Pulmonary Pathology Online
Pathology of Lymphangioleiomyomatosis
- Lymphangioleiomyomatosis (LAM) is a multi-system disease involving lung (cystic destruction), kidney (angiomyolipomas (AMLs)) and axial lymphatics (adenopathy and lymphangioleiomyoma) .
- It occurs in a sporadic form and in association with tuberous sclerosis complex (TSC),
- LAM is caused by the proliferation of abnormal smooth muscle-like LAM cells, which have TSC1 or TSC2 mutations.
In the rare condition of lymphangioleiomyomatosis (LAM) a bizarre proliferation of smooth muscle cells (LAM cells) in the lung involves the smooth muscle of lymphatics, arteries, veins, bronchioles, and alveolar walls.
Alveolar walls are weakened by the infiltrate and rupture, leading to focal emphysema and eventually gross honeycombing (and hence the recurrent pneumothoraces).
The resulting diffuse cystic appearance of the lung differs from the honeycomb lung in that the cysts are large (a few centimeters rather than a few millimeters in diameter), are lined by alveolar epithelium, and contain smooth muscle in their walls.
The lung is increased in size and losses elastic recoil. The functional effect is thus obstructive rather than restrictive, a situation equivalent to emphysema.
Pathology and pathogenesis:
- Bronchoalveolar obstruction by smooth muscle may lead to air trapping disrortion of the lung, or the proliferated parenchymal smooth muscle may itself distort the lung tissue.
- The proliferation of venous smooth muscle results in venous occlusion.
The smooth muscle proliferation is not neoplastic yet it infiltrates and occludes small veins, resulting in pulmonary haemorrhage (and therefore haemoptysis and haemosiderosis).
- The involvement of lymphatic channels by smooth muscle proliferation leads to lymph node enlargement in the thorax and elsewhere.
Smooth muscle proliferation of lymphatic ducts may lead to their occlusion and the accumulation of chyle in the pleural cavity (chylothorax).
An origin from pulmonary lymphatics is assumed because similar cells may replace lymph nodes (causing the chylous effusions).
- Hormonal factors are believed to be responsible as the condition never affects men or starts in women before the menopause or after the menopause.
- The condition is not familial, yet identical changes are found in the lungs in the familial condition of the tuberous sclerosis, in which case the patient is almost always female.
Since identical lesions of the lung and lymph nodes may be seen in tuberous sclerosis.
However, the other typical brain and skin lesions of tuberous sclerosis are absent, and no evidence exists of a family history.
The molecular basis of this disorder has been delineated over the past five years and it is now known to be a consequence of mutations in the tuberous sclerosis genes.
This knowledge, combined with advances in our understanding of the signalling pathways regulated by these genes, has given rise to potential molecular therapies that hold great promise for treating this devastating disease.
Lymphangioleiomyomatosis is exclusively a disease of women, almost always of child-bearing age.
Dyspnea is the usual presenting symptom.
The condition is characterized clinically by slowly progressive breathlessness and basal shadowing, and three distinctive complications: multiple pneumothoraces, haemoptyses and chylous effusions.
Histologically, there is an interstitial proliferation of short spindle cells that resemble fibroblasts but have the ultrastructural features of smooth muscle and stain for muscle actin.
They also stain for the melanocyte marker HMB45 and have been likened to the of the benign clear cell (sugar) tumour of the lung and angiomyolipomas. (Extrarenal Angiomyolipoma)
Although coexisting renal and pulmonary involvement in tuberous sclerosis is rare, it is important to recognize lymphangioleiomyomatosis as a pulmonary involvement of angiomyolipoma with tuberous sclerosis.
Oophorectomy has produced good results. Treatment is by oestrogen antagonists or progestogens but the response is unpredictable and does not always correlate with the presence of oestrogen receptors.
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