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Selection bias occurs when the relationship between the exposure and outcomes
changes from group to group due to systematic differences in the characteristics of
the groups. In RCTs, this is typically controlled for by the randomization process,
which should be blinded. Selection bias is much harder to control for in retrospective studies. A common method by which a statistical adjustment is made is to perform a propensity score analysis. However, propensity score analysis is not without
criticism and in some instances may actually worsen the imbalance [15]. A more
in- depth discussion of propensity score analysis is beyond the scope of text and
readers should refer to other sources when faced with a paper that utilizes this
technique.
Measurement bias comes in many forms and may result from a bias in the measured exposure such as recall bias, a bias over time in the way the exposure is
dened such as a change in diagnostic criteria or a bias in the way the observer
performing measurements.
Confounding leads to an error in the interpretation of the interaction between
exposure and outcome due to inadequate control of other variables. A confounding
variable is one that is associated with both the exposure and the outcome but is not
found in the causal pathway between the exposure and the outcome. For example,
in a study of rectal cancer patients with neoadjuvant chemoradiation being the exposure and survival being the outcome, age of the patient is a confounding variable
because age is likely associated with the likelihood of neoadjuvant therapy administration and older patients are at greater risk of mortality. Confounding is an incredibly common and difcult issue in retrospective observational studies. Hence,
investigators frequently use statistical models of different complexities to attempt to
control for as much confounding as possible and it is the task of the reader to determine whether enough confounding is accounted for to make the reported interaction
between exposure and outcome believable. [16]
Z. Xu and B. Sklow
Was theStudy Large Enough andContinued Long Enough
toMake theResults Believable?
Sample size and power was discussed earlier in the context of controlling for random error. To reiterate, a study or trial needs to be large enough to have a high
chance of detecting a statistically signicant difference between groups.
A study needs to be continued for long enough and participants need to have
been adequately followed up on for the effect of the exposure to be seen in the outcome. This obviously depends on the type of exposure and type of outcome. If we
return to a rectal cancer example, a study looking at the impact of an enhanced
recovery protocol on hospital length of stay after proctectomy may only need a follow up of a couple of weeks. However, if the study was instead looking at the impact
of an enhanced recovery protocol on 30-day readmission, we would expect that the
majority of the patients have at least a follow up of 30days in order to capture the
event of interest. In RCTs, careful attention has to be paid to the completeness of
follow up and dropout rate of participants. RCTs that exclude patients that were
1 Evaluating Evidence
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randomized but dropout risk biasing their results, therefore it is standard practice to
analyze the results based on an intent-to-treat. This means that all patients that were
randomized are analyzed as part of the original group they were randomized to. The
reader should conrm that the paper clearly states an intention-to-treat analysis and
also conrm the patient numbers within the results to the number of patients
randomized.
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Final Thoughts
We all strive to make clinical decisions based on the best clinical evidence available.
The analytical process involved in determining whether a study result can be applied
to clinical practice is complex and should be interpreted in the context of what is
already known. This unfortunately necessitates a discussion of publication bias.
Publication bias may be the most difcult bias to overcome because it relates to hidden information that the reader never gets a chance to assess. Negative studies are
less likely to get published and if they are published, frequently end up in lower
impact journals or never make it to manuscript from their abstract form. This unfortunately leads to a bias towards overestimation of treatment effects and is a bias that
may not be correctable no matter how high quality the studies or the subsequent
meta-analyses are [17].
Another common phenomenon in clinical literature is a heavy reliance on
p- values and condence intervals. This has resulted in a lack of true critical evaluation of methodology and biases. The use of p-values is increasingly being criticized
because of the potential impact of sample size on it. With a large enough sample
size, the p-value will almost certainly be signicant, even though the effect size
could be very small. Alternatively, a study with a small sample size may have insignicant p-values, but there could be a very large effect that wasn’t detected. While
both p-values and condence intervals rely on statistical inference and sample size,
condence intervals at least provide more information about the degree of uncertainty. [18]
Personal View
When evaluating a study or manuscript, the rst question to ask is what is the
hypothesis or question the study is trying to answer? If not familiar with the evidence on the subject matter, a search of the literature might be required to determine
if there is novelty of the study or if it provides a signicant contribution to the literature. The other question one should ask is the study designed to answer the proposed
hypothesis or question the authors are asking.
Assuming the study design is solid, we would then want to know if the study was
powered adequately to show a difference or to prove or disprove the hypothesis.
Was a power calculation done? In the results section, are the tables and gures formatted properly with appropriate labels? Do the results make sense and are the data
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Z. Xu and B. Sklow
relative to the hypothesis? There are some studies where there is data overload and
not relative to the question being asked. Does the study suffer from any kind of bias
that could affect the results (e.g. selection bias, etc)? We would also want to know if
the length of follow up was appropriate to answer the proposed question. For example, most studies looking at cancer survival have at least 5year follow up as 5year
survival is the standard in the literature.
As far as the Discussion is concerned, is the length appropriate? It is not uncommon for the Discussion section to be too long and wordy, with extraneous information provided that is not relevant. The Discussion should highlight the novelty of the
ndings and contain references to other previously published studies that are relevant. Lastly, are the conclusions appropriate and are they supported by the data? A
study can only draw conclusions based on the data within the study and a conclusion must be supported by the data.
Conclusion
Ultimately, it is up to clinicians to weigh the risks and benets of their decisions and
apply the available evidence to each unique patient, which may be the greatest challenge to evidence-based medicine. So much of EBM is provisioned on the importance of empirical data that it is easy to forget that to an individual patient, if they
are unable to benet from the study, it doesn’t matter how high quality it was. The
only way to do this effectively is to have a framework of skills to evaluate the evidence, integrate knowledge, and adapt evidence to the needs of our patients.
References
1. Thoma A, Eaves FF.A brief history of evidence-based medicine (EBM) and the contributions
of Dr David Sackett. Aesthet Surg J. 2015;35:NP261–3.
2. Greenhalgh T.How to read a paper: the basics of evidence-based medicine and healthcare.
Wiley; 2019.
3. Arya S, Schwartz TA, Ghaferi AA.Practical guide to meta-analysis. JAMA Surg. 2020;155:430.
4. Brasel K, Haider A, Haukoos J.Practical guide to survey research. JAMA Surg. 2020;155:351.
5. Brooke BS, Kaji AH, Itani KMF.Practical guide to cost-effectiveness analysis. JAMA Surg.
2020;155:250.
6. Davidson GH, Haukoos JS, Feldman LS.Practical guide to assessment of patient-reported
outcomes. JAMA Surg. 2020;155:432.
7. Dossett LA, Kaji AH, Dimick JB.Practical guide to mixed methods. JAMA Surg. 2020;155:254.
8. Merkow RP, Schwartz TA, Nathens AB.Practical guide to comparative effectiveness research
using observational data. JAMA Surg. 2020;155:349–50.
9. Neuman HB, Kaji AH, Haut ER.Practical guide to implementation science. JAMA Surg.
2020;155:434.
10. Scott JW, Schwartz TA, Dimick JB.Practical guide to health policy evaluation using observational data. JAMA Surg. 2020;155:353.
11. Segev DL, Haukoos JS, Pawlik TM. Practical guide to decision analysis. JAMA Surg.
2020;155:436.
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12. Van Spall HG, Toren A, Kiss A, Fowler RA. Eligibility criteria of randomized controlled
trials published in high-impact general medical journals: a systematic sampling review.
JAMA. 2007;297(11):1233–40.
13. Popovic A, Huecker MR.Study Bias. In: StatPearls. StatPearls Publishing; 2022.
14. Ambrosius WT.Topics in biostatistics. Humana Press; 2007.
15. Guo S, Fraser M, Chen Q.Propensity score analysis: recent debate and discussion. J Soc Soc
Work Res. 2020;11:463–82.
16. Groenwold RHH, Van Deursen AMM, Hoes AW, Hak E. Poor quality of reporting confounding bias in observational intervention studies: a systematic review. Ann Epidemiol.
2008;18:746–51.
17. Guyatt GH, etal. GRADE guidelines: 5. Rating the quality of evidence—publication bias. J
Clin Epidemiol. 2011;64:1277–82.
18. Greenland S, etal. Statistical tests, P values, condence intervals, and power: a guide to misinterpretations. Eur J Epidemiol. 2016;31:337–50.
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Part I
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Inflammatory Bowel Disease
Surgical vs Medical Management
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ofSymptomatic Anal Fistulas inPatients
withCrohn’s Disease
MeganObi andAmyL.Lightner
Introduction
Crohn’s disease (CD) is a chronic inammatory disease that predominantly affects
the gastrointestinal (GI) tract. Along with ulcerative colitis (UC), it is thought to
affect 1.3% of the US population and has an annual incidence of 3 to 20 cases per
100,000 [1, 2]. It is characterized by transmural inammation of the GI tract that
results in intestinal wall damage. One of the most common presentations of CD is
the formation of perianal stulas which affects anywhere from 5–40% of CD
patients worldwide [3]. Unlike idiopathic anal stula that develop from occlusion
and infection of the anal glands resulting in cryptoglandular abscesses, stulas
related to CD are thought to be due to the inammatory process characteristic of CD
which penetrates the mucosal lining of the GI tract [4]. Perianal stulizing disease
is quite heterogeneous in its presentation, and complete remission is notoriously
challenging to achieve. Therefore, medical and surgical management are both necessary to optimize patient outcomes.
CD patients with perianal stulizing disease report signicantly impaired quality
of life, overall health, and physical and sexual function [5, 6]. At presentation of
perianal disease, several patients may not have an underlying diagnosis of CD, as
perianal stulas can be the initial presenting phenotype in 10% of CD patients [7].
The presence of a perianal stula alone predicts increased utilization of medications
2
M. Obi
Department of General Surgery, Digestive Disease Surgical Institute, Cleveland Clinic,
Cleveland, OH, USA
e-mail: Obim2@ccf.org
A. L. Lightner (*)
Department of Colorectal Surgery, Digestive Disease Surgical Institute, Cleveland Clinic,
Cleveland, OH, USA
© The Author(s), under exclusive license to Springer Nature
Switzerland AG 2023
K. Umanskiy, N. Hyman (eds.), Difcult Decisions in Colorectal Surgery,
Difcult Decisions in Surgery: An Evidence-Based Approach,
https://doi.org/10.1007/978-3-031-42303-1_2
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M. Obi and A. L. Lightner
such as steroids and immunosuppressants, frequent re-hospitalizations, and likelihood of undergoing at least one surgical procedure given that perianal disease signals a more severe phenotype of CD.Because the recurrence rate of perianal stulas
is as high as 60% [3], treatment has required a multidisciplinary approach with
medicine optimizing the underlying disease process and surgery attempting to anatomically close the internal opening(s).
PICO Table
Patients
Patients with stula
in ano due
to Crohn’s Disease
Intervention Comparator
Surgery Medical
therapy
Outcome
Fistula healing, fecal continence, stula
stula recurrence
Diagnosis
Diagnosis of perianal stulas is commonly initiated by identication of symptoms
commonly associated with stula presence. Patients can describe new onset rectovaginal discharge, perianal erythema or skin irritation, perianal pain, air or stool in
the urine, and pain with defecation. Systemic symptoms such as fever are rarely
associated with perianal stulas [8]. A thorough history and physical exam evaluating for skin tags, hemorrhoidal tissue, ssures, bleeding, and palpation and inspection of areas of uctuance and external tract openings is the rst step in diagnosis.
Subsequent imaging may be a helpful next step in better characterizing the underlying stula anatomy. In certain scenarios, this can be a useful adjunct to guide
treatment.
The most common diagnostic imaging modalities include magnetic resonance
imaging (MRI) and endoanal ultrasonography (EUS). MRI is typically the initial
modality utilized for both diagnosis and follow-up as it is non-invasive, and can
serve to help locate stula tract openings that would otherwise be difcult to nd on
EUA as well as diagnose more complex stula tracts and locate clinically “silent”
uid collections [9]. T2- weight sequencing with fat suppression is the optimal
imaging technique but gadolinium-enhanced T1 weighted sequences can help differentiate uid/ gas from granulation tissue [10]. EUS, like MRI, can help identify
internal tract openings as well as provide more anatomical details of the sphincter
complex. It is limited though by its inability to identify ischioanal fossa or supralevator abscesses given poor penetration [11].
Computerized tomography (CT) and stulography are modalities that have been
previously considered in the diagnosis of perianal stulas, but have been deemed to
have poor accuracy and thus are not typically recommended for diagnostic purposes. There are few exceptions when stulography can provide additional information in the setting of particularly complex stulas [12]. In addition, in an acute
clinical setting where CT may be more readily available, it can be a useful modality
to expedite diagnosis and treatment of less clinically “silent” disease.
2 Surgical vs Medical Management of Symptomatic Anal Fistulas in Patients…
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Fig. 2.1 Axial image of a T2- weighted MRI of the pelvis demonstrating a complex transsphincteric stula and gluteal abscess (yellow arrows). (Adapted from Sharma etal. [14])
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The gold standard for diagnosis and treatment is an exam under anesthesia
(EUA). EUAs can be performed in either lithotomy or prone positioning and have
the benet of allowing for immediate treatment as well as diagnosis [12].
Experienced colorectal surgeons have been found to have up to 90% accuracy in
their ability to detect and classify perianal disease [11]. Use of Hydrogen peroxide
can be an adjunct with EUAs to help identify internal openings when not immediately clear, and this method can be utilized with EUS as well. The combination of
imaging and EUA results in a near 100% chance of accurate diagnosis [13] (Fig.2.1).
Park’s Classification
In the setting of CD, the incidence of perianal stulas increases as the transmural
disease process extends distally, and is most common in the setting of Crohn’s proctitis. Fistulas can tract to numerous locations including the perianal skin, rectum and
bowel (entero-enteric), vagina (rectovaginal), bladder (entero-vesical) and intraabdominally (entero-intra-abdominal), and can be singular or numerous in presentation [3]. Specically in CD, anovaginal stulas are less common than anoperineal
stulas [15–17].
In 1976, Sir Alan Park published his classication system for perianal stulas
that is still widely utilized today [17]. He described four types of stulas: intersphincteric, trans-sphincteric, suprasphincteric, and extrasphincteric [18].
Intersphincteric and trans-sphincteric stulas are the most common stulas
accounting for about 45% and 30% of all perianal stulas respectively.
Intersphincteric stulas penetrate through the internal sphincter but spare the
external sphincter whereas trans-sphincteric stulas affect both the internal and
external sphincters exiting below the level of the puborectalis muscle into the
ischiorectal fossa. Suprasphincteric stulas occur in about 20% of cases and penetrate the internal sphincter and tract between in the plane between both
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M. Obi and A. L. Lightner
sphincters superiorly over the puborectalis and external sphincter before extending out to the perineum. Horseshoe abscesses form as this tract passes over the
puborectalis into the supralevator space and downward into the ischiorectal fossa
creating an abscess cavity round the rectum. Lastly, extrasphincteric stulas are
amongst the rarest occurring in about 5% of cases. These stulas pass from the
rectum, through the levators and ischiorectal fat encompassing the sphincter complexes, to exit via the perianal skin. Supercial stulas were not a part of Park’s
original classication as they have no communication with the sphincter complex,
but are more commonly associated with CD or post anorectal procedures (i.e.
hemorrhoidectomy or sphincterotomies) [17] (Fig.2.2).
While the Park classication remains the most widely utilized, it fails in providing
information related to complexity of disease and presence of proctitis [20]. The
American Gastroenterological Association (AGA) proposed a classication system in
2003 that divided stulas into two categories: simple or complex. Complex stulas can
be high trans-sphincteric stulas (tract runs through the upper two thirds of the external
anal sphincter), have multiple external openings, multiple tracts, be associated with
stricturing disease, or be related to active proctitis [21]. While this system has signicant prognostic value, it does not help with determining effective individualization of
treatment. Additional classications such as the St James University Hospital
Classication (1996), the Hughes- Cardiff classication (1978), and the MilliganMorgan classication (1934) have been proposed but have not been used extensively
due to their inability to translate to daily practice and lack of descriptive ability [20].
Levator
Ani
Internal sphincter
Fig. 2.2 Parks’ classication of perianal stulas. (1) Supercial Fistula (2) Intersphinteric, Parks
type 1 (3) Transphincteric, Parks type 2 (4) Suprasphincteric, Parks type 3 (5) Extrasphincteric,
Parks type 4. (Adapted from Park etal. [19])
5
External sphincter
1
3
2
4
2 Surgical vs Medical Management of Symptomatic Anal Fistulas in Patients…
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Treatment Goals
The primary treatment goal is resolution of symptoms associated with the stula
(i.e. drainage and pain) and complete closure of the stula tract without impairing
continence. Ultimately, the goal is to improve patient quality of life while avoiding
treatment complications including incontinence or major colorectal surgery that can
result in stoma creation or bowel resection.
From a clinical standpoint, the Perianal Diseases Activity Index (PDAI) exists to
assess quality of life and disease severity in regards to clinical improvement and
response to therapy. It utilizes a 5-point Likert scale with scores of greater than 4
representing active stula disease with an accuracy of 87% [22, 23]. The Perianal
Crohn’s Disease Activity Index (PCDAI) was also created to specically assess features of perianal CD such as abscess, stula, ssures and/or ulcers, stenosis and
concomitant disease to determine disease severity as well as assess surgical success
[22, 24]. Radiographic healing can also be determined in conjunction with clinical
resolution. Typically, MRI is the modality of choice and healing is dened as lack
of presence of a stula tract or internal opening [25]. While complete stula closure
may remain the primary goal, in some instances symptom control may have to be
enough to avoid multiple procedures that negatively affect a patient’s quality of life.
Thorough discussions and shared decision making between the physician and
patient are thus necessary to ensure proper individual balance between remission
and symptom relief.
Medical Management
Corticosteroids
Corticosteroids do not play a signicant role in the treatment of stulizing CD.Prior
studies found that corticosteroids had increased risk of worsened discharge and as
well as increased surgical needs [26]. Corticosteroid use has only been proposed in
the treatment of luminal disease with the caveat that any concern for perianal sepsis
was addressed and well controlled prior to utilization [4].
Aminosalicylate (ASA) Derivatives
Aminosalicyclates (i.e. sulfasalazine and mesalazine) have also been found to have
no role in the treatment of stulizing CD.Studies have demonstrated no clinical
improvement for the treatment of CD despite their usefulness in the ulcerative colitis patient population [27].
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