In a 34-year-old man with known rectosigmoid mucinous adenocarcinoma presenting with new abdominal distension, diffuse abdominal pain, bloating, nausea, a single episode of vomiting, and worsening severe back pain, what is the most likely acute problem and what immediate management steps should be taken?

Acute Bowel Obstruction in Rectosigmoid Cancer Patient

This patient most likely has acute bowel obstruction from tumor progression or metastatic disease, and requires immediate CT scan with IV contrast followed by urgent surgical consultation. 1

Immediate Diagnostic Approach

The combination of abdominal distension (present in 65.3% of obstruction cases with positive likelihood ratio of 16.8), diffuse abdominal pain, nausea, vomiting, and bloating in a patient with known rectosigmoid mucinous adenocarcinoma strongly suggests acute bowel obstruction. 1 The worsening severe back pain over 3 months, now acutely aggravated, raises concern for either spinal metastases with cord compression or retroperitoneal tumor extension. 2

Critical Warning Signs Present

• Diffuse abdominal pain with distension indicates potential complete obstruction 1
• Acute worsening of chronic back pain in cancer patients warrants immediate spinal imaging 2
• The absence of fever, tachycardia, or peritoneal signs is reassuring against immediate perforation or ischemia, but does not exclude obstruction 1

Urgent Imaging Protocol

Order CT scan with IV contrast immediately—this is the diagnostic standard with ~90% accuracy and can identify obstruction site, cause, and signs of ischemia. 1 The CT will:

• Locate the site and character of obstruction (tumor recurrence, metastatic disease, or adhesions) 1
• Detect pneumoperitoneum or free fluid indicating perforation requiring emergent surgery 1
• Assess for tumor progression or peritoneal carcinomatosis 1
• Identify strangulation or ischemia that mandates immediate operative intervention 1

If back pain is severe or associated with any neurological symptoms, obtain MRI of entire spine with gadolinium to exclude spinal cord compression from metastases. 2 In cancer patients with new-onset back pain, 30% of spinal cord compression referrals occur on Fridays, emphasizing the need for expedited workup. 2

Immediate Supportive Management

While awaiting imaging, initiate the following:

• Intravenous isotonic crystalloid resuscitation to correct hypovolemia from third-spacing and vomiting 2
• Nasogastric tube placement for decompression to prevent aspiration and analyze gastric contents (feculent aspirate suggests distal obstruction) 2
• NPO status with bowel rest 2
• Anti-emetics: metoclopramide as first-line (but only if obstruction is incomplete) or haloperidol as alternative 3
• Correct electrolyte abnormalities, particularly potassium 2

Critical Medication Warning

Do NOT administer loperamide, antispasmodics, or prokinetics like metoclopramide if complete obstruction is present—these can cause perforation or toxic megacolon. 1, 4 Wait for CT confirmation of partial versus complete obstruction before using prokinetics. 4

Laboratory Evaluation

Obtain immediately:

• Complete blood count (leukocytosis suggests ischemia or perforation) 2
• Serum lactate (elevated indicates tissue hypoperfusion and intestinal ischemia) 1
• Renal function and electrolytes (assess for pre-renal failure) 2
• Arterial blood gas if patient appears ill (low pH and bicarbonate suggest ischemia) 2
• Procalcitonin and CRP if available (correlate with intestinal necrosis and mortality) 1

Surgical Indications

Immediate surgical consultation is mandatory. 1 Surgery is indicated if CT demonstrates:

• Pneumoperitoneum or free intraperitoneal fluid (perforation) 1
• Signs of intestinal ischemia or strangulation 1
• Complete obstruction with compromised bowel loop 1
• Diffuse peritonitis on examination 1

In hemodynamically unstable patients, proceed directly to exploratory laparotomy without waiting for CT. 1

Special Considerations for Mucinous Adenocarcinoma

Mucinous adenocarcinoma of the rectosigmoid comprises 5-20% of rectal cancers and commonly presents at advanced stage with extramural progression. 5 This histology:

• Is readily identified on MRI with characteristic mucin signal 5
• Carries worse prognosis compared to non-mucinous adenocarcinoma of same stage 5
• Often presents with peritoneal carcinomatosis causing obstruction 1

The recent unsuccessful colonoscopy attempt (if applicable) increases risk of iatrogenic perforation and mandates urgent CT evaluation. 1

Management if Obstruction is Malignant and Inoperable

If imaging reveals extensive peritoneal disease or the patient is not a surgical candidate:

• Octreotide 150-300 mcg subcutaneously twice daily or continuous infusion (high efficacy for malignant obstruction) 1, 6
• Dexamethasone up to 60 mg/day (discontinue if no improvement in 3-5 days) 1, 3
• Glycopyrrolate for secretion control 3
• Consider venting gastrostomy for refractory symptoms if no extensive peritoneal or gastric serosal disease 2, 6

Back Pain Management Priority

If spinal MRI confirms cord compression, administer dexamethasone 96 mg IV immediately before imaging confirmation if clinical suspicion is high. 2 However, this high-dose regimen carries 29% side effect rate including GI perforation (14% serious complications), so use cautiously in setting of possible bowel obstruction. 2 Standard dose dexamethasone (10-16 mg) may be safer in this dual-pathology scenario. 2

Common Pitfalls to Avoid

• Do not attribute all symptoms to cancer progression—approximately one-third of symptoms in cancer patients are unrelated to oncologic disease 1
• Do not delay imaging for "conservative management trial" in cancer patients with acute obstruction 1
• Do not use opioids liberally before establishing diagnosis—they worsen ileus and mask peritoneal signs 2
• Do not assume partial obstruction will resolve—60% of large bowel obstructions in this population are from cancer progression requiring intervention 2, 1