What is the clinical term for muscle atrophy over the Sacroiliac (SI) joints and lumbar muscles?

Paraspinal Muscle Atrophy in Axial Spondyloarthritis

The clinical term for muscle atrophy over the sacroiliac (SI) joints and lumbar muscles is paraspinal muscle atrophy associated with axial spondyloarthritis (axSpA).

Understanding Paraspinal Muscle Atrophy in Axial Spondyloarthritis

Paraspinal muscle atrophy in axSpA is characterized by:

• Loss of muscle mass and strength in the paraspinal muscles surrounding the sacroiliac joints and lumbar spine, occurring as a complication of the inflammatory disease process 1
• Often associated with fatty degeneration of the paraspinal muscles, which can be visualized on MRI as areas of increased signal intensity on T1-weighted images 1
• May be present alongside other radiographic features of axSpA, such as sacroiliitis, syndesmophytes, and vertebral corner inflammatory or fatty lesions 1

Imaging Features and Diagnosis

The detection of paraspinal muscle atrophy typically involves:

• MRI is the gold standard for evaluating both inflammatory changes and muscle atrophy in axSpA patients 1

  • T1-weighted sequences show muscle volume loss and fatty replacement
  • Fluid-sensitive sequences (STIR or T2-weighted fat-suppressed) demonstrate active inflammatory changes 1

• CT may be helpful in patients unable to undergo MRI, showing:

  • Muscle volume loss
  • Fatty infiltration of paraspinal muscles
  • Associated bone erosions and structural changes 1

• Conventional radiography has limited value in directly assessing muscle atrophy but may show:

  • Syndesmophytes and other structural changes of the spine
  • Sacroiliitis and SI joint abnormalities 1

Clinical Significance and Associations

Paraspinal muscle atrophy in axSpA has important clinical implications:

• Associated with disease progression and structural damage in axSpA 1
• Contributes to functional limitations, decreased mobility, and impaired quality of life 2, 3
• May be more prevalent in advanced stages of axSpA, particularly in patients with radiographic evidence of disease 1
• Often correlates with disease duration and severity 4

Pathophysiological Mechanisms

The development of paraspinal muscle atrophy in axSpA involves:

• Chronic inflammation leading to muscle protein degradation and impaired protein synthesis 5, 2
• Activation of catabolic pathways including the ubiquitin-proteasome system and autophagy lysosome system 2
• Disuse atrophy due to pain-related movement limitations and decreased physical activity 3, 6
• Possible direct inflammatory effects on muscle tissue adjacent to affected joints 4

Management Implications

Recognition of paraspinal muscle atrophy has treatment implications:

• Early diagnosis and treatment of axSpA may help prevent or minimize muscle atrophy 1
• Physical therapy and exercise programs should be incorporated into treatment plans to maintain muscle mass and function 3
• Anti-TNF-α and other biologic therapies for axSpA may help reduce inflammation and potentially slow muscle atrophy progression 1
• Regular imaging follow-up (typically no more frequently than every 2 years) to monitor disease progression and muscle changes 1

Clinical Pearls and Pitfalls

Important considerations in evaluating paraspinal muscle atrophy:

• Muscle atrophy may be present even in early or non-radiographic axSpA, though it's more common in advanced disease 1
• Differential diagnosis should include other causes of muscle atrophy such as disuse, neurogenic causes, and age-related sarcopenia 6
• When requesting MRI to evaluate for axSpA and muscle atrophy, specify the need for appropriate sequences (including fat-suppressed fluid-sensitive sequences) 1
• Isolated spine involvement without SI joint abnormalities occurs in 6-23% of axSpA cases, so evaluation should include both areas 1