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1
Question:

A 24-year-old man undergoes pulmonary function testing to evaluate dyspnea on exertion.  He has no other symptoms except chronic low back pain.  The pain is worse at night and improves upon arising and with exercise.  He only takes naproxen as needed for back discomfort.  He smokes 1 pack of cigarettes per day and drinks a 6-pack of beer each weekend.  His erythrocyte sedimentation rate is 77 mm/hr.  The following values are obtained:

Vital capacity75% of predicted
FEV1/FVC95%
FRC110% of predicted

(FEV1: Forced expiratory volume in 1 second; FVC: Forced vital capacity; FRC: Functional residual capacity)

Which of the following best explains the pulmonary function test findings in this patient?

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Explanation:

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Symptoms/signs of ankylosing spondylitis

  • Low back pain (onset age <40, insidious onset, improves with exercise but not with rest, pain at night)
  • Hip & buttock pain
  • Limited chest expansion & spinal mobility
  • Enthesitis (inflammation at the site of insertion of a tendon to the bone)
  • Systemic symptoms (eg, fever, chills, fatigue, weight loss)
  • Acute anterior uveitis (unilateral pain, photophobia, blurry vision)

Chronic low back pain in an otherwise young healthy man, pain at night, improvement of pain with activity, and elevated erythrocyte sedimentation rate are suggestive of ankylosing spondylitis (AS).  AS is a chronic inflammatory disease of the axial skeleton characterized by progressive stiffness of the spine, sacroiliitis on radiographs, and positive serology for HLA-B27 in the majority of patients.  Some patients also develop extraarticular features, such as anterior uveitis, inflammatory bowel disease, and cardiac involvement with aortic regurgitation.

Patients with AS can develop limitations in lung expansion due to diminished chest wall and spinal mobility.  Pulmonary function tests (PFT) may reveal a mildly restrictive pattern with reduced vital capacity (VC) and total lung capacity (TLC) but normal FEV1/FVC ratio.  Functional residual capacity (FRC) and residual volume (RV) are normal or increased due to fixation of the rib cage in an inspiratory position.

(Choice B)  Patients with AS may occasionally develop pulmonary fibrosis, which also causes a restrictive pattern with reduced FVC and FEV1 and a normal or increased FEV1/FVC ratio.  However, FRC, TLC, and RV are also reduced.

(Choices C and E)  PFT in patients with obstructive lung disease typically reveals a reduced FEV1 (<80% of predicted) and FEV1/FVC (<70%) ratio.  This patient's PFTs are more consistent with restrictive lung disease.

(Choice D)  Pulmonary hypertension can develop as a result of long standing obstructive or severe restrictive lung disease.  Patients with idiopathic pulmonary arterial hypertension can have a mild decrease in FEV1 or FVC; however, the lung volumes are usually normal.

Educational objective:
Patients with ankylosing spondylitis can develop restrictive lung disease due to diminished chest wall and spinal mobility.  Pulmonary function tests show a mildly restrictive pattern with reduced vital capacity and total lung capacity but normal FEV1/FVC.