Morphology and pathophysiology of target anatomical sites for ablation procedures in patients with atrial fibrillation

2024 European Heart Rhythm Association/Heart Rhythm Society/Asia Pacific Heart Rhythm Society/Latin American Heart Rhythm Society expert consensus statement on catheter and surgical ablation of atrial fibrillation

In the last three decades, ablation of atrial fibrillation (AF) has become an evidence-based safe and efficacious treatment for managing the most common cardiac arrhythmia. In 2007, the first joint expert consensus document was issued, guiding healthcare professionals involved in catheter or surgical AF ablation. Mounting research evidence and technological advances have resulted in a rapidly changing landscape in the field of catheter and surgical AF ablation, thus stressing the need for regularly updated versions of this partnership which were issued in 2012 and 2017. Seven years after the last consensus, an updated document was considered necessary to define a contemporary framework for selection and management of patients considered for or undergoing catheter or surgical AF ablation. This consensus is a joint effort from collaborating cardiac electrophysiology societies, namely the European Heart Rhythm Association, the Heart Rhythm Society, the Asia Pacific Heart Rhythm Society, and the Latin American Heart Rhythm Society.

Catheter ablation using pulmonary vein isolation with versus without left atrial posterior wall isolation for persistent atrial fibrillation: an updated systematic review and meta-analysis

BACKGROUND

Pulmonary vein isolation (PVI) is the cornerstone of catheter ablation of atrial fibrillation (AF); however, the results are suboptimal for persistent AF. The left atrial posterior wall (LAPW) is thought to be a major additional area in initiation and perpetuation of persistent AF. Therefore, adjunctive ablation of the posterior wall may reduce AF recurrence in patients with persistent AF.

OBJECTIVE

The objective of this study was to compare outcomes of catheter ablation in patients with persistent AF using PVI alone versus a combination of PVI and LAPW isolation.

METHODS

Literature search was conducted in PubMed, PubMed Central, Scopus, and Embase since inception to February 2023. Screening of studies was done via Covidence software. Risk of bias assessment was done using appropriate tools. Data extraction and a narrative synthesis were carried out accordingly.

RESULTS

Ten studies were included, of which five were randomized controlled trials. PVI with LAPW ablation group had significantly lower recurrence of overall atrial tachyarrhythmia (OR 0.47, CI 0.32-0.70) and AF (OR 0.39, CI 0.23-0.69). In sensitivity analysis, freedom from atrial arrhythmias was noted to be significantly higher in the PVI with LAPW ablation group (OR 2.22, CI 1.36-3.64). However, there was no significant difference in occurrence of atrial flutter (OR 1.36, CI 0.86-2.14) or with periprocedural adverse events (OR 1.10, CI 0.60-1.99).

CONCLUSION

LAPW ablation, in addition to PVI, significantly improves the rates of arrhythmia freedom and reduces the recurrence of overall atrial tachyarrhythmia. There was no significant difference in atrial flutter or periprocedural adverse events.

2024 European Heart Rhythm Association/Heart Rhythm Society/Asia Pacific Heart Rhythm Society/Latin American Heart Rhythm Society expert consensus statement on catheter and surgical ablation of atrial fibrillation

In the last three decades, ablation of atrial fibrillation (AF) has become an evidence-based safe and efficacious treatment for managing the most common cardiac arrhythmia. In 2007, the first joint expert consensus document was issued, guiding healthcare professionals involved in catheter or surgical AF ablation. Mounting research evidence and technological advances have resulted in a rapidly changing landscape in the field of catheter and surgical AF ablation, thus stressing the need for regularly updated versions of this partnership which were issued in 2012 and 2017. Seven years after the last consensus, an updated document was considered necessary to define a contemporary framework for selection and management of patients considered for or undergoing catheter or surgical AF ablation. This consensus is a joint effort from collaborating cardiac electrophysiology societies, namely the European Heart Rhythm Association, the Heart Rhythm Society, the Asia Pacific Heart Rhythm Society, and the Latin American Heart Rhythm Society.

2024 European Heart Rhythm Association/Heart Rhythm Society/Asia Pacific Heart Rhythm Society/Latin American Heart Rhythm Society expert consensus statement on catheter and surgical ablation of atrial fibrillation

In the last three decades, ablation of atrial fibrillation (AF) has become an evidence-based safe and efficacious treatment for managing the most common cardiac arrhythmia. In 2007, the first joint expert consensus document was issued, guiding healthcare professionals involved in catheter or surgical AF ablation. Mounting research evidence and technological advances have resulted in a rapidly changing landscape in the field of catheter and surgical AF ablation, thus stressing the need for regularly updated versions of this partnership which were issued in 2012 and 2017. Seven years after the last consensus, an updated document was considered necessary to define a contemporary framework for selection and management of patients considered for or undergoing catheter or surgical AF ablation. This consensus is a joint effort from collaborating cardiac electrophysiology societies, namely the European Heart Rhythm Association, the Heart Rhythm Society (HRS), the Asia Pacific HRS, and the Latin American HRS.

The autonomic nerves around the vein of Marshall: a postmortem study with clinical implications

This study aims to analyze the vein of Marshall (VOM) in human autopsy hearts and its correlation with clinical data to elucidate the morphological substrates of atrial fibrillation (AF) and other cardiac diseases. Twenty-three adult autopsy hearts were studied, assessing autonomic nerves by immunohistochemistry with tyrosine hydroxylase (sympathetic nerves), choline acetyltransferase (parasympathetic nerves), growth-associated protein 43 (neural growth), and S100 (general neural marker) antibodies. Interstitial fibrosis was assessed by Masson trichrome staining. Measurements were conducted via morphometric software. The results were correlated with clinical data. Sympathetic innervation was abundant in all VOM-adjacent regions. Subjects with a history of AF, cardiovascular cause of death, and histologically verified myocardial infarction had increased sympathetic innervation and neural growth around the VOM at the mitral isthmus. Interstitial fibrosis increased with age and heart weight was associated with AF and cardiovascular cause of death. This study increases our understanding of the cardiac autonomic innervation in the VOM area in various diseases, offering implications for the development of new therapeutic approaches targeting the autonomic nervous system.

2024 European Heart Rhythm Association/Heart Rhythm Society/Asia Pacific Heart Rhythm Society/Latin American Heart Rhythm Society expert consensus statement on catheter and surgical ablation of atrial fibrillation

In the last three decades, ablation of atrial fibrillation (AF) has become an evidence-based safe and efficacious treatment for managing the most common cardiac arrhythmia. In 2007, the first joint expert consensus document was issued, guiding healthcare professionals involved in catheter or surgical AF ablation. Mounting research evidence and technological advances have resulted in a rapidly changing landscape in the field of catheter and surgical AF ablation, thus stressing the need for regularly updated versions of this partnership which were issued in 2012 and 2017. Seven years after the last consensus, an updated document was considered necessary to define a contemporary framework for selection and management of patients considered for or undergoing catheter or surgical AF ablation. This consensus is a joint effort from collaborating cardiac electrophysiology societies, namely the European Heart Rhythm Association, the Heart Rhythm Society, the Asia Pacific Heart Rhythm Society, and the Latin American Heart Rhythm Society .

The autonomic nervous system in atrial fibrillation-pathophysiology and non-invasive assessment

The autonomic nervous system plays a crucial role in atrial fibrillation pathophysiology. Parasympathetic hyperactivity result in a shortening of the action potential duration, a reduction of the conduction wavelength, and as such facilitates reentry in the presence of triggers. Further, autonomic remodeling of atrial myocytes in AF includes progressive sympathetic hyperinnervation by increased atrial sympathetic nerve density and sympathetic atrial nerve sprouting. Knowledge on the pathophysiological process in AF, including the contribution of the autonomic nervous system, may in the near future guide personalized AF management. This review focuses on the role of the autonomic nervous system in atrial fibrillation pathophysiology and non-invasive assessment of the autonomic nervous system.

Adjunct posterior wall isolation reduces the recurrence of atrial fibrillation in patients undergoing cryoballoon ablation: A systematic review and meta-analysis

BACKGROUND

Recurrence rates of atrial fibrillation (AF) remain high even after complete wide area circumferential pulmonary vein isolation (PVI). In recent years adjunct posterior wall isolation (PWI) has been performed in patients with more persistent forms of AF but the benefits remain unclear.

AIM

The objective of this meta-analysis was to evaluate the efficacy of adjunct posterior wall isolation in reducing recurrence rates of AF using cryoballoon ablation (CBA).

METHODS

We searched PubMed, Google Scholar, Clinicaltrials.gov and Cochrane CENTRAL. We included studies comparing PVI to PVI + PWI in patients with either persistent or paroxysmal AF (PAF) undergoing CBA. After data extraction and quality assessment of the studies, we assessed recurrence rates of atrial tachy-arrhythmias (AF, atrial flutter, and atrial tachycardia) as well as total ablation time and procedural adverse events. Risk ratio (RR), mean difference (MD), and 95% confidence interval (CI) were calculated using Review Manager.

RESULTS

Concomitant PWI exhibited a substantial reduction in the risk of AF recurrence (RR: 0.51; 95% CI: 0.42-0.63, p < .00001), as well as all atrial arrhythmias (RR: 0.58; 95% CI: 0.49-0.68, p < .00001). On subgroup analysis, in patients with only PAF, adjunct PWI resulted in significant reduction in recurrence risk of AF (RR: 0.56; 95% CI: 0.41-0.76, p = .0002) as well. There was no significant difference in adverse events between both groups (RR: 0.90; 95% CI: 0.44-1.86; p = .78), whereas total ablation time was significantly increased in PVI + PWI group (MD: 21.75; 95% CI: 11.13-32.37, p < .0001).

CONCLUSION

Adjunct PWI when compared to PVI alone decreases recurrence rates of atrial fibrillation after CBA of patients with persistent as well as paroxysmal atrial fibrillation.

Selective anatomical catheter ablation of left atrial ganglionated plexus for vasovagal syncope: Left superior and right anterior ganglionated plexus ablation

BACKGROUND

Vasovagal syncope (VVS) is one of the most common causes of syncope. Traditional treatment has not achieved satisfactory results. The purpose of this study was to assess the feasibility and efficacy of selective anatomical catheter ablation of left atrial ganglionated plexus (GP) as a therapeutic strategy for the patients of symptomatic VVS.

METHODS

A total of 70 patients with at least once recurrent syncopal episode of VVS with a positive head-up tilt test were enrolled. They were divided into GP ablation group and control group. Patients in GP ablation group received anatomical catheter ablation of left superior ganglionated plexus (LSGP) and right anterior ganglionated plexus (RAGP). Patients in the control group were guideline-directed conventional therapy. The primary endpoint was VVS recurrences. The secondary endpoint was the recurrence of syncope and prodrome events.

RESULTS

There were no statistical differences in clinical characteristics between the ablation group (n = 35) and the control group (n = 35). Over a follow-up of 12 months, the ablation group had significantly lower syncope recurrence compared with the control group (5.7% vs. 25.7%, p = .02), and the ablation group had significantly lower syncope and prodrome recurrence compared with the control group (11.4% vs. 51.4%, p < .001). In GP ablation, 88.6% of the patients showed significant vagal response during LSGP ablation, and 88.6% of the patients showed significantly increased heart rate during RAGP ablation.

CONCLUSIONS

For patients with recurrent VVS, selective anatomical catheter ablation of LSGP and RAGP is superior to conventional therapy in reducing syncope recurrence.

Review article: diagnosis, pathophysiology and management of atrial fibrillation in cirrhosis and portal hypertension

BACKGROUND

Atrial fibrillation (AF) is the most common arrhythmia and its management in cirrhosis can be challenging due to the altered hepatic metabolism of medications and increased risk of bleeding.

AIMS

To provide a comprehensive overview of the diagnosis, pathophysiology and management of AF in patients with cirrhosis from both a cardiology and a hepatology perspective.

METHODS

An extensive literature search was performed using the terms 'atrial fibrillation' and 'cirrhosis'. Guideline documents and consensus statements were explored.

RESULTS

The prevalence of AF in patients with cirrhosis ranges between 6.6% and 14.2%, while the incidence of new-onset AF in the post-operative period after liver transplant ranged between 6.8% and 10.2%. AF in patients with cirrhosis is associated with adverse outcomes in both pre-transplant and post-transplant settings, including an increased risk of stroke when compared to the general population. We review the pathogenesis of AF in general and in cirrhosis. This review also provides guidance on the management of AF, including the use of anticoagulation and rate versus rhythm control. In the absence of strict contraindications, all patients with cirrhosis and AF should be anticoagulated. The use of DOACs is preferred over vitamin K antagonists. In patients with a high bleeding risk, a DOAC with an approved antidote may be preferred.

CONCLUSIONS

Atrial fibrillation is increased in patients with cirrhosis. AF management requires careful consideration of treatment options. Since patients with cirrhosis were excluded from all major randomised clinical trials, dedicated research on the pathophysiology and management of AF in cirrhosis is needed.

Adjunctive Left Atrial Posterior Wall Isolation in Treating Atrial Fibrillation: Insight From a Large Secondary Analysis

OBJECTIVES

This study evaluated the role of adjunctive left atrium posterior wall isolation (PWI) in preventing atrial fibrillation (AF) recurrence.

BACKGROUND

The left atrium posterior wall is an arrhythmogenic substrate that contributes to the development of AF.

METHODS

This was an updated pooled analysis that included clinical data regarding PWI plus pulmonary vein isolation (PVI) in treating AF. The primary outcome was recurrence of atrial tachyarrhythmias.

RESULTS

A total of 26 studies with 3,287 patients with AF (age 61.7 ± 10.8 years) were included. Mean follow-up was 15.2 ± 8.4 months. Procedural success to achieve PWI was 92.8%. In paroxysmal AF, adjunctive PWI did not reduce the recurrence of all atrial arrhythmias (P = 0.21) or AF (P = 0.37); however, in persistent AF, adjunctive PWI was associated with substantially lower recurrence of all atrial arrhythmias (risk ratio: 0.74; P < 0.001) and AF (risk ratio: 0.67; P = 0.01), particularly when randomized data were included. Subgroup analyses based on meta-regression demonstrated that patients with older age, a larger left atrial diameter, and persistent AF benefited more significantly from the adjunctive PWI. Adjunctive PWI using either radiofrequency or a cryoballoon reduced AF recurrence, whereas using radiofrequency seemed to be associated with higher recurrence rate of atrial tachycardias and/or atrial flutter. Non-BOX adjunctive PWI rather than BOX PWI was associated with significantly lower recurrence of AF. The incidence of procedural adverse events between the PVI+PWI (3.2%) and PVI (2.8%) groups was low and similar. PVI+PWI needed longer ablation and/or procedural time but had similar fluoroscopy time.

CONCLUSIONS

Adjunctive PWI can be achieved in most patients without compromising safety. Patients with persistent AF appear to benefit from this approach. The ablation technology and/or approach may affect the clinical outcome of PWI.

Clinical outcomes of adjunctive posterior wall isolation in persistent atrial fibrillation: A meta-analysis

BACKGROUND

Pulmonary vein isolation (PVI) is the cornerstone of atrial fibrillation (AF) ablation but the recurrence rate remains relatively high in persistent patients with AF. Therefore, posterior wall isolation (PWI) in addition to PVI has been proposed to increase freedom from AF.

OBJECTIVE

To evaluate the success of adjunctive PWI in persistent AF.

METHODS

We searched electronic database using specific terms. The primary outcomes are recurrence rate of AF and recurrence of atrial arrhythmias. The secondary outcomes were atrial flutter/tachycardia (AFL/AT), procedure time, fluoroscopy time, and procedure related complications. Estimated risk ratios (RRs) and 95% confidence intervals (CIs) were evaluated.

RESULTS

Six studies were included (1334 patients with persistent AF). Adjunctive PWI resulted in a significant reduction in the recurrence rate of AF compared with patients who had PVI only (19.8% vs 29.1%; RR, 0.64; 95% CI, 0.42-0.97; P < .04; I²  = 76%). There was a significant reduction in the recurrence rate of all atrial arrhythmia (30.8% vs 41.1%; RR, 0.75; 95% CI, 0.60-0.94; P < .01; I²  = 60%). Compared with PVI only, adjunctive PWI did not increase the rate of AFL or AT (11.6% vs 13.9%; RR, 0.85; 95% CI, 0.54-1.32; P < .46; I²  = 47%) or the rate of procedure related complications (4.6% vs 3.6%; RR, 1.25; 95% CI, 0.72-2.17; P < .44; I²  = 0%).

CONCLUSION

In patients with persistent AF, adjunctive PWI was associated with decreased recurrence of AF and atrial arrhythmias compared with PVI alone without an increased risk of AFL or AT or procedure related complications.

Individualized ablation strategy guided by live simultaneous global mapping to treat persistent atrial fibrillation

Atrial fibrillation (AF) is the most common clinical arrhythmia encountered. Catheter ablation has become the first-line therapy for symptomatic drug-refractory paroxysmal and persistent AF. Although pulmonary vein electrical isolation is still the cornerstone of the ablation strategy, the clinical outcome particularly in treating persistent AF is suboptimal. Significant efforts have been applied with live global chamber mapping of AF aimed to identify patient-specific drivers and/or maintainers located outside of the pulmonary veins to further improve the outcome of catheter ablation. Within this review, we present an overview of contemporary global chamber AF mapping technologies and characteristics, with a particular focus on global, noncontact, dipole density mapping illustrated with a clinical case of persistent AF ablation using this novel methodology.

Longstanding persistent accelerated idioatrial rhythm: Benign sinus node-like rhythm or insidious rhythm?

INTRODUCTION

Detailed description of longstanding persistent accelerated idioatrial rhythm (AIAR) is lacking. This observational study investigated the clinical manifestations, electrophysiological characteristics, diagnosis, treatment, and prognosis of this unusual arrhythmia.

METHODS AND RESULTS

Fifteen patients (11 males; average age 25.9 ± 15.7 years) suspected with longstanding persistent AIAR were enrolled in our study. All patients had electrocardiogram (ECG), 24-hour Holter monitoring, isoproterenol provocation test, echocardiogram, and exercise treadmill test. Electrophysiological study (EPS) and catheter ablation were performed if necessary. The above noninvasive tests would be repeated during follow-up. Among the patients, 10 were asymptomatic; 5 had concomitant paroxysmal atrial tachycardia. Two asymptomatic patients had impaired left ventricular function. AIAR was observed throughout 24-hour Holter monitoring, showing chronotropic profile similar to sinus rhythm (SR). Such AIAR exhibited competitive property with SR when provoked by isoproterenol or during treadmill test. Twelve patients had EPS and 8 of them had successful ablation to eliminate AIAR. During a medium follow-up of 3.7 years, all patients were in well clinical course and preserved left ventricular dysfunction, and 3 patients spontaneously reverted to SR at 10-year follow-up.

CONCLUSIONS

Longstanding persistent AIAR is an unusual entity of atrial arrhythmias and in most situations a benign rhythm requiring no treatment. The clinical course will be worsened when AIAR develops rapid focal firing, is associated with focal atrial tachycardias or results in tachycardia-mediated cardiomyopathy, but can be resolved via catheter ablation.

Expression of connexin 43, ion channels and Ca2+-handling proteins in rat pulmonary vein cardiomyocytes

Atrial fibrillation (AF) is the most common cardiac arrhythmia. AF is thought to be triggered by ectopic beats, originating primarily in the myocardial sleeves surrounding the pulmonary veins (PVs). The mechanisms underlying these cardiac arrhythmias remain unclear. To investigate this, frozen sections of heart and lung tissue from adult rats without arrhythmia were obtained in different planes, stained with Masson's trichrome, and immunolabeled for connexin 43 (Cx43), caveolin-3 (Cav3), hyperpolarization-activated cyclic nucleotide-gated channel 4 (HCN4), Na_v1.5, Kir2.1, and the calcium handling proteins sarcoplasmic/endoplasmic reticulum calcium-ATPase 2a (SERCA2a) and ryanodine receptor 2 (RyR2). Transverse sections offered the best view of the majority of the PVs in the tissue samples. Cx43 was observed to be expressed throughout the atria, excluding the sinoatrial and atrioventricular nodes, and in the myocardial sleeves of the PVs. In contrast, HCN4 was only expressed in the sinoatrial and atrioventricular nodes. The immunodensity of Cav3, Na_v1.5, Kir2.1, SERCA2a and RyR2 in the PVs imaged was similar to that in atria. The results suggest that in the absence of arrhythmia, the investigated molecular properties of the ion channels of rat PV cardiomyocytes resemble those of the working myocardium. This indicates that ectopic beats originating in the myocardial sleeves of the PVs occur only under pathological conditions.

Is ganglionated plexi ablation during Maze IV procedure beneficial for postoperative long-term stable sinus rhythm?

BACKGROUND

We investigated the role of surgical ablation targeting the autonomous nervous system during a Cox-Maze IV procedure in the maintenance of sinus rhythm at long-term follow-up.

METHODS

The patient population consisted of 519 subjects with persistent or long-standing persistent atrial fibrillation (AF) undergoing radiofrequency Maze IV during open heart surgery between January 2006 and July 2013 at three institutions without (Group 1) or with (Group 2) ganglionated plexi (GP) ablation. Recurrence of atrial fibrillation off-antiarrhythmic drugs was the primary outcome. Predictors of AF recurrence were evaluated by means of competing risk regression. Median follow-up was 36.7 months.

RESULTS

The percentage of patients in normal sinus rhythm (NSR) off-antiarrhythmic drugs did not differ between groups (Group 1-75.5%, Group 2-67.8%, p = 0.08). Duration of AF ≥ 38 months (p = 0.01), left atrial diameter ≥ 54 mm (0.001), left atrial area ≥ 33 cm(2) (p = 0.005), absence of connecting lesions (p= 0.04), and absence of right atrial ablation (p < 0.001) were independently associated with high incidence of AF recurrence. In contrast the absence of GP ablation was not a significant factor (p = 0.12).

CONCLUSIONS

GP ablation did not prove to be beneficial for postoperative stable NSR. A complete left atrial lesion set and biatrial ablation are advisable for improving rhythm outcomes. Randomized controlled trials are necessary to confirm our findings.

Persistent lone atrial fibrillation: clinicopathologic study of 19 cases

BACKGROUND

The extent to which atrial myocardium is remodeled in patients with persistent lone atrial fibrillation (LAF) is largely unknown.

OBJECTIVE

The purpose of this study was to perform a clinicopathologic investigation in patients with persistent LAF.

METHODS

We characterized structural and molecular remodeling in atrial biopsies from 19 patients (17 males, mean age 49 years) with persistent (>7 days; n = 8) or long-lasting persistent (>1 year; n = 11) LAF who underwent surgical ablation. Atrial tissue from 15 autopsy samples without clinicopathologic evidence of heart disease served as controls.

RESULTS

Morphometric analysis showed cardiomyocyte hypertrophy and greater amounts of myolytic damage and interstitial fibrosis in persistent LAF patients compared to controls (P <.0001). Atrial tissue levels of heme oxygenase-1 and 3-nitrotyrosine were increased in persistent LAF patients (P <.001), consistent with oxidative stress. Levels of superoxide dismutase-2, interleukin-8, interleukin-10, tumor necrosis factor-α, and thiobarbituric acid reactive substance were greater in controls than in persistent LAF patients. Immunoreactive signal for connexin43 was reduced more frequently in persistent LAF patients than controls. There was no correlation between features of structural or molecular remodeling and clinical parameters, including persistent LAF duration.

CONCLUSION

In persistent LAF patients, the atria are modified by structural remodeling and molecular changes of oxidative stress. Tissue changes in persistent LAF appear to occur early after its onset and are qualitatively no different than those observed in patients with atrial fibrillation related to conventional risk factors. These findings suggest that different types of atrial fibrillation are associated with the same spectrum of tissue lesions. Early intervention to restore sinus rhythm in persistent LAF patients may prevent irreversible tissue change, especially interstitial fibrosis.

Atrial fibrillation from the pathologist's perspective

Atrial fibrillation (AF), the most common sustained cardiac arrhythmia encountered in clinical practice, is associated with increased morbidity and mortality. Electrophysiologically, it is characterized by a high rate of asynchronous atrial cell depolarization causing a loss of atrial contractile function and irregular ventricular rates. For a long time, AF was considered as a pure functional disorder without any structural background. Only in recent years, have new mapping and imaging techniques identified atrial locations, which are very often involved in the initiation and maintenance of this supraventricular arrhythmia (i.e. the distal portion of the pulmonary veins and the surrounding atrial myocardium). Morphological analysis of these myocardial sites has demonstrated significant structural remodeling as well as paved the way for further knowledge of AF natural history, pathogenesis, and treatment. This architectural myocardial disarrangement is induced by the arrhythmia itself and the very frequently associated cardiovascular disorders. At the same time, the structural remodeling is also capable of sustaining AF, thereby creating a sort of pathogenetic vicious circle. This review focuses on current understanding about the structural and genetic bases of AF with reference to their classification, pathogenesis, and clinical implications.