What is the most likely diagnosis in a 45-year-old woman with chronic low back pain radiating to the right buttock and posterior thigh, aggravated by running and prolonged sitting, and whose pain is reproduced by the FAIR test (resisted external rotation with the hip extended and internally rotated) and by resisted abduction/external rotation while seated?

Piriformis Syndrome

The most likely diagnosis is piriformis syndrome, based on the classic presentation of buttock pain radiating down the posterior thigh, aggravated by prolonged sitting (toilet sitting), and the positive FAIR test (flexion, adduction, internal rotation) combined with pain on resisted abduction and external rotation.

Clinical Reasoning

The clinical presentation here is pathognomonic for piriformis syndrome:

• Pain pattern: Chronic buttock pain extending down the posterior thigh (not below the knee, which would suggest true sciatica from disc herniation)
• Aggravating factors: Running and prolonged sitting are classic triggers 1, 2
• Positive provocative tests:
  • FAIR test positive (resisted external rotation with hip extended and internally rotated) - this has sensitivity of 0.881 and specificity of 0.832 at 3 standard deviations 1
  • Pain with resisted abduction/external rotation while seated - another hallmark finding 3, 4

Why Not the Other Diagnoses?

Herniated lumbar disc would typically present with:

• Pain radiating below the knee in a dermatomal pattern 5
• Positive straight leg raise test (30-70 degrees)
• Possible neurologic deficits (weakness, sensory changes, reflex changes)
• This patient lacks these features

Greater trochanteric pain syndrome presents with:

• Lateral hip pain, not buttock pain
• Tenderness over the greater trochanter
• Pain with lying on the affected side
• Different examination findings

Legg-Calvé-Perthes disease:

• Occurs in children (ages 4-10), not a 45-year-old adult
• Presents with hip/groin pain and limp
• Not applicable to this case

Pseudoclaudication (neurogenic claudication from spinal stenosis):

• Pain with walking/standing, relieved by sitting or spinal flexion 5
• This patient's pain worsens with sitting - opposite pattern

Diagnostic Approach

The modified FAIR test is the most reliable clinical diagnostic tool 1, 2:

1. Position the hip in flexion, adduction, and internal rotation
2. Have patient resist external rotation
3. Reproduction of buttock/posterior thigh pain confirms diagnosis

Prevalence: Piriformis syndrome accounts for 5-6% of all low back, buttock, and leg pain cases, and up to 17.2% of chronic low back pain patients 2, 3, 6.

Key Clinical Pearls

Common pitfalls to avoid:

• Piriformis syndrome is often a diagnosis of exclusion and frequently overlooked 3, 6
• The pain does NOT typically radiate below the knee (unlike true sciatica from disc herniation)
• Neurologic examination is usually normal 4
• Straight leg raise test is typically negative 4
• Tenderness over the sciatic notch and buttock is characteristic 3

Confirmatory findings:

• Direct palpation of piriformis muscle reproduces symptoms 4
• Freiberg sign (passive internal rotation with hip extended) may be positive
• Pace sign (resisted abduction with hip flexed) may be positive 4

Management Implications

Once diagnosed, treatment follows a stepwise approach 1, 2, 7:

1. Conservative therapy (first-line): 79% of patients improve ≥50% with this approach 1

   • Physical therapy focusing on piriformis stretching
   • NSAIDs for pain control
   • Activity modification (avoid prolonged sitting)

2. Piriformis muscle injection (if conservative fails):

   • Local anesthetic ± corticosteroid
   • Both diagnostic and therapeutic 2, 6
   • Can confirm diagnosis if symptoms resolve

3. Surgery (last resort): Reserved for refractory cases after all conservative measures fail 1, 3, 7

   • 68.8% show ≥50% improvement with surgery 1
   • Consider only after minimum 3-6 months of failed conservative treatment

Important caveat: While imaging (MRI) is typically normal and not required for diagnosis, it should be considered if red flags are present or if the patient fails to respond to initial treatment, to exclude other pathology 6, 7.