Bronchiectasis: Epidemiology and Pathophysiology

00:00 Welcome back.

00:02 Today we're going to be discussing bronchiectasis.

00:05 So big picture, bronchiectasis is one of the four basic types of chronic obstructive pulmonary disease or COPD.

00:12 What's going on in this case is permanent damage to the distal bronchiolar airways.

00:18 Think of the airways, the bronchi, as the conduits leading down into the most distal long parenchyma, that is, into the alveoli.

00:25 In bronchiectasis, the conduits have been damaged to the point that they are permanently dilated.

00:31 So now that we have this general overview of what bronchiectasis is, we're going to go through all the segments you see here in our roadmap for this topic.

00:39 We'll start with the epidemiology.

00:41 Who gets this disease? In the United States, the prevalence is estimated to be somewhere between 350,000 and 500,000 cases per year.

00:50 Although bronchiectasis can occur at any age, there's an increased incidence with older age, which makes sense when you think about the cumulative effect of lung insults over a lifetime.

01:01 It also appears to be more common in women.

01:04 Okay, so that's the who.

01:06 Now let's dive into the why.

01:07 What are the actual causes? Looking at this table, you'll immediately notice it's a pretty long list.

01:13 A lot of things that can lead to bronchiectasis.

01:16 Do not worry about memorizing every single thing.

01:18 You'll have access to the complete table in the downloadable materials.

01:22 What I want to do here is focus on the really high yield stuff, the big players that you'll see most often.

01:28 When researchers looked at this condition on a global scale, studying over 27,000 patients, a very clear pattern emerged.

01:34 The most frequent cause, at about 30%, was post-infectious.

01:39 This makes perfect sense, right? A patient gets a really bad lung infection, it damages the airways, and they don't heal properly.

01:48 Right behind that, at around another 30%, is idiopathic.

01:52 And remember, that's just our fancy medical term for we don't know the cause.

01:56 So, for a huge chunk of patients, we can't find a specific reason.

02:01 After those two, a significant cause, especially worldwide, is damage left over from tuberculosis or post-tuberculous bronchiectasis at about 15%.

02:11 And then you have cases related to underlying COPD and asthma, which make up about another 5% each for bronchiectasis.

02:19 To get a real foothold, you almost always need a combination of two things.

02:24 First, there's usually some kind of initiating infectious or other damaging insult.

02:29 Things like a noxious inhalation.

02:32 And then second, the body responds to the injury.

02:35 It activates the immune system.

02:37 And the main dog in this fight is the neutrophil.

02:40 And in that response, lies the key to the whole process.

02:44 The neutrophils are a bit too aggressive.

02:46 In their response to bacteria or other insult, they release a host of powerful enzymes, including elastase, as well as other destructive substances like reactive oxygen species.

02:56 And while these are intended to kill any invaders or to clean up necrotic debris, they can also cause some pretty substantial collateral damage.

03:04 These substances end up breaking down normal, healthy walls of the airways, leading to their destruction.

03:11 At the same time, a variety of inflammatory mediators released by the neutrophils and other secondarily recruited inflammatory cells increases airway mucus production as well as its viscosity.

03:24 Although that's theoretically designed to help trap more potential pathogens, it's not all that easy to clear anymore, especially with airways that have lost their elasticity.

03:34 As a result, this thick mucus ironically becomes the perfect setup to actually retain more bacteria, which sets up this really nasty, vicious cycle.

03:45 The trapped bacteria lead to more infection.

03:49 The infection calls in more neutrophils.

03:51 The neutrophils cause more damage, and as they die, they release their DNA contents, adding to the mucus sludge.

03:58 This traps even more bacteria, and around and around it goes.

04:03 Ultimately, this cycle is what destroys the airways.

04:07 The walls of the bronchus are eventually degraded and thus become permanently dilated, which is how we get to that end-stage picture you now see on the left.