How should a 26‑year‑old woman taking azelastine‑fluticasone nasal suspension, an etonogestrel (Nexplanon) 68 mg implant, ferrous sulfate 325 mg delayed‑release tablets, and topiramate 25 mg daily be evaluated and managed for new abdominal and back pain?

Evaluation and Management of Abdominal and Back Pain in a 26-Year-Old Woman on Topiramate and Nexplanon

This patient requires urgent evaluation for ectopic pregnancy and assessment of potential Nexplanon-topiramate drug interaction that may have compromised contraceptive efficacy, followed by imaging to identify the source of her abdominal and back pain.

Immediate Priority: Rule Out Pregnancy and Ectopic Pregnancy

• Obtain a urine or serum β-hCG immediately before any further workup, as this patient is on a contraceptive regimen with documented reduced efficacy 1, 2.
• Topiramate at 25 mg daily causes a significant drug interaction with etonogestrel implants: at standard epilepsy dosages (400 mg/day), 30.8% of patients fall below the ovulatory suppression threshold (serum etonogestrel <90 pg/mL), and even at lower doses there is a dose-dependent reduction in etonogestrel concentrations 1.
• Although this patient is on a lower dose (25 mg), the interaction is established and dose-related, creating real contraceptive failure risk 1, 2.
• If β-hCG is positive, immediately perform transvaginal ultrasound to localize the pregnancy, as ectopic pregnancy is a life-threatening cause of abdominal and back pain in reproductive-age women 3.

Imaging Strategy Once Pregnancy Is Excluded

Order CT abdomen and pelvis with IV contrast as the primary imaging modality for nonlocalized abdominal pain in this non-pregnant patient 3.

• CT with IV contrast is the most appropriate initial study for evaluating nonlocalized abdominal pain, as it screens for the broadest range of pathologies including appendicitis, ovarian pathology, urolithiasis, inflammatory bowel disease, and abscess 3.
• Ultrasound abdomen may be considered first in younger patients to avoid radiation, but has limited sensitivity for many abdominal pathologies compared to CT, particularly in the presence of bowel gas 3.
• If ultrasound is performed first and results are negative or inconclusive, proceed immediately to CT with IV contrast for definitive evaluation 3.
• MRI without contrast can achieve 99% accuracy for acute abdominal pathology but is typically reserved for pregnant patients or as a problem-solving tool when CT is contraindicated 3.

Specific Diagnostic Considerations

Gynecologic Causes

• Ovarian torsion must be considered given her age and hormonal contraceptive use; this requires urgent pelvic ultrasound with Doppler if suspected clinically 3.
• Functional ovarian cysts can occur even with Nexplanon, though less commonly than with other contraceptive methods 4.
• Pelvic inflammatory disease should be considered if she has risk factors or cervical motion tenderness on examination 3.

Gastrointestinal Causes

• Appendicitis, diverticulitis, inflammatory bowel disease, small bowel obstruction, and cholecystitis are all within the differential for nonlocalized abdominal and back pain 3.
• CT with IV contrast will identify these conditions with high sensitivity 3.

Urologic Causes

• Urolithiasis commonly presents with back pain radiating to the abdomen and is readily identified on CT 3.
• Urinary tract infection or pyelonephritis should be evaluated with urinalysis 3.

Management of the Topiramate-Nexplanon Interaction

Counsel the patient that her current contraceptive may have reduced efficacy and recommend either:

1. Discontinuing topiramate if clinically feasible (discuss with prescribing provider regarding indication—likely migraine prophylaxis at this dose) 1, 2, OR
2. Adding barrier contraception (condoms) for the duration of topiramate therapy 1, 2, OR
3. Switching to a non-interacting contraceptive method such as:
   • Copper IUD (no hormonal interaction) 2
   • Levonorgestrel IUD (minimal systemic absorption, no documented interaction with topiramate) 2
   • Depot medroxyprogesterone acetate (DMPA) injections (no documented interaction) 2

• Do not rely on the current Nexplanon alone for contraception while she continues topiramate, as enzyme-inducing antiepileptic drugs including topiramate significantly reduce etonogestrel levels 1, 2.

Common Pitfalls to Avoid

• Failing to obtain β-hCG before imaging: Never assume contraceptive efficacy in a patient on interacting medications; pregnancy must be definitively excluded 1, 2.
• Ordering plain abdominal radiographs: These have poor sensitivity for most causes of acute abdominal pain and should not be the primary imaging modality 3.
• Delaying imaging in favor of observation: Nonlocalized abdominal and back pain in a young woman requires prompt imaging to avoid missing time-sensitive diagnoses like appendicitis, ovarian torsion, or ectopic pregnancy 3.
• Not addressing the drug interaction: Failing to counsel about reduced contraceptive efficacy exposes the patient to ongoing pregnancy risk 1, 2.
• Assuming Nexplanon complications: While implant-site reactions and nerve injury from insertion are documented, these present with localized arm symptoms, not abdominal/back pain 5, 6.

Additional Considerations

• Ferrous sulfate (iron supplementation) suggests possible anemia; check CBC to assess for anemia that might indicate chronic blood loss from a gastrointestinal source 3.
• Azelastine-fluticasone nasal spray does not interact with Nexplanon or topiramate and is not relevant to her abdominal pain 7, 8, 9.
• If back pain is musculoskeletal and imaging is negative, consider ibuprofen 400 mg three times daily as first-line therapy, using the lowest effective dose for the shortest duration 10.