Shrinking lung syndrome as a manifestation of pleuritis: a new model based on pulmonary physiological studies

Abstract

Objective: The pathophysiology of shrinking lung syndrome (SLS) is poorly understood. We sought to define the structural basis for this condition through the study of pulmonary mechanics in affected patients.

Methods: Since 2007, most patients evaluated for SLS at our institutions have undergone standardized respiratory testing including esophageal manometry. We analyzed these studies to define the physiological abnormalities driving respiratory restriction. Chest computed tomography data were post-processed to quantify lung volume and parenchymal density.

Results: Six cases met criteria for SLS. All presented with dyspnea as well as pleurisy and/or transient pleural effusions. Chest imaging results were free of parenchymal disease and corrected diffusing capacities were normal. Total lung capacities were 39%-50% of predicted. Maximal inspiratory pressures were impaired at high lung volumes, but not low lung volumes, in 5 patients. Lung compliance was strikingly reduced in all patients, accompanied by increased parenchymal density.

Conclusion: Patients with SLS exhibited symptomatic and/or radiographic pleuritis associated with 2 characteristic physiological abnormalities: (1) impaired respiratory force at high but not low lung volumes; and (2) markedly decreased pulmonary compliance in the absence of identifiable interstitial lung disease. These findings suggest a model in which pleural inflammation chronically impairs deep inspiration, for example through neural reflexes, leading to parenchymal reorganization that impairs lung compliance, a known complication of persistently low lung volumes. Together these processes could account for the association of SLS with pleuritis as well as the gradual symptomatic and functional progression that is a hallmark of this syndrome.

Figure 1. Maximal Inspiratory Pressures Measured by Esophageal Manometry at Low and High Lung Volumes

Maximal inspiratory pressures at functional residual capacity (FRC) and at total lung capacity (TLC). The gray boxes represent normal values for maximal inspiratory pressures as measured by the esophageal balloon (MIP_es). MIP_es are recorded by the esophageal balloon catheter as negative pressures since they are measured in the thorax during inspiration; however, the absolute value is reported.

Figure 2. Chest CT Reformatted Images from a Case 1

Volume-rendered images depicting lung volumes during symptomatic SLS and after recovery in Case 1. Irregularity of the surface of both lungs during symptomatic disease (left panel) reflects reduced lung volumes due to pleural effusions and potentially other factors. Resolution of pleural effusions in the recovery phase (right panel) was accompanied by improved lung volumes and densities.

Figure 3. Model of the Pathophysiology of Shrinking Lung Syndrome

We propose that SLS begins with pleural inflammation due to the underlying rheumatic disease. Activation of local neural reflexes, and/or volitional splinting because of pain, leads to chronic hypoinflation of the lung, which gradually impairs lung compliance through undefined parenchymal changes. The less compliant lung is more difficult to inflate, leading to a slowly progressive spiral of declining inflation until the positive feedback cycle is halted, likely by the patient's central respiratory drive.