1: Crepts are the hallmark sign of alveolar wall thickening which needs to be opened up with force during inspiration, hence producing cracklings sounds. Normal alveoli are thin like a tissue paper and don’t produce any sound during opening (during inspiration) or collapsing (during expiration). Only thickened alveoli produce opening sound during inspiration and this is crepitation. Thickening of alveolar wall can be due to any reason such as fluid (pulmonary edema, pneumonia), fibrosis (chronic infection, ILD), granuloma, cancer cells etc.
2: fine crepts indicate that the disease process is limited within the wall of alveoli and hasn’t spilled into the alveolar lumen, hence the sound produced is not too loud n coarse. If the disease process spills into the alveolar lumen as well then more force is needed to open the alveoli and produces louder n coarse crepts. If the whole lumen is filled up then the alveoli won’t be able to open or close and hence no crepts but this will produce bronchial breathing. Bronchial breathing means the alveolar lumens are either completely filled up or the alveoli are collapsed to the point that no lumen is available.
3: Fine crepts are due to pulmonary edema, interstitial pneumonia (viral and atypical pneumonia are mostly limited within the wall as organisms are weaker and usually don’t spill into lumen), early to middle stages of ILD (fibrosis limited within the wall). All the causes of Fine crepts are almost always bilateral.
4: Coarse Crepts are due to typical bacterial pneumonia (organisms are strong and infection often spills into lumen too), chronic infections (such as TB, fungus etc), malignancies. Any cause if Fine crackles in severe form can also spill into lumen and change into coarse crackles such as severe n extensive ILD may produce Coarse Crepts as well. Almost all the causes of Coarse Crepts are often unilateral or asymmetrical if bilateral.
5: Unilateral Crepts or asymmetrical Crepts are almost always Coarse, whereas Fine crepts are almost always bilateral n symmetrical.
6: Rattle is crackling like sound audible even without Stethoscope and is produced by secretions in the airways which move with the airflow. Rattle is not the same as Crepts and indicates different pathology.
7: Crepts are never a normal finding, always indicate that alveoli (also called as lung parenchyma or interstitium or alveolar barrier for gas exchange etc) is abnormal n thickened.
8: Other features which will indicate the same pathology i.e. thickening of alveolar wall are reticulonodular shadowing or consolidations etc on X-ray /CT/HRCT, & widened A-a gradient on ABG. Only alveolar pathology where A-a gradient is reduced is rupture of alveolar capillaries into alveolar lumen, hence A-a gradient is a very useful test to differentiate pulmonary Vasculitis from other alveolar diseases causing thickening of alveolar wall.