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Difficulty: MediumRheumatology and Immunology

An occupational health screening includes spirometry for a 42-year-old male construction worker who reports slowly progressive exertional breathlessness over the past year. He denies chronic cough, sputum production, or wheezing. The patient has a 10-year history of daily lower back stiffness that is worst upon waking and improves after a few hours of physical labor. He does not smoke and has no history of cardiovascular disease. Vital signs are within normal limits. Physical examination reveals a stooped posture and a significantly reduced range of motion of the lumbar spine. Chest expansion during deep inspiration is markedly limited to 1.5 cm (normal is greater than 5 cm). Cardiac auscultation is normal, and breath sounds are clear bilaterally. Pulmonary function testing demonstrates a forced vital capacity (FVC) of 65% of predicted, a forced expiratory volume in 1 second (FEV1) of 70% of predicted, and an FEV1/FVC ratio of 86%. Diffusing capacity of the lung for carbon monoxide (DLCO) is normal. Which of the following is the most likely primary cause of this patient's respiratory symptoms?

  1. Fixation of the costovertebral and costosternal jointsAnswer
  2. B
    Interstitial fibrosis of the upper lung lobes
  3. C
    Decreased left ventricular compliance
  4. D
    Airway hyperresponsiveness and smooth muscle constriction
  5. E
    Destruction of the alveolar septa and loss of elastic recoil

Answer

Fixation of the costovertebral and costosternal joints
The correct answer accurately links the pathophysiology of ankylosing spondylitis (AS) to the patient's respiratory symptoms and objective test results. AS causes progressive inflammation and eventual ossification of the syndesmophytes, leading to fusion of the spine and thoracic cage joints. Fixation of the costovertebral and costosternal joints drastically reduces chest wall compliance. This mechanical restriction prevents adequate lung expansion, producing an extrapulmonary restrictive pattern on pulmonary function tests (decreased FVC, normal or elevated FEV1/FVC ratio). Because the lung parenchyma and alveoli themselves remain structurally intact, gas exchange is unaffected, which is why the diffusing capacity for carbon monoxide (DLCO) remains normal.

Step-by-Step Solution

1
Analyze the patient's clinical history and physical exam.
The patient is a young male with a 10-year history of inflammatory back pain (worst in morning, improves with activity), stooped posture, restricted lumbar mobility, and severely limited chest expansion (1.5 cm).
This constellation of findings is the classic presentation of ankylosing spondylitis (AS).
2
Interpret the pulmonary function tests (PFTs).
FVC is 65% (decreased), FEV1 is 70% (decreased), FEV1/FVC ratio is 86% (normal/high), and DLCO is normal.
A decreased FVC with a normal or elevated FEV1/FVC ratio indicates a restrictive ventilatory defect. A normal DLCO combined with a restrictive pattern specifically narrows the differential down to an extrapulmonary cause (e.g., chest wall restriction or neuromuscular weakness) rather than an intrinsic parenchymal lung disease.
3
Correlate the extrapulmonary restrictive PFT pattern with the primary diagnosis of ankylosing spondylitis.
The chronic inflammation in AS leads to enthesitis and subsequent ankylosis (fusion) of the axial skeleton, including the costovertebral and costosternal joints.
This fusion physically restricts the outward expansion of the rib cage during inspiration, explaining the patient's low FVC, normal DLCO, and exertional breathlessness.

Key Concept

Pulmonary manifestations of ankylosing spondylitis typically involve a restrictive ventilatory defect with a normal DLCO, caused by decreased chest wall compliance due to costovertebral and costosternal joint fusion.
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Topics

Rheumatology and Immunology

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