To present a rare case of anal sphincter myotonia in a patient with Myotonic Dystrophy Type 1 (MD1) complicated by megacolon and refractory constipation.  

A 52-year-old woman with MD1, diagnosed in 2023, and comorbidities including right breast cancer (T3N0), thyroid goiter, hirsutism, and a history of type II myocardial infarction in early 2024. She has experienced lifelong constipation, which worsened over the past year, likely related to decreased physical activity due to progressive muscle weakness. Despite various treatments for constipation, including laxatives and enemas, she had minimal response. In September 2024, the patient was admitted with severe abdominal distension, with 15 days of refractory constipation. Abdominal CT revealed significant colonic distension without small bowel involvement, a massive fecaloma, located in the sigmoid/rectum. An anorectal manometry showed oscillating short-wave contractions throughout the anal canal, hypocontractility of the internal sphincter, reduced rectal sensitivity, and absence of the anorectal inhibitory reflex, suggestive of anal sphincter myotonia.  Despite initial attempts to relieve symptoms through enemas, the patient achieved only limited improvement. Surgical consultation attributed her symptoms to sphincter hypertonia  likely caused by neurological involvement from MD1. Digital rectal manipulation resulted in immediate release of gas, reducing abdominal distension. However, the chronic constipation remained largely refractory to treatment, underscoring the complex gastrointestinal involvement of MD1.  We started the treatment for myotonia with Lamotrigine.

This case highlights anal sphincter myotonia as a rare and severe gastrointestinal manifestation of MD1, leading to megacolon  and refractory constipation. Multidisciplinary management—including neurology, proctology, and surgery—is essential to address such complex cases. Recognizing these rare complications early can prevent delayed diagnosis, improve symptom control, and enhance patient outcomes.