LARS Syndrome: Causes, Classification, and Diagnosis

Low Anterior Resection Syndrome (LARS) is a collection of bowel dysfunction symptoms that significantly impact quality of life after rectal cancer surgery. Patients may experience fecal incontinence, urgency, frequent bowel movements, or incomplete evacuation, making daily activities difficult. While common after sphincter-preserving procedures, its severity varies widely.

Understanding the underlying mechanisms, identifying risk factors, and utilizing effective classification and diagnostic tools are key to managing LARS.

Mechanisms That Lead to Symptoms

LARS symptoms stem from anatomical, neurological, and functional disruptions following rectal cancer surgery. A primary cause is the loss of rectal reservoir capacity. The rectum normally stores stool for controlled defecation, but when part of it is removed and the remaining bowel is reconnected, stool retention diminishes. The neorectum, often formed from the sigmoid colon, lacks the original rectum’s elasticity, leading to increased stool frequency and urgency.

Nerve damage also plays a significant role. The autonomic and somatic nerves that regulate rectal sensation and sphincter function can be affected during surgery, particularly if the pelvic autonomic plexus is disrupted. This impairs stool sensation, leading to unpredictable bowel movements and incontinence. Studies show greater nerve injury correlates with more severe symptoms, emphasizing the importance of nerve-preserving techniques (Emmertsen & Laurberg, 2013, Annals of Surgery).

Altered colonic motility further exacerbates symptoms. Resection and reconnection of bowel segments can disrupt peristalsis, resulting in erratic stool propulsion. Some patients experience clustering—multiple bowel movements in rapid succession—due to uncoordinated motility. This is especially common in those with very low anastomoses, as the remaining bowel struggles to adapt. Research indicates that altered transit times contribute to both urgency and incomplete evacuation, complicating symptom management (Battersby et al., 2016, British Journal of Surgery).

Risk Factors

Several factors influence the likelihood and severity of LARS, with surgical variables playing a key role. The level of anastomosis is a major determinant, as lower reconnections near the anal sphincter often result in more pronounced symptoms. When the anastomosis is closer to the dentate line, bowel control is further impaired due to reduced rectal capacity and disrupted anorectal reflexes. A systematic review (Colorectal Disease, Keane et al., 2020) found that patients with an anastomosis within 5 cm of the anal verge had significantly higher LARS scores than those with more proximal reconnections.

Radiotherapy further increases risk, particularly when administered preoperatively. Pelvic irradiation can cause fibrosis of the bowel wall, reducing compliance and worsening motility dysfunction. Radiation-induced damage to the enteric nervous system also alters rectal sensation, increasing fecal urgency and incontinence. A prospective study (The Lancet Oncology, Marijnen, 2015) found that patients who received neoadjuvant radiotherapy had nearly double the incidence of severe LARS compared to those who underwent surgery alone.

Patient-specific characteristics also play a role. Older individuals often experience greater symptom severity due to age-related declines in anorectal function, including weakened sphincter tone and reduced rectal compliance. Women, particularly those with a history of vaginal childbirth, may have additional pelvic floor dysfunction, compounding bowel irregularities. A multicenter analysis (Diseases of the Colon & Rectum, Chen et al., 2017) found that female patients exhibited higher rates of incontinence and clustering, likely due to pre-existing pelvic floor laxity. These findings highlight the need for individualized risk assessment.

Classification Approaches

Assessing LARS severity requires structured classification methods. The most widely used system is the LARS Score, a validated questionnaire that quantifies symptom burden based on five factors: incontinence for flatus, incontinence for liquid stool, frequency of bowel movements, clustering, and urgency. Patients are categorized into three groups—no LARS (0-20), minor LARS (21-29), and major LARS (30-42). This tool strongly correlates with patient-reported quality of life, making it a cornerstone of clinical assessment.

However, the LARS Score does not fully capture the physiological mechanisms underlying symptoms, prompting the development of additional classification models. Some researchers propose dividing LARS into two subtypes: incontinence-dominant and evacuation-dominant. Incontinence-dominant cases involve severe urgency and frequent stool leakage, often linked to impaired sphincter control and reduced rectal compliance. Evacuation-dominant cases are characterized by incomplete emptying and constipation-like symptoms, typically arising from dysregulated colonic motility and poor neorectal adaptation. Recognizing these subtypes allows for targeted treatment strategies.

Further refinement comes through objective physiological testing. Anorectal manometry assesses sphincter pressure and rectal sensory thresholds, helping differentiate between structural and neurological impairments. Balloon expulsion tests and colonic transit studies identify delayed evacuation patterns versus hyperactive motility. While not routinely used in all clinical settings, these methods aid in personalizing treatment, particularly for refractory cases.

Common Tools for Diagnosis

Diagnosing LARS requires both subjective assessments and objective tests to capture the full spectrum of bowel dysfunction. Patient-reported outcome measures (PROMs) serve as the foundation, with the LARS Score being the most widely used tool. This questionnaire quantifies symptom severity and helps stratify patients into different categories. However, while PROMs gauge the impact on daily life, they do not pinpoint the physiological disturbances causing symptoms.

To complement subjective assessments, anorectal manometry evaluates sphincter function and rectal sensory thresholds. This test measures anal sphincter pressures, revealing whether incontinence stems from muscular weakness or neurological impairment. Rectal compliance can also be assessed by inflating a balloon within the rectum, determining whether the neorectum retains adequate storage capacity. Patients with reduced compliance often struggle with urgency and frequent bowel movements, making this test particularly useful.

Colonic transit studies refine the diagnostic picture by assessing motility patterns. Radiopaque marker studies or scintigraphy track stool movement through the colon, identifying whether stool propulsion is overly rapid or delayed. This distinction is important, as patients with clustering symptoms often exhibit erratic transit, while those with incomplete evacuation may have slowed motility. Understanding these patterns allows for more targeted interventions, such as prokinetic agents for delayed transit or bulking agents for rapid stool passage.