LARS Syndrome: Causes, Classification, and Diagnosis

Low anterior resection syndrome, usually called LARS, is a cluster of bowel problems that can develop after sphincter-preserving surgery for rectal cancer. The symptoms include fecal incontinence, frequent bowel movements, urgency, clustering of stools (multiple trips to the bathroom in a short window), and a persistent feeling of incomplete emptying. A meta-analysis estimated that roughly four in ten patients who undergo this type of surgery develop the severe form, known as major LARS, and for many of them the problems persist for years.¹ The condition is recognized as one of the most significant quality-of-life trade-offs in modern rectal cancer treatment, and some patients ultimately choose a permanent colostomy rather than live with the symptoms.²

Why LARS Happens

During a low anterior resection, the surgeon removes the cancerous portion of the rectum and then reconnects the remaining bowel to the anal canal or a short rectal stump. The surgery saves the patient from needing a permanent stoma, but the rectum is not just a passive tube. It serves as a reservoir that holds stool, senses when it is full, and coordinates with the anal sphincters to control when you go. Removing part or most of the rectum shrinks that reservoir, which is one reason patients feel the urge to go more often and have difficulty holding stool.

The damage goes deeper than just losing storage capacity. During the operation, the nerves that supply the internal anal sphincter and the pelvic floor can be stretched or cut. Research has shown that rectal mobilization injures the nerve supply to the internal sphincter, and further direct damage to the sphincter happens when instruments are passed through the anal canal to create the new connection.³ Both of these injuries lower the resting pressure of the internal sphincter, which is the muscle responsible for keeping the anus closed at rest without you having to think about it.

When an intersphincteric resection is performed, meaning the internal sphincter itself is partly or fully removed to reach a very low tumor, the structural damage is even more direct. Resting anal pressure drops further, and the lower it goes, the more severe the LARS tends to be. Sensation in the anal canal also diminishes because the sensory branch of the pudendal nerve and parts of the autonomic nerve network can be disrupted, making it harder for the patient to tell the difference between gas, liquid, and solid stool.⁴ The net result is a bowel that holds less, squeezes differently, and sends garbled signals about what is inside it.

Newer imaging work has begun quantifying another piece of the puzzle: the rebuilt segment of bowel has reduced compliance, meaning its walls do not stretch the way the original rectum did. Researchers using MRI defecography have found that lower intestinal compliance correlates with higher LARS scores, suggesting this stiffness is a major driver of the defecation problems patients experience.⁵

Risk Factors That Raise the Odds

Not everyone who has a low anterior resection develops major LARS. Several characteristics of the surgery and the patient’s treatment history push risk up or down. The evidence is clearest for a handful of factors.

How Low the Surgery Goes

The height of both the tumor and the resulting anastomosis (the new connection) matters enormously. Tumors sitting closer to the anal opening require more extensive removal of the rectum, leaving less reservoir behind and bringing the surgical instruments closer to the sphincter. One cross-sectional study found that patients whose tumors were less than 8 cm from the anal opening had about three times the risk of LARS compared to those with higher tumors.⁶ A separate analysis showed that the probability of major LARS declined almost linearly as anastomotic height increased, falling from over 50 percent at the lowest connections to under 10 percent at higher ones.⁷ Two meta-analyses have confirmed that low tumor height and low anastomotic height are consistent, strong predictors of LARS.⁸

Radiation Therapy

Patients who receive radiation before surgery, especially a long course of neoadjuvant radiotherapy, face substantially higher LARS risk. A meta-analysis of ten studies found that long-course preoperative radiotherapy roughly tripled the odds of major LARS.⁹ A separate observational study of 100 patients confirmed radiotherapy as an independent risk factor, with more than half of the entire cohort scoring in the major-LARS range.¹⁰ Radiation injures the pelvic floor muscles, the anal sphincters, and the nerve fibers that supply them, adding to the damage from surgery itself.¹¹

Temporary Stomas and Anastomotic Leaks

Many patients receive a temporary diverting stoma (usually an ileostomy) after their rectal surgery. The stoma diverts the fecal stream away from the new connection while it heals, and it is reversed weeks or months later. Counter-intuitively, having a temporary stoma is itself associated with worse bowel function afterward. A systematic review and meta-analysis found that patients with a diverting ileostomy had roughly three times the odds of developing major LARS compared to those without one. Longer time to stoma closure also correlated with worse outcomes, with a mean difference of about two and a half months between the major-LARS group and the no-LARS group.¹² Another study confirmed that having a defunctioning stoma roughly doubled the adjusted odds of major LARS, though this particular analysis did not find a clear link between how long the stoma was in place and worse outcomes.¹³

The picture gets more complicated when you account for why patients get a stoma in the first place. Patients with very low anastomoses or who received radiation are more likely to get a temporary stoma, so the stoma’s apparent effect may partly reflect those underlying risk factors. One large study found that after correcting for confounders like anastomotic height, leakage, and radiotherapy, the stoma’s independent link to incontinence weakened considerably.¹⁴ Still, disuse of the downstream bowel during the diversion period is thought to contribute to the problem, possibly through changes in the gut lining and local nerve function.

Anastomotic leakage, where the new bowel connection fails to heal properly and leaks, is another consistent risk factor. The resulting inflammation and scarring around the connection further compromise sphincter function and reservoir compliance. Both meta-analyses in the source literature identify leakage as a significant independent predictor of major LARS.⁹

Surgical Technique

The type of minimally invasive approach may also matter. A propensity-matched study comparing robotic and laparoscopic surgery for low rectal cancer found that patients in the robotic group had a lower rate of major LARS at six, twelve, and eighteen months after surgery. Multivariate analysis in that study identified laparoscopic surgery (as opposed to robotic) as an independent risk factor, alongside tumor location, intersphincteric resection, neoadjuvant therapy, and anastomotic leakage.¹⁵ Robotic platforms offer finer instrument articulation in the narrow pelvis, which may translate into less nerve and sphincter damage, though this is still being studied across larger trials.

Interestingly, age and sex have not consistently shown up as risk factors. One meta-analysis found no significant difference in LARS rates between men and women, or between younger and older patients.⁸ This makes the surgical and treatment-related factors even more central to risk prediction.

How LARS Is Classified

The standard tool for measuring LARS severity is the LARS score, a five-item questionnaire developed and validated in 2012. It asks about incontinence for gas, incontinence for liquid stool, stool frequency, clustering of bowel movements, and urgency. Each answer carries a weighted score, and the total ranges from 0 to 42. A score of 0 to 20 means no LARS, 21 to 29 means minor LARS, and 30 to 42 means major LARS. The score showed good sensitivity and specificity for identifying patients with major LARS and correlates well with overall quality of life.¹⁶

The beauty of the LARS score is its simplicity. A patient can fill it out in a few minutes, and it gives clinicians a clear way to categorize severity and track changes over time. Before this tool existed, studies used a patchwork of different bowel function questionnaires, making it hard to compare results across institutions. The LARS score has since been translated into dozens of languages and has become the de facto standard in rectal cancer research.

An international consensus process later refined the conceptual definition of LARS itself, identifying eight core symptoms and eight consequences of those symptoms that together capture the essential features of the syndrome.¹⁷ This broader definition acknowledges that LARS is more than just leakage and frequency; it includes the emotional, social, and physical fallout from unpredictable bowel behavior.

Predicting LARS Before It Happens

Clinicians have an obvious interest in predicting who will develop severe LARS before the surgery takes place. The POLARS (Pre-Operative LARS) tool was designed for this purpose, combining preoperative variables like tumor height, planned procedure type, and radiation status into a predicted LARS score. In theory, it would help surgeons counsel patients about what to expect and might even influence decisions about whether to attempt sphincter preservation in borderline cases.

In practice, POLARS has underperformed. A Polish study found that the predicted LARS category matched the actual category in only about 39 percent of patients. In three-quarters of the mispredicted cases, patients ended up doing better than the tool expected, landing in a lower LARS category at follow-up.¹⁸ A Swedish population-based validation was similarly underwhelming: of 255 patients who reported major LARS on the actual questionnaire, the POLARS tool correctly identified only 80 of them, giving it a sensitivity of just 31 percent for major LARS.¹⁹

The weak predictive performance likely reflects how many variables contribute to LARS that are impossible to know before surgery: exactly how much nerve damage will occur, whether the anastomosis will leak, how the patient’s body will adapt to a smaller reservoir, and individual variation in pelvic anatomy. For now, preoperative counseling relies more on known risk factors (low tumors, planned radiation, expected stoma use) than on a single predictive formula.

How LARS Is Diagnosed

There is no blood test or single scan that diagnoses LARS. The diagnosis is clinical, based on a patient’s symptom report after sphincter-preserving rectal surgery. In practice, the LARS score questionnaire is the primary diagnostic instrument. A patient who scores 30 or higher has major LARS; one who scores 21 to 29 has minor LARS.

Beyond the questionnaire, a thorough clinical assessment includes a detailed bowel history, a physical exam (including digital rectal exam to assess sphincter tone), and, in some cases, additional investigations. Anorectal manometry measures the pressures generated by the anal sphincters and can confirm reduced resting pressure. Endoanal ultrasound can visualize structural defects in the sphincter muscles. MRI defecography is a newer approach that captures the bowel in motion, allowing clinicians to assess how well the reconstructed rectum expands and contracts. Research using this technique has found that poor rectal compliance, as measured on dynamic MRI, tracks closely with worse LARS scores.⁵

These advanced tests are not always performed. Many clinicians use the LARS score alone for initial classification, reserving manometry or imaging for patients whose symptoms are severe, atypical, or not responding to conservative treatment. The practical reality is that most rectal cancer survivors do not see a dedicated bowel-function specialist unless their symptoms are bad enough to prompt a referral.

How Symptoms Evolve Over Time

One of the most common questions patients have is whether things will get better. The short answer is: partly, and mostly in the first six months. A prospective two-year study tracked patients from one month after surgery and found that about 76 percent had major LARS at one month. By six months, that number dropped to about 59 percent. After that initial improvement, the rate of major LARS essentially plateaued, with no statistically significant changes between six and twelve months or between twelve and twenty-four months. Among those who still had major LARS at six months, only about one in five improved by the two-year mark.²⁰

Even further out, the evidence suggests that LARS is a chronic condition for many patients. A study with an average follow-up of seven and a half years found that just over a quarter of patients still had major LARS at that point, and roughly seven percent had complete fecal incontinence. The authors described bowel dysfunction after low anterior resection as a chronic condition requiring long-term support.²¹ So while things do tend to improve from the immediate postoperative period, patients should be prepared for the possibility that some degree of altered bowel function is permanent.

The Quality-of-Life Toll

LARS is not just about bowel symptoms in isolation. It ripples outward into nearly every aspect of daily life. A cross-sectional study found that patients with LARS scored significantly worse on global health status and across physical, role, cognitive, and social function scales compared to those without it. Patients with major LARS also reported more diarrhea, fatigue, insomnia, and pain.²²

Qualitative research has fleshed out what these numbers mean in human terms. Patients describe psychological and emotional distress, strained relationships, physical limitations, and sharply curtailed social and leisure activities. Those with major bowel dysfunction were especially likely to report disruption to their role as a sexual partner and effects on their sleep. Some patients described a gradual process of learning self-empowerment and adapting to a “new normal,” but the adjustment is not easy or quick.²³

The practical impact is worth spelling out. People with severe LARS often plan their entire day around bathroom access. Leaving the house means knowing where every restroom is. Social events become anxiety-laden. Work can be disrupted by sudden urgency. The unpredictability of symptoms, more than their severity on any single day, is often what patients find hardest to live with. A person who has twelve bowel movements a day but can predict when they will happen copes better than someone who has six but never knows when the next one is coming.

The Gut Microbiome Connection

An emerging area of research is the relationship between LARS and the gut microbiome. The bacterial communities living in the bowel are disrupted by surgery, radiation, and diversion through a stoma. A recent randomized trial tested whether reinfusing the liquid from the diverted stoma back into the downstream bowel (essentially keeping the unused segment of gut exposed to its normal contents during the diversion period) could reduce LARS severity after stoma reversal. Patients who received the reinfusion had less severe LARS and improved anal function. Microbiota analysis showed that patients with and without LARS harbored different bacterial species, though overall community diversity was not dramatically different between groups.²⁴

This line of research is still early, but it hints that the problem is not entirely structural. If the microbial environment of the reconstructed bowel contributes to symptoms, interventions targeting the microbiome (probiotics, dietary changes, or techniques like stoma effluent reinfusion) could eventually become part of LARS management alongside pelvic floor rehabilitation and irrigation therapy. The finding also raises the possibility that the disuse of the downstream bowel during stoma diversion is itself contributing to the worse outcomes seen in patients who have temporary stomas, not just through muscular atrophy but through microbial changes as well.

Transanal Irrigation as a Treatment Signal

While this article focuses on causes, classification, and diagnosis, it is worth noting one treatment that illustrates how dramatically symptoms can respond to targeted intervention. Transanal irrigation, where the patient flushes the bowel with water using a specialized device, has shown striking results. One study found that at baseline, about two-thirds of patients had major LARS. After irrigation therapy, 80 percent reported no LARS at all, with the mean LARS score dropping sharply.²⁵ The technique works by allowing the patient to empty the bowel on their own schedule, essentially replacing the unpredictable urgency and clustering with a controlled evacuation. It does not fix the underlying nerve damage or reservoir loss, but it gives the patient back something LARS takes away: predictability.