The American Society of Colon and Rectal Surgeons Clinical Practice Guidelines for the Management of Anorectal Abscess, Fistula-in-Ano, and Rectovaginal Fistula.

Abstract

The American Society of Colon and Rectal Surgeons (ASCRS) is dedicated to ensuring high-quality patient care by advancing the science and prevention and management of disorders and diseases of the colon, rectum, and anus. The Clinical Practice Guidelines Committee is composed of ASCRS members who are chosen because they have demonstrated expertise in the specialty of colon and rectal surgery. This committee was created to lead international efforts in defining quality care for conditions related to the colon, rectum, and anus and develop clinical practice guidelines based on the best available evidence. Although not proscriptive, these guidelines provide information on which decisions can be made and do not dictate a specific form of treatment. These guidelines are intended for the use of all practitioners, health care workers, and patients who desire information on the management of the conditions addressed by the topics covered in these guidelines. These guidelines should not be deemed inclusive of all proper methods of care nor exclusive of methods of care reasonably directed toward obtaining the same results. The ultimate judgment regarding the propriety of any specific procedure must be made by the physician considering all the circumstances presented by the individual patient. STATEMENT OF THE PROBLEM A generally accepted explanation for the cause of anorectal abscess and fistula-in-ano is that an abscess results from obstruction of an anal gland and that a fistula is caused by chronic infection and epithelialization of the abscess drainage tract.1–4 Anorectal abscesses are described by the anatomic space in which they develop; ischiorectal (also called ischioanal) abscesses are the most common followed by intersphincteric, supralevator, and submucosal locations.5–8 Anorectal abscess occurs more often in males than females, and although an abscess may develop at any age, the peak incidence is among 20- to 40-year-olds.4,8–12 In general, an abscess is treated with prompt incision and drainage.4,6,10,13 The diagnosis and treatment of necrotizing soft tissue infections and Fournier’s gangrene are beyond the scope of this guideline. Fistula-in-ano is an epithelialized tract that connects the perianal skin with the anal canal. In patients with an anorectal abscess, 30% to 70% present with a concomitant fistula-in-ano, and, of those who do not, approximately 30% to 50% will ultimately be diagnosed with a fistula in the months to years after abscess drainage.2,5,8–10,13–16 Although an anorectal abscess is described by the anatomic space in which it forms, a fistula-in-ano is classified in terms of its relationship with the internal and external anal sphincters (eg, the Parks classification; Table 1).16 In general, intersphincteric and transsphincteric fistulas are more frequently encountered than suprasphincteric, extrasphincteric, and submucosal tract locations.9,17–19 Anal fistulas may also be classified as “simple” or “complex.”20,21 Complex anal fistulas include transsphincteric fistulas that involve greater than 30% of the external sphincter, suprasphincteric, extrasphincteric, or horseshoe fistulas and anal fistulas associated with IBD, radiation, malignancy, preexisting fecal incontinence, or chronic diarrhea.20–22 Recurrent or branching fistulas may also be described as complex. Given the attenuated nature of the anterior sphincter in women, anterior fistulas deserve special consideration and may also be considered complex. Simple anal fistulas have none of these complex features and, in general, include intersphincteric and low transsphincteric fistulas that involve less than 30% of the external sphincter. TABLE 1. - Parks classification of fistula-in-ano Fistula type Description Submucosal Superficial fistula tract. Does not involve any sphincter muscle. Intersphincteric Crosses the internal sphincter and then has a tract to the perianal skin. Does not involve any external anal sphincter muscle. Transsphincteric Tracks from the internal opening at the dentate line via the internal and external anal sphincters and then terminates in the perianal skin or perineum. Suprasphincteric Courses superiorly into the intersphincteric space over the top of the puborectalis muscle and then descends through the iliococcygeus muscle into the ischiorectal fossa and into the perianal skin. Extrasphincteric Passes from the perineal skin through the ischiorectal fossa and levator muscles and then into the rectum and lies completely outside the external sphincter complex. Adapted from Parks et al.16 Distinct from cryptoglandular processes, anorectal abscess and fistula-in-ano can be manifestations of Crohn’s disease. Among patients with Crohn’s disease, fistula-in-ano has an incidence rate of 10% to 20% in population-based studies and 50% in longitudinal studies; meanwhile, nearly 80% of patients with Crohn’s disease who were cared for at tertiary referral centers may have a history of fistula-in-ano.23,24 In Crohn’s disease, anorectal abscesses and fistulas seem to result from penetrating inflammation rather than from infection of an anorectal gland.25 Patients with fistulas related to Crohn’s disease are typically managed with a multidisciplinary approach.26 Rectovaginal fistulas (RVFs), a unique subset of fistulas in many respects, may be classified as “low,” with a tract between the distal anal canal (at or below the dentate line) and the inside of the posterior fourchette; “high,” with a tract connecting the upper vagina (at the level of the cervix) with the rectum; and “middle” with a tract that lies in between these levels.27–29 The terms “anovaginal fistula” and “low rectovaginal fistula” may be used interchangeably. RVFs may also be classified as “simple” or “complex.” Simple RVFs have a low, small-diameter (<2 cm) communication between the anal canal and vagina and typically result from obstetrical injury or infection.29 “Complex” RVFs involve a higher tract between the rectum and vagina, are of a larger diameter, or result from radiation, cancer, or complications of pelvic surgical procedures.30–33 RVFs most commonly occur as a result of obstetric injury29 but may also occur in the setting of Crohn’s disease,25 malignancy, or infection,32 or as a complication of a failed colorectal anastomosis,33 an anorectal operation,34 or radiation therapy.35 The surgical treatment of a particular fistula is influenced by the patient’s presenting symptoms, unique anatomy of the fistula tract, quality of the surrounding tissues, and previous attempts at fistula repair.36 This guideline addresses the management of cryptoglandular fistulas, RVFs, and anorectal fistulas in the setting of Crohn’s disease. MATERIALS AND METHODS These guidelines were built on the last clinical practice guidelines for the management of anorectal abscess and fistula-in-ano published in 2016.37 An organized search was performed of MEDLINE, PubMed, Embase, and the Cochrane Database of Systematic Reviews between December 1, 2015, and November 5, 2021. Key word combinations using MeSH terms included abscess, fistula, fistula-in-ano, anal, rectal, perianal, perineal, rectovaginal, anovaginal, seton, fistulotomy, stem cell, advancement flap, ligation of intersphincteric fistula tract (LIFT), fistula plug, fistula glue, video-assisted anal fistula treatment (VAAFT), fistula laser closure (FiLaC), over-the-scope clip (OTSC) device, and Crohn’s disease. The search was restricted to English-language articles and studies of adult patients. Directed searches using embedded references from primary articles were performed in selected circumstances, and other sources including practice guidelines and consensus statements from relevant societies were also reviewed. The 841 screened articles were evaluated for their level of evidence, favoring clinical trials, meta-analysis/systematic reviews, comparative studies, and large registry retrospective studies during single-institutional series, retrospective reviews, and peer-reviewed, observational studies. A final list of 269 sources was evaluated for methodologic quality, the evidence base was analyzed, and a treatment guideline was formulated by the subcommittee for this guideline (Fig. 1). The final grade of recommendation and level of evidence for each statement were determined using the Grades of Recommendation, Assessment, Development, and Evaluation system (Table 2). When the agreement was incomplete regarding the evidence base or treatment guideline, consensus from the committee chair, vice chair, and 2 assigned reviewers determined the outcome. Members of the ASCRS Clinical Practice Guidelines Committee worked in joint production of these guidelines from inception to final publication (Table 3). The entire Clinical Practice Guidelines Committee reviewed the recommendations formulated by the subcommittee. Final recommendations were approved by the ASCRS Executive Council. In general, each ASCRS Clinical Practice Guideline is updated every 5 years. No funding was received for preparing this guideline, and the authors have declared no competing interests related to this material. This guideline conforms to the Appraisal of Guidelines for Research and Evaluation checklist. TABLE 2. - The GRADE system: grading recommendations Description Benefit versus risk and burdens Methodologic quality of supporting evidence Implications 1A Strong recommendation, high-quality evidence Benefits clearly outweigh risks and burdens or vice versa RCTs without important limitations or overwhelming evidence from observational studies Strong recommendation, can apply to most patients in most circumstances without reservation 1B Strong recommendation, moderate-quality evidence Benefits clearly outweigh risks and burdens or vice versa RCTs with important limitations (inconsistent results, methodologic flaws, indirect or imprecise) or exceptionally strong evidence from observational studies Strong recommendation, can apply to most patients in most circumstances without reservation 1C Strong recommendation, low- or very-low quality evidence Benefits clearly outweigh risks and burdens or vice versa Observational studies or case series Strong recommendation but may change when higher-quality evidence becomes available 2A Weak recommendation, high-quality evidence Benefits closely balanced with risks and burdens RCTs without important limitations or overwhelming evidence from observational studies Weak recommendation, best action may differ depending on circumstances or patients’ values or societal values 2B Weak recommendation, moderate-quality evidence Benefits closely balanced with risks and burdens RCTs with important limitations (inconsistent results, methodologic flaws, indirect or imprecise) or exceptionally strong evidence from observational studies Weak recommendation, best action may differ depending on circumstances or patients’ values or societal values 2C Weak recommendation, low- or very-low quality evidence Uncertainty in the estimates of benefits, risks, and burdens; benefits, risks, and burdens may be closely balanced Observational studies or case series Very weak recommendations; other alternatives may be equally reasonable GRADE = Grades of Recommendation, Assessment, Development, and Evaluation; RCT = randomized controlled trial.Adapted from Guyatt et al.38 Used with permission. TABLE 3. - What is new in the 2022 ASCRS Clinical Practice Guidelines for the management of anorectal abscess, fistula-in-ano, and rectovaginal fistula? 2022 New recommendations 11. Minimally invasive approaches to treat fistula-in-ano that use endoscopic or laser closure techniques have reasonable short-term healing rates but unknown long-term fistula healing and recurrence rates. Grade of recommendation: weak recommendation based on low-quality evidence, 2C. 19. Anorectal fistula associated with Crohn’s disease is typically managed with a combination of surgical and medical approaches. Grade of recommendation: strong recommendation based on moderate-quality evidence, 1B. 25. Local administration of mesenchymal stem cells is a safe and effective treatment for selected patients with refractory anorectal fistulas in the setting of Crohn’s disease. Grade of recommendation: weak recommendation based on moderate-quality evidence, 2B. 2022 Updated recommendations 5. Antibiotics should typically be reserved for patients with an anorectal abscess complicated by cellulitis, systemic signs of infection, or underlying immunosuppression. Grade of recommendation: weak recommendation based on moderate-quality evidence, 2C→2B. 9. A cutting seton may be used selectively in the management of complex cryptoglandular anal fistulas. Grade of recommendation: weak recommendations based on low-quality evidence, 2B→2C. 10. The anal fistula plug and fibrin glue are relatively ineffective treatments for fistula-in-ano. Grade of recommendation: strong recommendation based on moderate-quality evidence, 2B→1B. 21. Draining setons are typically useful in the multimodality therapy of fistulizing anorectal Crohn’s disease and may be used for long-term disease control. Grade of recommendation: strong recommendation based upon moderate-quality evidence, 1C→1B. 22. Symptomatic, simple, low anal fistulas in carefully selected patients with Crohn’s disease may be treated by lay-open fistulotomy. Grade of recommendation: weak recommendation based on low-quality evidence, 1C→ 2C. 23. Endorectal advancement flaps and the LIFT procedure may be used to treat fistula-in-ano associated with Crohn’s disease. Grade of recommendation: strong recommendation based on moderate-quality evidence, 2B→1B. ASCRS = American Society of Colon and Rectal Surgeons; LIFT = ligation of intersphincteric fistula tract. FIGURE 1.: PRISMA literature search flow sheet. PRISMA = Preferred Reporting Items for Systematic Reviews and Meta-Analyses.Initial Evaluation of Anorectal Abscess and Fistula 1. A disease-specific history and physical examination should be performed evaluating symptoms, relevant history, abscess and fistula location, and presence of secondary cellulitis. Grade of recommendation: strong recommendation based on low-quality evidence, 1C. Anorectal abscess is usually diagnosed on the basis of a patient’s history and physical examination. Anorectal pain and swelling are common with superficial abscesses, whereas spontaneous drainage and fever occur less often.8–10,38 Deeper abscesses, including those in the supralevator or high ischiorectal spaces, may present with pain referred to the perineum, lower back, or buttocks.6,39,40 Evaluation of the anus and perineum may reveal erythema, calor, fluctuance, cellulitis, or tenderness on palpation or may be relatively unrevealing, particularly in patients with intersphincteric or deeper abscesses,6,10,40,41 and digital rectal examination and anoscopy/proctoscopy are occasionally needed to clarify the diagnosis. The differential diagnosis of anorectal abscess may include fissure, hemorrhoid thrombosis, pilonidal disease, hidradenitis, anorectal neoplasia, Crohn’s disease, and sexually transmitted infections.6,42,43 Patients who present with anal fistula typically report intermittent anorectal swelling and drainage. Relevant information about baseline anal sphincter function, history of anorectal operations, family history of IBD, obstetric history, and associated GI, genitourinary, or gynecologic pathology should typically be included in the patient’s history. Inspection of the perineum should involve noting the specific findings of an abscess, surgical scars, anorectal deformities, signs of possible anorectal Crohn’s disease, and the presence of an external fistula opening. Gentle probing of an external opening, when tolerated, may help confirm the presence of a fistula tract but should be done with care to avoid creating false tracts.43 Goodsall’s rule, that an anterior fistula-in-ano has a radial tract and a posterior fistula has a curvilinear tract to the anus, has generally proven to be accurate for anterior fistulas but is less accurate in cases with a posterior fistula.44–47 2. Routine use of diagnostic imaging is not typically necessary for patients with anorectal abscess or fistula. However, imaging may be considered in selected patients with an occult anorectal abscess, recurrent or complex anal fistula, immunosuppression, or anorectal Crohn’s disease. Grade of recommendation: strong recommendation based on moderate-quality evidence, 1B. In a retrospective study of 113 patients with anorectal abscess, the overall sensitivity of CT for detecting an abscess was 77% and the sensitivity of CT in immunosuppressed patients was 70%.48 An advantage of MRI over CT is its ability to identify anorectal abscesses and associated fistula tracts. In a study of 54 patients with anorectal Crohn’s disease, in which MRI and operative/clinical findings were compared, all the abscesses and 82% of the fistulas were correctly identified by MRI.49 In a 2014 prospective study of 50 patients with suspected anorectal fistula, MRI had a 95% sensitivity, 80% specificity, and 97% positive predictive value in detecting and grading the primary fistula tract.50 In a retrospective study of 136 patients specifically looking at the role of MRI in the preoperative assessment of fistula patients, Konan et al51 found that MRI identified “significant” findings defined as secondary (blind) tracts, horseshoe abscesses, or abscesses undiagnosed by physical examination or examination under anesthesia in 34% of patients. In this study, MRI provided significant findings more frequently for complex fistulas than for simple fistulas (54% vs 5%; p < 0.001). Additionally, the proportion of patients who had significant MRI contributions increased with increasing Parks grade (5% for grade 1; 48% for grade 2; 86% for grade 3; 87.5% for grade 4). A prospective trial published in 2019, including 126 patients, assessed the utility of 3-dimensional endoanal ultrasound (EAUS) and MRI in both simple (n = 68) and complex (n = 58) anal fistulas and reported comparable accuracy for the 2 modalities in cases of a simple fistula; however, MRI had significantly higher sensitivity evaluating secondary extensions in complex fistulas (97% vs 74%; p = 0.04).52 Endoanal ultrasound, in 2 or 3 dimensions and with or without peroxide enhancement, may be useful in the management of patients with abscess or fistula, and studies demonstrate concordance between EAUS and operative findings in 73% to 100% of cases.53–55 Tantiphlachiva et al56 found that preoperative EAUS may help preserve anorectal function in patients undergoing anal fistula surgery. This study retrospectively evaluated pre- and postoperative Fecal Incontinence Severity Scores in 109 patients who underwent preoperative EAUS and in 230 patients without preoperative imaging and found significantly worse Fecal Incontinence Severity Scores in the group that did not undergo preoperative EAUS at a mean follow-up of 34 weeks. The potential added value of combining diagnostic modalities to enhance the accuracy of anal fistula assessment was exemplified in a 2001 blinded study of 34 patients with anorectal Crohn’s disease in which EAUS was accurate in 91% of patients, MRI was accurate in 87% of patients, and examination under anesthesia was accurate in 91% of patients, whereas 100% accuracy was achieved when any 2 techniques were combined.57 The sensitivity, accuracy, and utility of transperineal ultrasound (TPUS), a noninvasive alternative to EAUS, have also been studied in patients with anorectal abscess, anoperineal fistulas, and RVFs.58–61 A prospective study of 23 patients with Crohn’s disease comparing the diagnostic accuracies of EAUS, TPUS, and MRI with operative findings found that the diagnostic accuracy of all 3 modalities was nearly identical.62 The authors concluded that TPUS might be considered first-line imaging because of its availability, low cost, and noninvasive nature, yet because of its operator dependency and lack of high-quality supporting data, this imaging technique has not gained widespread popularity. Anorectal Abscess 3. Patients with acute anorectal abscess should be treated promptly with incision and drainage. Grade of recommendation: strong recommendation based on low-quality evidence, 1C. The primary treatment of anorectal abscess remains surgical drainage. In general, the incision should be made large enough to provide adequate drainage while taking care not to injure the anal sphincter complex. The perianal incision should be kept as close as possible to the anal verge to minimize the length of a subsequent fistula tract should one develop. Alternatively, a surgical drain (eg, Pezzer, Malecot) can be placed into the abscess cavity63,64 if this provides adequate drainage, although this technique typically does not address loculations within an abscess cavity and generally omits primary fistulotomy. Small comparative analyses have shown comparable efficacy and higher patient satisfaction with drain placement compared to incision and drainage.65–67 Once an abscess has been drained, randomized trials report equivalent or superior abscess resolution rates with less pain and faster healing in patients whose wounds were left unpacked.68,69 After drainage, abscesses may recur in up to 44% of patients, most often within 1 year of initial treatment.2,10,70 Inadequate drainage, the presence of loculations or a horseshoe-type abscess, and not performing a primary fistulotomy are risk factors for recurrent abscess (primary fistulotomy is further addressed in recommendation no. 4).10,71,72 Abscess location generally determines whether a patient should have internal versus external drainage. Intersphincteric abscesses should typically be drained through the intersphincteric groove or into the anal canal via an internal sphincterotomy.69 Similarly, it is usually preferable to drain supralevator abscesses originating from the complicated extension of an intersphincteric abscess internally by incising the rectal wall to prevent fistula formation. Meanwhile, supralevator abscesses because of cephalad extension of an ischiorectal abscess should typically be drained externally through the perianal skin.16,71 These approaches to abscess drainage may help prevent complex fistula formation. Abscesses that cross the midline (ie, horseshoe) can be challenging to manage. These abscesses most often involve the deep postanal space and extend laterally into the ischiorectal spaces.40,71 Under these circumstances, primary lay-open fistulotomy should typically be avoided because these fistulas tend to be transsphincteric. The Hanley procedure, a technique that drains the deep postanal space and uses counter incisions to address the ischiorectal spaces, is effective in the setting of a horseshoe abscess,71 although it may negatively impact anal sphincter function.40,71 A modified Hanley technique using a posterior midline partial sphincterotomy to unroof the postanal space plus seton placement has a high rate of abscess resolution and has been reported to better preserve anorectal function compared to other operative interventions.40,72,73 4. Abscess drainage with concomitant fistulotomy may be performed in selected patients with simple anal fistulas. Grade of recommendation: weak recommendation based on moderate-quality evidence, 2B. Although 30% to 70% of patients with anorectal abscesses present with a concomitant fistula-in-ano,10,11 the role of primary fistulotomy at the time of abscess drainage remains controversial. Although a fistulotomy may effectively address the offending crypt, edema and inflammation from the suppurative process may increase the risk of causing a false tract when probing a fistula and can make it difficult to accurately assess the anatomy, potentially causing the surgeon to underestimate the degree of sphincter involvement. Small, randomized studies evaluating primary fistulotomy have reported varied results with regard to fistula recurrence and fecal incontinence.12,74,75 Schouten and van Vroonhoven,12 in a randomized controlled trial, found that of 36 patients treated with primary fistulectomy and partial internal sphincterotomy only 3% had recurrence, whereas 39% reported postoperative sphincter disturbance at a median follow-up of 42 months; meanwhile, of 34 patients treated with incision and drainage alone, 41% had recurrence and 21% reported postoperative functional disturbance. Bokhari and Lindsey,74 in a retrospective review of 128 patients treated with either fistulotomy or sphincter preservation, found that after treatment, major incontinence was significantly more common in patients who had a complex fistula (13%) compared to those who had a simple fistula (5%). A 2010 Cochrane review that included 479 patients pooled from 6 randomized controlled trials demonstrated that sphincter division (via fistulotomy or fistulectomy) at the time of incision and drainage was associated with a significantly decreased likelihood of abscess recurrence, persistence of fistula or abscess, or need for subsequent surgery (relative risk, 0.13; 95% CI, 0.07–0.24) but an increased, albeit not statistically significant, incidence of continence disturbance at 1-year follow-up.75 Notably, the randomized trials included in this meta-analysis excluded patients with complex fistulas, recurrent abscesses, IBD, preexisting incontinence, or history of anorectal surgery and included patients with low fistulas. Given the potential negative consequences of a fistulotomy, some surgeons have advocated performing a partial fistulotomy with placement of a draining seton through the remaining tract. A retrospective review evaluated the outcomes of 26 patients with low transsphincteric fistulas who underwent partial fistulotomy and then draining seton placement (23 patients were male). Postoperatively, patients who had preserved anal sphincter function underwent a staged, completion fistulotomy. This study reported that at 1 year, all 24 patients who underwent staged fistulotomy reported no fistula or abscess recurrence or incontinence, supporting the approach of temporary seton placement followed by staged fistulotomy in selected patients with a low transsphincteric fistula.76 When a simple fistula is encountered during incision and drainage of an anorectal abscess, fistulotomy may be performed in selected patients provided that the anticipated benefit of healing outweighs the potential risk of fecal incontinence.1,4,5 However, placing a draining seton to treat a fistula discovered at the time of incision and drainage requires patients to proceed with a staged procedure to address their fistula.4,11,77 5. Antibiotics should typically be reserved for patients with an anorectal abscess complicated by cellulitis, systemic signs of infection, or underlying immunosuppression. Grade of recommendation: weak recommendation based on moderate-quality evidence, 2B. In general, administering antibiotics after performing an incision and drainage of a routine, uncomplicated anorectal abscess in a healthy patient does not improve healing or reduce the recurrence rate and is typically not recommended. However, antibiotics may be used selectively in patients with an anorectal abscess complicated by cellulitis, systemic illness, or underlying immunosuppression.4,10,13,78,79 Given the available evidence, the grade of this clinical practice guideline recommendation was changed from a 2C grade in 2016 to a 2B grade. A retrospective study of 172 patients with “uncomplicated” anorectal abscess who underwent incision and drainage with (n = 64) or without (n = 108) subsequent oral antibiotic therapy for 5 to 7 days reported that 9% of all patients required repeat surgery related to anorectal infection, but there was no significant difference between the groups in this regard.80 Patients with surrounding cellulitis, induration, or signs of systemic sepsis who did not receive antibiotics had a 2-fold increase in the rate of recurrent abscess compared with patients who received antibiotics, although this did not meet statistical significance. The authors also concluded that routinely culturing abscesses does not affect management or outcomes.80 A 2017 study evaluated the impact of postoperative antibiotics on fistula formation after incision and drainage of an anorectal abscess. In this single-blinded, randomized trial by Ghahramani and colleagues,81 307 patients were treated with incision and drainage with or without a 7-day postoperative course of ciprofloxacin and metronidazole. At 3-month follow-up, 14% of patients in the antibiotic treatment group developed an anal fistula versus 30% in the control group (p < 0.001). Contrary to this study, Sözener et al13 studied 334 patients in a randomized, placebo-controlled, double-blinded multicenter trial who showed no protective effect of antibiotics with regard to anal fistula formation. Although routinely culturing anorectal abscesses is not considered clinically useful, methicillin-resistant Staphylococcus aureus has been reported in up to 33% of patients.80,82,83 When methicillin-resistant Staphylococcus aureus is isolated from an anorectal abscess, a combination of abscess drainage and antibiotics directed against the organism is typically recommended for patients with systemic signs of sepsis, leukocytosis, or leukopenia.84 Microbial cultures should also be considered in cases of recurrent infection or nonhealing wounds.80 Data suggest that antibiotics play an important role in treatment for neutropenic or otherwise immunosuppressed patients with an anorectal abscess.85–87 Although patients with a higher absolute neutrophil count (ie, >1000/mm3) and fluctuance on examination typically have high resolution r