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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 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 go in during inhalation thereby reducing the lung volume, in extra-pulmonary, the structures around the lung have been damaged which doesn't allow for chest expansion think about how hard it'd be to take a breath when you have someone sitting on your chest during inhalation both the diaphragm and intercostals muscles located in between your ribs contract to pull the ribs up and out and expand the chest cavity.  

This creates a tiny vacuum that pulls the lungs open the air reaches the alveoli and this is where the majority of gas exchange happens 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 overall shape during exhalation, both the diaphragm and the intercostals 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 inner Citium 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's 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 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 happens 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 and it happens in silica miners and sandblasters who inhale silica particles which also settle in the upper lobe to the lung this can cause fibrotic nodule formation in addition to macrophage dysfunction causing an increased risk of tuberculosis infections, the last type is asbestosis which is seen in construction and shipyard workers because asbestos has been used in building materials for a long time often asbestos fibers settle in the lower lobes and on the inner pleural membrane which surrounds the lung white thick patches along the pleural membrane called pleural plaques can commonly be seen on chest x-ray along the lower portions of the chest but how they're formed is still unclear. 

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 huge immune response as more immune cells are recruited by cytokines released by these immune cells damage the alveolar epithelium fibroblasts then arrive on the scene to try to repair the damage by depositing extracellular matrix composed of collagen now the second type of interstitial restrictive lung disease is Sarcoidosis  (https://muhammadfahadhealthcare.blogspot.com/2022/04/sarcoidosis.html) which is a systemic disease with an underlying mechanism that's not well understood what we do know is that in Sarcoidosis immune cells try to destroy the pathogens or particles but aren't always successful often times a pathogen simply gets surrounded by a cluster of macrophages and other immune cells in a sort of standoff 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 lead to fibrosis. Another type of restrictive lung disease is hypersensitivity pneumonitis which is caused by an immune reaction to various small particles things as bird droppings mold or dust every time there's an exposure the immune cells release cytokines and inflammatory mediators which damages the lung and causes 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 which is 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 force vital capacity to all decrease interestingly the fev1 to 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 arterial start to constrict which effectively shuttles blood away from the most damaged areas of the lung, but if the damage is widespread then there's widespread vasoconstriction of pulmonary arterials 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 this can lead to right ventricular hypertrophy.

All of which is a process called core pulmonary extrapulmonary 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 pectus excavatum which is when the chest wall folds inward or a person might 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 (https://muhammadfahadhealthcare.blogspot.com/2022/06/myasthenia-gravis.html) an autoimmune condition that destroys receptors at the neuromuscular junction and prevents 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 disease 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 to 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 anti-fibrotic therapies like colchicines management of extra-pulmonary causes of restrictive lung disease depends on the underlying condition.

All right as a quick recap interstitial restrictive lung diseases happen when the lung tissue becomes fibrotic and firm which reduces the total lung capacity forced vital capacity and forced expiratory volume in one second but usually results in an increased or normal fev1 to FVC ratio, diagnosis is made based on spirometry and chest imaging and corticosteroids are the first-line treatment, extra-pulmonary restrictive lung diseases can be from causes like abnormalities in the chest wall or diaphragm as well as pleural effusions and neuromuscular diseases.

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