Restrictive lung diseases

Definitions & Key takeaways

Restrictive lung diseases are a group of lung conditions that make it difficult for the lungs to expand fully, leading to a decrease in the amount of air that can be inhaled. This results in a decrease in lung function, leading to difficulty breathing and reduced oxygenation of the body's tissues.

Common causes of restrictive lung diseases include interstitial lung diseases such as sarcoidosis, idiopathic pulmonary fibrosis, and asbestosis, as well as neuromuscular conditions such as muscular dystrophy and scleroderma. Diagnosis is made based on spirometry and chest imaging, and treatment involves bronchodilators and corticosteroids to reduce inflammation and improve lung function, as well as oxygen therapy to increase oxygen levels in the body. In some cases, lung transplantation may be necessary.

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Restrictive lung diseases are just as they sound, restrictive. There are two types of restrictive lung diseases, interstitial and extra-pulmonary.
For the interstitial type, it refers to the lung tissue itself being damaged. Imagine a lung being hard and stiff like tough rubber, that lung tissue won’t easily allow air to enter during inhalation, thereby reducing the lung volume.
In the extra-pulmonary type, the structures around the lung are damaged and that prevents chest expansion. Think about how hard it would be to take a breath when you have someone sitting on your chest.
During inhalation, both the diaphragm and intercostal muscles located in between your ribs contract to pull the ribs up and out and expand the chest cavity.
This creates a vacuum which pulls the lungs open. The air reaches the alveoli and this is where the majority of gas exchange occurs in the lungs.
Between the alveoli, there’s connective tissue made up of proteins like elastin fibers, which give the lungs their rubber-band like properties, and collagen, which gives the lungs their firmness and their overall shape.
During exhalation, both the diaphragm and the intercostal muscles relax to allow the chest wall to fall and return the chest cavity back to normal.
At the same time, the elastin and collagen fibers in the interstitium allow the lung to spring back and push the air back out.
There are a number of ways to measure the volume of air as it is inhaled and exhaled from the lungs. For example, total lung capacity is the total amount of air that the lungs can hold.
Tidal volume is the volume of air inhaled during normal inhalation and the functional residual capacity is the total amount of air left in the lungs after a normal exhalation.
There’s also the forced vital capacity, or FVC, which is the maximum amount of air exhaled after a full inhalation, and the forced expiratory volume in one second, or FEV1, which is the amount of air forcibly breathed out in one second after a maximum inhalation.
The normal ratio of FEV1 to FVC is 0.8, meaning that 80% of the air should be able to get forced out in the first second.
In interstitial restrictive lung diseases, the lung tissue gets damaged in various ways. The first large category of these diseases is called pneumoconiosis, which is a broad category that includes the diseases that result from occupational exposures.
Coal workers’ pneumoconiosis occurs in coal miners who inhale tiny particles of coal or carbon dust which mainly settle in small airways in the upper lobes of the lung.
Silicosis is another type of pneumoconiosis which occurs in silica miners and sandblasters who inhale silica particles which also settle in the upper lobes of the lung.
A third type of pneumoconiosis is asbestosis and that typically occurs in construction workers and shipyard workers because asbestos has been used in building materials for many years.
Often, asbestos fibers settle in the lower lobes and on the pleural membrane which surrounds the lung, forming white thick patches called pleural plaques.
All of these different particles - carbon dust, silica particles, and asbestos fibers - can cause an immune reaction to develop.
When macrophages pick up these particles, they release chemokines like interleukin-1 and interleukin-18, which cause a large immune response.
As more immune cells are recruited, cytokines released by these immune cells damage the alveolar epithelium. Fibroblasts then arrive on scene to try and repair the damage by depositing extracellular matrix composed of collagen.
A second type of interstitial restrictive lung disease is sarcoidosis which is a systemic disease with an underlying mechanism that’s not well understood.
What we know is that in sarcoidosis, immune cells try to destroy the pathogens or particles but aren’t always successful.
Often times pathogens simply get surrounded by a cluster of macrophages and other immune cells - in a sort of stand-off - called a granuloma.
These granulomas accumulate in lymph nodes throughout the body, but particularly in the hilum of the lung - or central part of the lung.
In severe cases of sarcoidosis, immune cells start releasing pro-inflammatory cytokines that leads to fibrosis. A third type of restrictive lung disease is hypersensitivity pneumonitis which is caused by an immune reaction to various small particles - things like bird droppings, mold, or dust.
Every time there’s an exposure, the immune cells release cytokines and inflammatory mediators which damages the lung and cause a bit more fibrosis.
Over time with chronic exposure to these particles, and chronic inflammation, the entire lung can become fibrotic. With all types of interstitial restrictive lung disease, healthy lung tissue starts getting replaced by collagen fibers - a process called fibrosis.
Fibrotic lungs are stiff and firm and can’t stretch and expand properly, making it hard for them to fill with as much air.
That causes the total lung capacity, forced expiratory volume in one second and forced vital capacity to all decrease. Interestingly, the FEV1/FVC ratio actually stays about the same or increases because the FVC is reduced more than the FEV1 because a fibrotic lung provides elastic recoil and allows the air to be pushed out faster during the first second of expiration.
Over time, as lung function declines, there can be hypoxia. To adapt, the pulmonary arterioles, start to constrict - effectively shuttling blood away from the most damaged areas of the lung.
But if the damage is widespread, then there’s widespread vasoconstriction of pulmonary arterioles, and that increases pulmonary vascular resistance in general.
Increased resistance, increases pressures - called pulmonary hypertension - and that makes it hard for the right ventricle to pump out blood – and lead to right ventricular hypertrophy - a process called cor pulmonale.
Extra-pulmonary causes of restrictive lung disease typically involve damage to the ribcage and diaphragm or the nerves which innervate those structures.
When that happens the chest wall can’t expand, and that prevents the lung from fully expanding. For example, someone might be born with a pectus excavatum which is when the chest wall folds inwards or a person may be obese which can put a lot of pressure on the chest wall, preventing it from expanding.
Another cause is a pleural effusion, where fluid builds up around the lungs and prevents them from fully inflating. Another cause is myasthenia gravis, an autoimmune condition which destroys receptors at the neuromuscular junction preventing the neurons from effectively controlling muscles like the diaphragm.
If the diaphragm isn’t able to move properly the lungs won’t be able to expand. Finally, a catch-all for any type of restrictive lung disease which doesn’t have a clear cause is called idiopathic restrictive lung disease.
Symptoms of interstitial restrictive lung diseases include a cough and progressively worsening shortness of breath. A diagnosis of interstitial restrictive lung disease is usually done with spirometry which shows a decreased total lung capacity, FVC, and FEV1, but a normal or increased FEV1/FVC ratio.
Imaging shows hazy whiteness in multiple lung fields, which are called diffuse ground glass opacities. Generally speaking, treatment of interstitial restrictive lung disease often includes corticosteroids and other immunosuppressive drugs as well as antifibrotic therapies like colchicine.
Management of extra-pulmonary causes of restrictive lung disease depends on the underlying condition. Alright, as a quick recap, interstitial restrictive lung diseases occur when lung tissue becomes fibrotic and firm which reduces the total lung capacity, forced vital capacity and forced expiratory volume in one second and but an increased or normal FEV1/FVC ratio.
Diagnosis is made based on spirometry and chest imaging, and corticosteroids are the first line treatment. Extrapulmonary restrictive lung diseases can be from causes like abnormalities in the chest wall or diaphragm, as well as pleural effusions, and neuromuscular diseases.