Preservation of lung volumes in the presence of increased interstitial markings is a radiographic hallmark of LAM that helps distinguish it from most other interstitial lung diseases, in which alveolar septal and interstitial expansion tend to increase the lungs elastic recoil properties and decreased lung volumes. Computed tomography
The high-resolution computed tomography (HRCT) chest scan is better than the chest radiograph to detect cystic parenchymal disease and is almost always abnormal at the time of diagnosis, even when the chest radiograph and pulmonary function assessments are normal. The typical CT shows diffuse round, bilateral, thin-walled cysts of varying sizes ranging from 1 to 45 mm in diameter. The numbers of cysts varies in LAM from a few to almost complete replacement of normal lung tissue. The profusion of cysts tends to be milder in patients with TSC-LAM than S-LAM, perhaps explained in part because TSC-LAM patients typically receive earlier screening. Pleural effusions are seen on CT in 12% of patients with S-LAM and 6% of patients with TSC-LAM. Other CT features include linear densities (29%), hilar or mediastinal lymphadenopathy (9%), pneumothorax, lymphangiomyoma, and thoracic duct dilation. Ground-glass opacities (12%) suggest the presence of interstitial edema due to lymphatic congestion. In patients with TSC, nodular densities on HRCT may represent multifocal micronodular pneumocyte hyperplasia (MMPH) made up of clusters of hyperplastic type II pneumocytes. MMPH may be present in males or females with TSC in the presence or absence of LAM, but not in patients with S-LAM.
It is not unusual for DLCO to be reduced out of proportion to forced expiratory volume in 1 second (FEV1). Reduction in DLCO and increase in residual volume are generally considered to be LAMs earliest physiologic manifestations.Cardiopulmonary exercise testing in a much larger cohort of patients with LAM revealed a reduced maximal oxygen consumption (VO2 max) and anaerobic threshold in 217 patients. Exercise-induced hypoxemia was found even in patients who did not have resting abnormalities in FEV1 and DLCO. In most patients, exercise was thought to be ventilation limited, owing to airflow obstruction and increased dead-space ventilation.Disease progression is usually accompanied by a progressive obstructive ventilatory defect. Decline in FEV1 is the most commonly used parameter to monitor disease progression. Although resting pulmonary hypertension appears to be unusual in LAM, pulmonary arterial pressure often rises with low levels of exercise, related in part to hypoxemia. One study reported an increase in intraparenchymal shunts in dyspneic patients with LAM, which may contribute to resting and exercise hypoxemia. Pathology
Grossly, LAM lungs are enlarged and diffusely cystic, with dilated air spaces as large as several centimeters in diameter. Microscopic examination of the lung reveals foci of smooth muscle-like cell infiltration of the lung parenchyma, airways, lymphatics, and blood vessels associated with areas of thin-walled cystic change. LAM lesions often contain an abundance of lymphatic channels, forming an anastomosing meshwork of slit-like spaces lined by endothelial cells.