dorsum sellae. ILSA provided a 9.4 × 4.6 mm mean surgical corridor, just enough room for a standard clip applier. This space was limited by the carotid bifurcation inferiorly, the lenticulostriate arteries medially and laterally, and the optic tract superiorly. There was no difference between opticocarotid and carotid-oculomotor windows, in terms of clip position (+8.9 vs +8.6 mm, respectively; P = .78). In contrast, ILSA provided significantly improved superior exposure, compared with either approaches (mean stylet position: +14.3 mm; P = .005). The exposure benefit afforded by ILSA was consistent across all 10 hemispheres, ranging from +2.5 to +8 mm. For high-riding distal basilar trunk aneurysms that cannot be reached via the frontotemporal orbitozygomatic approach, ILSA can provide a viable route of access. Vascular neurosurgeons should be familiarized with this approach.

脳底動脈本幹部破裂動脈瘤に対する急性期コイル塞栓術

Basilar artery trunk aneurysm (BATA) is an aneurysm that occurs between the union of the bilateral vertebral arteries and the superior cerebellar artery. Dissection, in addition to the formation of an aneurysm itself, has been considered a cause of BATA. Direct surgery is difficult in many cases; however, recently, endovascular treatment (EVT) has been used for BATA rupture.Between November 2007 and October 2015, acute-phase EVT was performed in five cases of ruptured BATA in our hospital. The male-to-female ratio and mean age of the patients were 1:4 and 63.6 years, respectively. The mean (range) maximum diameter of the aneurysm was 9.24 mm (3.2-17.8 mm). Four aneurysms were multilocular and irregular in shape, while two were partially thrombosed. During EVT, the balloon remodeling technique was used in four cases and a stent was used in one case as an adjunctive technique. We encountered two technical problems, namely failure during the delivery of the stent system and in-stent thrombosis. Three cases were exacerbated during the perioperative period and resulted in a modified Rankin score of 6.In our cases, EVT did not contribute adequately to improving the prognosis of BATA. This study suggests that to perform coil embolization safely and effectively for ruptured BATA, establishing not only the appropriate procedural strategies, including perioperative antiplatelet drugs management, but also a treatment plan that considers disease severity and systemic condition is important.

Magnetic resonance imaging characteristics of ischemic stroke in an Afro-Caribbean population: A 1-year prospective MRI study on 534 consecutive patients

Few population-based MRI studies on stroke, particularly in African-descent populations, are available. Based on a 1-year Afro-Caribbean population-based study MRI, ischemic stroke characteristics were extensively analyzed. All strokes occurring in Martinique (390,371inhabitants) were prospectively included. Ascertainment was based, whenever possible, on MRI. All patients were categorized as single- (subclassified as cortical, cortical-subcortical, subcortical, lacunar) or multiple-lesion pattern, and vascular (single, multiple or junctional) territory. Brain parenchyma was evaluated, based on visualization of macrobleeds, microbleeds, white-matter hyperintensities or stroke sequelae. Etiology was classified according to TOAST criteria. Among 596ischemic stroke patients included, 534 (295men, 239women; mean age, 71 [range 23-110] years) underwent MRI (median delay 1day). Four hundred and eighty-eight had single-type lesion (14.8% cortical, 42.4% cortical-subcortical, 14.5% subcortical, 16.6% lacunar), involving anterior cerebral (4%), middle cerebral (63.7%), posterior cerebral artery (10.4%) or basilar trunk (11.7%) territories, with 10.3% simultaneously involving multiple territories and 4.9% junctional infarction. Etiologies were LAA (11.2%), SVD (10.7%), CE (29.6%), rare (4.5%) or undetermined (44.1%). Our prospective, consecutive, ischemic stroke series gives a comprehensive description of ischemic stroke imaging patterns and etiologic distributions in an Afro-Caribbean population with high socio-economic status. Our patients' stroke characteristics are close to those of European-descent populations.

A dissecting aneurysm of a basilar perforating artery

We report an additional case of a ruptured basilar trunk perforator aneurysm, for which an endovascular treatment was initially planned, but aborted due to the spontaneous regression of the aneurysm. Thus, a conservative management consisting on a close follow-up was decided that confirmed the favorable radiological outcome. Spontaneous regression of such aneurysm should be well-known by neurosurgeons and neuroradiologists in order to prevent the potential iatrogenic effects of the related treatment modalities.

Endovascular reconstructive treatment of vertebrobasilar fusiform aneurysms by multiple overlapping stents with or without coils

To explore the feasibility, safety and efficacy of endovascular reconstructive treatment by multiple overlapping stents with or without coils. Totally 17 patients of vertebrobasilar fusiform aneurysms treated by multiple overlapping stents with or without coils between September 2011 and September 2015 in Department of Neurosurgery, Beijing Hospital were retrospectively reviewed. There were 15 male and 2 female patients with mean age of (47±13) years. Clinical manifestations included subarachnoid hemorrhage in 5 patients, cerebral ischemia or infarction in the posterior circulation in 5 patients, mass effect accompanied with brainstem infarction in 1 patients, headache or dizziness in 4, and incidental findings in 2 patients. Aneurysm located in intracranial vertebral artery in 11 patients, vertebrobasilar junction in 2 patients, and basilar trunk in 4 patients. Mean aneurysmal size was 18.5 mm×8.0 mm(length×width). Sole stenting with overlapping stents was performed in 8 patients, and overlapping stents with coils was applied in 9 patients. Post-procedural hemorrhage occurred in a patient with a giant basilar trunk fusiform aneurysm and led to death. The 16 surviving patients were clinically followed up for a mean of 21.1 months. One patient had a modified Rankin score of 4, and the other patients had a good recovery. Among 15 patients with a mean angiographic follow-up of 8.6 months, 9 patients had their aneurysms further thrombosed, 3 patients were stable, and 3 patients with a recurred aneurysm. Final complete occlusion was achieved in 7 patients. Reconstructive treatment by overlapping stents with or without coils is feasible and relative safe in vertebrobasilar fusiform aneurysms.

Laying on the cause of stroke?

A 62-year-old man developed sudden slurring of speech, dysphagia and vertigo and in the emergency department he rapidly became stuporous. A CT angiogram of head showed occlusion of the distal...

脳底動脈本幹部大型広頚動脈瘤に対するステントを併用したmultiple catheterでのコイル塞栓術

脳底動脈本幹部大型広頚動脈瘤に対するステントを併用したmultiple catheterでのコイル塞栓術

A rare case of persistent hypoglossal artery associated with contralateral proximal subclavian stenosis.

The persistent hypoglossal artery is rare vascular anomalies. We report the case of a 50-year old man with right hypoglossal artery, ipsilateral hypoplasic internal carotid artery, associated with left proximal subclavian stenosis with subclavian steal syndrome. Power-Doppler-Ultra-Sonography spectral images obtained after the patient exercised the left arm showed mid-systolic deceleration with retrograde late-systolic velocities. A Computed Tomography Angiography demonstrated a proximal stenosis of the left SA, a mild right ICA hypoplasia and an anomalous artery arising from right ICA at C2–C3 level, entering the cranium via the hypoglossal canal and joining the basilar artery. Usually the presence of PHA may be completely asymptomatic, and detected as an incidental finding by CTA or MRA, but in our case its diagnosis is extremely important because it is often the only vessel supplying blood to the basilar trunk and posterior circulation.

A Case of Repeatedly Recurrent Basilar Trunk Dissecting Aneurysm Successfully Treated by the “Coil Sandwich Technique”

A Case of Repeatedly Recurrent Basilar Trunk Dissecting Aneurysm Successfully Treated by the “Coil Sandwich Technique”

Bypass Surgery for the Treatment of Dolichoectatic Basilar Trunk Aneurysms: A Work in Progress.

The treatment of dolichoectatic basilar trunk aneurysms has been ineffectual or morbid due to nonsaccular morphology, deep location, and involvement of brainstem perforators. Treatment with bypass surgery has been advocated to eliminate malignant hemodynamics and to stabilize aneurysm growth. To validate that flow alteration with bypass and parent artery occlusion favorably impacts aneurysm progression. Surgical management evolved in 3 phases, each with different hemodynamic alterations. During a 17-year period, 37 patients with dolichoectatic basilar trunk aneurysms were retrospectively identified, of whom 21 patients were observed, 12 treated immediately, and 4 selected for treatment after clinical progression. In phase 1, flow reversal was overly thrombogenic, despite heparin (N = 5, final mortality, 100%). In phase 2, flow reduction with intracranial-to-intracranial bypass was safer than flow reversal, but did not prevent progressive aneurysm enlargement (N = 3, final mortality 67%). In phase 3, distal clip occlusion of the basilar trunk aneurysm preserved anterograde flow in the aneurysm without rupture, but reduced flow threatened perforator patency, despite treatment with clopidogrel (N = 8, final mortality 62%). Shifting treatment strategy for dolichoectatic basilar trunk aneurysms improved surgical (80% to 50%) and final mortalities (100% to 62%), with stabilization of aneurysms in the phase 3 survivors. Good outcomes are determined by perforator preservation and mitigating aneurysm thrombosis. Occlusion techniques with increased distal run-off seem to benefit perforators. The treatment of dolichoectatic basilar trunk aneurysms can advance through concentrated management in dedicated centers, concerted efforts to study morphology and hemodynamics with computational methods, and widespread collection of registry data. 4D PC-MRI, time-resolved phase-contrast MRIAICA, anterior inferior cerebellar arteryCE-MRA, high-resolution contrast-enhanced MR angiographyEC-IC, extracranial-to-intracranial bypassMCA, middle cerebral arteryMR, magnetic resonancemRS, modified Rankin ScalePCA, posterior cerebral arteryPICA, posterior inferior cerebellar arterySCA, superior cerebellar arterySTA, superficial temporal arteryVA, vertebral artery.

Early Recruitment of Cerebral Microcirculation by Neuronal Nitric Oxide Synthase Inhibition in a Juvenile Ischemic Rat Model

Background: The development of collateral circulation is proposed as an inherent compensatory mechanism to restore impaired blood perfusion after ischemia, at least in the penumbra. We have studied the dynamic macro- and microcirculation after ischemia-reperfusion in the juvenile rat brain and evaluated the impact of neuronal nitric oxide synthase (nNOS) inhibition on the collateral flow. Methods: Fourteen-day-old (P14) rats were subjected to ischemia-reperfusion and treated with either PBS or 7-nitroindazole (7-NI, an nNOS inhibitor, 25 mg/kg). Arterial blood flow (BF) was measured using 2D-color-coded pulsed ultrasound imaging. Laser speckle contrast (LSC) imaging and sidestream dark-field videomicroscopy were used to measure cortical and microvascular BF, respectively. Results: In basal conditions, 7-NI reduced BF in the internal carotids (by ∼25%) and cortical (by ∼30%) BF, as compared to PBS. During ischemia, the increased mean BF velocity in the basilar trunk after both PBS and 7-NI demonstrated the establishment of collateral support and patency. Upon re-flow, BF immediately recovered to basal values in the internal carotid arteries under both conditions. The 7-NI group showed increased collateral flow in the penumbral tissue during early re-flow compared to PBS, as shown with both LSC imaging and side-stream dark-field videomicroscopy. The proportion of perfused capillaries was significantly increased under 7-NI as compared to PBS when given before ischemia (67.0 ± 3.9 vs. 46.8 ± 8.8, p < 0.01). Perfused capillaries (63.1 ± 17.7 vs. 81.1 ± 20.7, p < 0.001) and the BF index (2.4 ± 0.6 vs. 1.3 ± 0.1, p < 0.001) significantly increased under 7-NI given at the re-flow onset. Conclusions: Collateral support in the penumbra is initiated during ischemia, and may be increased during early re-flow by neuronal NOS inhibition (given in pre- and post-treatment), which may preserve brain tissue in juvenile rats.