What are back mice?

Back Mice: Understanding Episacroiliac Lipoma

Back mice are firm, rubbery, mobile subcutaneous nodules of herniated fat tissue through fascial layers, typically located in the sacroiliac, posterior superior iliac, and lumbar paraspinal regions, which can cause localized and radiating pain when symptomatic. 1

Clinical Characteristics

Back mice (also called episacroiliac lipoma or fibro-fatty nodules) have several distinguishing features:

• Physical characteristics:

  • Firm, rubbery, mobile nodules 1
  • Located primarily near the posterior superior iliac spine, sacroiliac, and lumbar paraspinal regions 2
  • Palpable subcutaneous masses 2

• Prevalence: Found in up to 16% of people 3

• Pain presentation:

  • Can cause localized low back pain
  • May present with radiating pain to the lower leg (37% of cases)
  • Can cause leg numbness or paresthesias (14% of cases) 4
  • May mimic other conditions with referred pain to distant locations like lower abdomen or trochanteric region 5

Pathophysiology

Back mice represent subfascial fat herniation through fascial defects:

• They consist of fatty tissue that has herniated through fascial layers 1
• Sonographic examination reveals these as subcutaneous nodules with characteristic location and appearance 2
• The pain mechanism may involve:
  • Tension on the fascia
  • Nerve entrapment
  • Referred pain patterns that can mimic other conditions 5

Differential Diagnosis

Back mice may be confused with other causes of low back pain:

• Myofascial pain (due to point tenderness)
• Herniated nucleus pulposus (due to radiating pain)
• Trochanteric bursitis
• Abdominal pain syndromes 1, 5

However, back mice can be distinguished by:

• Characteristic location
• Palpable, mobile nodules
• Absence of neurological deficit
• Response to direct injection 1

Treatment Options

Treatment approaches for symptomatic back mice include:

1. Injection therapy:

   • Local anesthetic with or without corticosteroid injection into the nodule 1, 4
   • Success rate of approximately 89% for lasting relief reported in one case series 4
   • May relieve both local and radiating symptoms

2. Multiple needling technique:

   • Repeated direct needling of the nodule following anesthetic injection
   • May be effective by lessening tension in the fibro-fatty nodule 5
   • Often provides long-lasting relief

3. Patient satisfaction:

   • 86% of patients in one case series reported being "satisfied" or "very satisfied" with injection treatment 4
   • Treatment is generally well-tolerated with no significant adverse events reported

Clinical Pearls and Pitfalls

• Underrecognition: Back mice are potentially under-recognized in clinical practice despite being well-characterized 1
• Misdiagnosis: The radiating pain patterns may lead to misdiagnosis and unnecessary treatments for other conditions 5, 4
• Persistent nodules: Injection therapy may relieve symptoms but often does not eliminate the nodules themselves 4
• Limited evidence: While case reports suggest effectiveness of treatment, there is limited high-quality evidence from randomized trials 1
• Evaluation timing: Median time to treatment in one case series was 8 weeks, but ranged from 3 weeks to 10 years, suggesting delayed diagnosis is common 4

Back mice should be considered in the differential diagnosis for patients with unexplained low back pain, particularly when accompanied by palpable nodules in characteristic locations, before proceeding to more expensive or invasive treatments.