Most Likely Diagnosis: Ankylosing Spondylitis
The most likely diagnosis is ankylosing spondylitis, given the characteristic presentation of morning stiffness across the whole back that improves with movement and exercise. 1
Key Diagnostic Features Present
The patient's presentation contains classic features that distinguish inflammatory from mechanical back pain:
- Morning stiffness across the whole back - This is a hallmark feature of ankylosing spondylitis rather than typical mechanical low back pain 1
- Improvement with exercise and movement - This is pathognomonic for inflammatory spondyloarthropathy, as mechanical pain typically worsens with activity 1
- Unilateral predominance (right side) - While the stiffness is diffuse, the pain pattern can be asymmetric initially 1
Distinguishing from Mechanical Low Back Pain
The American College of Physicians guidelines specifically identify ankylosing spondylitis as a condition requiring different management than nonspecific mechanical low back pain 1. The key distinguishing features include:
- Younger age - Ankylosing spondylitis typically presents in younger patients 1
- Morning stiffness - A cardinal feature 1
- Improvement with exercise - Mechanical pain worsens with activity, while inflammatory pain improves 1
- Alternating buttock pain - Often present in ankylosing spondylitis 1
- Awakening due to back pain during the second part of the night only - Another characteristic feature 1
Clinical Caveat
Clinicians should be aware that criteria for diagnosing early ankylosing spondylitis (before the development of radiographic abnormalities) are evolving. 1 This means that normal initial radiographs do not exclude the diagnosis, and MRI of the sacroiliac joints may be necessary to detect early inflammatory changes.
Why Not Mechanical Low Back Pain
Mechanical low back pain typically presents with pain that worsens with activity and improves with rest 2, 3. The patient's symptom pattern of improvement with movement directly contradicts this typical mechanical pain behavior and strongly suggests an inflammatory etiology instead.