Differential Diagnosis for Left Side Pain near Posterior Superior Iliac Spine (PSIS)

The patient's complaint of pain near the left posterior superior iliac spine (PSIS), especially when considering the axillary side, suggests a variety of potential causes. The location and nature of the pain can guide us toward a differential diagnosis. Here's an organized approach:

• Single Most Likely Diagnosis

  • Trochanteric Bursitis: This condition involves inflammation of the bursa over the greater trochanter of the femur, which can cause pain on the outer aspect of the hip and thigh. The pain can sometimes radiate to the area near the PSIS, especially if the patient has a history of hip or lower back issues that alter gait and put additional stress on the trochanteric bursa.

• Other Likely Diagnoses

  • Lower Back Strain or Sprain: Muscle strains or ligament sprains in the lower back can cause pain that radiates to the sides, potentially affecting the area near the PSIS.
  • Piriformis Syndrome: This condition, where the piriformis muscle irritates the sciatic nerve, can cause pain in the buttocks and hip area, sometimes extending to the posterior thigh and potentially near the PSIS.
  • Gluteus Medius Tendinopathy: Inflammation or degeneration of the gluteus medius tendon can cause pain on the outer aspect of the hip, which might be perceived near the PSIS.

• Do Not Miss Diagnoses

  • Appendicitis (if pain has shifted): Although appendicitis typically presents with pain that initially is diffuse or around the navel and then shifts to the right lower quadrant, atypical presentations can occur, especially in retrocecal appendicitis, where the pain might be felt more posteriorly.
  • Kidney Stones or Pyelonephritis: Pain from kidney issues can sometimes be referred to the lower back and sides, potentially near the PSIS, especially if the stones are moving or if there's an infection.
  • Osteomyelitis or Septic Arthritis: Infections of the bone or joint can cause severe pain and require prompt diagnosis and treatment to prevent serious complications.

• Rare Diagnoses

  • Avulsion Fracture: A fracture where a fragment of bone is pulled away by a tendon or ligament, potentially causing pain near the PSIS if the fracture occurs in the nearby bones.
  • Tumors (Bone or Soft Tissue): Although rare, tumors can cause localized pain and should be considered in the differential diagnosis, especially if the pain is persistent and unexplained by more common causes.

Each diagnosis should be considered in the context of the patient's full history, physical examination, and potentially, diagnostic imaging or laboratory tests to confirm the cause of the pain.