Pneumonia: Pathology review

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Pneumonia0:00–1:01

Two people came to your clinic one day. Mariah is a 54-year-old smoker, who came in with with productive cough with yellow sputum and left-sided chest pain.
Physical examination reveals fever, tachycardia, and tachypnea. Her lung sounds are barely audible, but it had crackles at the left base.
Next is Jeremy, a 64-year-old man who was hospitalized for a stroke 2 weeks ago. He recently developed a cough and right-sided chest pain.
He is tachycardic and has a fever of 38.4°C. Examination reveals fremitus, decreased breath sounds, and dullness to percussion in the right lower lung field.
Chest x-rays were performed which showed a left lower lobe infiltrate in Mariah’s case, and a right lower lobe infiltrate in Jeremy’s.
Now, both people have pneumonia. So pneumonia is an infection of the lung tissue.

Pathology1:01–1:35

Some microbes can overcome the innate defenses of the lungs and immune system to colonize the bronchioles or alveoli. These pathogens then triggers an inflammatory response.
Inflammatory cells, such as white blood cells, dead bacteria, proteins and fluid from the damaged tissue, form a fluid called exudate which can be coughed up and expelled from the body.
However they can also accumulate in the lungs, filling up the alveoli. We can divide pneumonia into “classic” pneumonia or “atypical pneumonia based on symptoms.

Signs and Symptoms1:35–3:35

So with classical pneumonia, high yield symptoms might include dyspnea, or shortness of breath, fatigue, and fever. Individuals might also develop pleuritic chest pain, which is characterized by sudden and intense sharp, stabbing, or burning pain in the chest when inhaling and exhaling; and productive cough with yellow sputum.
Besides these, High yield signs that might come up on your exam include dullness to percussion, which suggests that there’s a lung consolidation.
This occurs because the air in the alveoli is replaced by pus and fluid so the sound will lose its normal tympanic or drum-like quality.
There’s also tactile fremitus, which is when you can feel increased vibrations when you place your hand on the individual’s chest or back on the area with the consolidation and they say 99.
Similarly, if you auscultate the area with the consolidation, the “99” will sound louder and this is called bronchophony.
This is because sound waves and vibrations travels better through the fluid-filled consolidated tissue than air-filled healthy tissue.
For a similar reason if you put your stethoscope on the area with the consolidation, you can hear bronchial breath sound where the sound of air moving in the bronchi is transmitted clearly through the consolidated area.
This is higher pitched and louder than the normal vesicular breath sound where sound waves passes through more air filled alveoli.
Next is egophony which is increased resonance in the area with consolidation, so when asked to say the “E” it will sound like the letter “A.” Late inspiratory crackles can also be heard over the affected area.
Crackles are popping lung sounds made when air passes through the fluid in the collapsed alveoli. Finally, people with pneumonia will often have tachypnea and tachycardia.
Now, there are many ways to categorize pneumonia. One way would be by the causative microbes.

Causative Microbes3:35–7:55

Usually, pneumonia is caused by viruses and bacteria, and rarely fungi as well. In adults, the most common viral cause is influenza.
This type usually has an abrupt onset and tends to develop secondary bacterial pneumonia. Next is the typical pneumonia, or the classic bacterial pneumonia and its most often caused by either streptococcus pneumoniae, haemophilus influenzae, or staphylococcus aureus.
“Atypical or walking pneumonia” is also caused by bacteria, but the individual won’t develop the classic symptoms. In atypical pneumonia with mycoplasma pneumoniae, a bacteria without a cell wall, individuals usually present with insidious onset, headache, nonproductive cough, and sometimes, no fever.
If you see these symptoms in a question prompt, it’s safe to assume the answer is atypical pneumonia with mycoplasma pneumoniae.
Another clue is that it tends to occur in areas with many people packed together like in military recruits training camps and prisons.
Also common in these populations, we have chlamydial pneumonia, and it can be caused by Chlamydophila pneumoniae, Chlamydophila psittaci, and Chlamydia trachomatis.
Now, another cause of atypical pneumonia is legionella pneumophila, which causes more severe symptoms like headaches, mild cough, confusion, high fever, and watery diarrhea.
This bacteria can be found in water-systems or water-based cooling systems in hotels, which is why this infection is also common in travelers.
The next high yield concept is that people in certain age groups are more at risk for certain types of microbial infections.
In neonates, or those younger than 4 weeks, the most common cause of pneumonia are Group B streptococci and Escherichia coli.
In those 4 weeks to 18 years, the condition is typically caused by viruses like the Respiratory syncytial virus, but also by bacteria such as Chlamydia trachomatis, which is more common in those younger than 3, and Chlamydophila pneumoniae, which is usually found in school-aged children.
Streptococcus pneumoniae is also a common cause of pneumonia in this age group. Next, in those 18 to 40 years, the most common causes are Mycoplasma, Chlamydophila and Streptococcus pneumoniae, but also viruses like influenza.
The last age group is individuals over 40, especially the elderly over 65. The most frequent causes are Streptococcus pneumoniae, anaerobes, and viruses.
Ok besides age, remember that in IV drug users, pneumonia is usually caused by Staphylococcus aureus and Streptococcus pneumoniae.
These also common causes of pneumonia in people with cystic fibrosis, but a very high yield pathogen associated with CF is Pseudomonas aeruginosa.
Another special group is people who are immunocompromised. The more frequent causes include Staphylococcus aureus, enteric gram negative rods, and viruses.
HIV+ individuals with a CD4+ lymphocyte count under 200 cells per microliter can also suffer from AIDS-related opportunistic pathogens, like the fungus Pneumocystis jirovecii.
This type of pneumonia can cause respiratory failure or pneumothorax if left untreated, and was a common cause of death in AIDS patients.
Moving on, in those who’ve acquired pneumonia after a viral infection, the most common culprit is either Streptococcus pneumoniae, Staphylococcus aureus, or Haemophilus influenzae.
And finally, in post organ transplant patients who are on immunosuppressants, pneumonia with intranuclear and cytoplasmic inclusion bodies histologically points to opportunistic infection by Cytomegalovirus.

Comm. vs Hospital Acqr.7:55–9:18

Pneumonia can also be categorized by where it’s acquired. The most common is community-acquired pneumonia, and it’s when a person gets sick outside of a hospital or healthcare setting.
It is usually caused by Streptococcus pneumoniae. Ok, so next is hospital-acquired pneumonia or nosocomial pneumonia, which is when a person gets pneumonia when they are already hospitalized for something else for at least 2 days.
It tends to be more serious because these individuals often have weakened immune systems and the microbes in hospitals are often resistant to the common antibiotics.
Some high yield bacteria to remember are Methicillin-resistant Staphylococcus aureus or MRSA, which is usually carried by asymptomatic hospital staff; Legionella pneumophila, which can be found in water-systems or water-based cooling systems in hospitals is also common.
Another category is ventilator-associated pneumonia, which is a subset of hospital-acquired pneumonia. It often develops when ill individuals are intubated for more than 48 hours.
Oftentimes, bacteria like Pseudomonas aeruginosa and Staph. aureus can form a biofilm on the endotracheal tube.
Individuals on a ventilator can’t cough so over time microbes move from the tube directly into the lungs and cause pneumonia.

Aspiration Pneumonia9:18–10:33

Now, in addition to inhaling microbes, there are other ways to develop this disease. Aspiration pneumonia happens when you accidentally breathe in food, drinks, or even gastric contents.
Its gravity-dependant, meaning that the location where pneumonia will develop depends on your position. Remember that if you were to breathe in food while lying on your back, it would most likely involve the posterior segments of the upper lobe and the superior part of the lower ones.
If you are standing or sitting upright, the basilar segments would be affected. Now, aspiration pneumonia tends to occur in people with impaired gag and cough reflexes, so a high yield fact is that it’s more common in individuals who abuses drugs or alcohol or if they have brain injuries that cause swallowing issues.
Aspirated gastric contents can be particularly nasty because the stomach acid can cause a chemical irritation in addition to the infection.
Pulmonary infections in those alcohol use disorders include both anaerobic and aerobic bacteria, especially Klebsiella pneumoniae, which is also a common cause of pneumonia in those with diabetes.
This type of infection is often associated with lung necrosis and abscesses. Keep in mind that all of these types of pneumonia can also be characterized by where the infection occurs.

Localization10:33–12:07

In bronchopneumonia, the infection can be patchy throughout the lungs involving the bronchioles as well as the alveoli. In atypical or interstitial pneumonia, the infection is mainly just outside the alveoli in the interstitium.
And in lobar pneumonia, the infection causes complete consolidation of a whole lobe of the lung, meaning that the entire region is filled with fluid.
Usually, lobar pneumonia happens in stages, and this can be high yield. The first stage is congestion, and it happens between day 1 and 2.
This is where the blood vessels and alveoli start filling with excess fluid. The next stage is red hepatization, and it happens between days 3 and 4.
This is where exudate, which contains red blood cells, neutrophils, and fibrin starts filling the airspaces and makes them more solid.
The name hepatization refers to the lungs taking on a liver-like appearance due to the build up of reddish-brown exudate.
The third stage is gray hepatization, which happens around days 5 to 7. In this stage the lungs are still firm but they turned pale gray because the red blood cells in the exudate are starting to break down.
The last stage is called resolution, and this happens around day 8 and can continue for 3 weeks. In this stage the exudate gets digested by enzymes, ingested by macrophages, or coughed up.

Diagnosis12:07–15:18

Diagnosis typically begins with chest x-ray, and a very high yield fact is that the exudate appear as an infiltrate or opacity in individuals with pneumonia.
Also note that x-ray can help locate the infection. Bronchopneumonia typically show up as patchy areas that are spread out throughout the lung.
In lobar pneumonia, exudate is localized to a single lobe or set of lobes. In atypical or interstitial pneumonia, the infiltrate often spread throughout the lungs but is often concentrated in the perihilar region and looks reticular, or web-like.
There can also be more linear opacities around pulmonary vasculature, usually veins, or along the periphery of the lungs along the pleura, and these are called Kerley B lines and are a classic sign of pulmonary edema.
Then, sputum culture and Gram stain are needed to identify the pneumonia-causing bacteria. There are also some lab tests suggestive of pneumonia, like an elevated white blood cell count and elevated markers of inflammation like procalcitonin and C-reactive protein.
Now, the diagnosis for atypical pneumonia can be a bit different. In atypical pneumonia with mycoplasma pneumoniae, the X-ray often appears worse than the individual symptoms would suggest.
So if you see this in a question, think of mycoplasma pneumoniae! Diagnosis is then confirmed by serological tests, more specifically there’s a greater than four-fold increase in IgM titer.
Next, remember mycoplasma pneumoniae requires an acellular medium, called Eaton agar enriched with cholesterol, to grow.
Another thing to know is that mycoplasma pneumoniae leads to what is known as cold autoimmune hemolytic anemia. This condition is characterized by autoantibodies that react to the cold and bind to the erythrocyte membrane, leading to premature erythrocyte destruction.
Diagnosis is made by a positive Coombs test, where an antibody against the autoantibody is added to the blood, causing abnormal clumping.
Next, diagnosis of legionella pneumophila infection can be confirmed by legionella urine antigen test. Another clue is that there’s a high number of neutrophils but the absence of organisms on sputum Gram stain, since the bacteria have a cell wall that inhibits staining.
Also keep in mind that it requires a buffered charcoal yeast extract medium supplemented with L-cysteine and iron. Chlamydia infection is usually diagnosed via PCR, nucleic acid amplification, and by Giemsa stain, where the bacteria is seens as cytoplasmic inclusions.
And finally, Pneumocystis jirovecii can be seen as disc-shaped bodies on a methenamine silver stain of lung tissue, obtained by either bronchoalveolar lavage or lung biopsy.

Treatment15:18–16:33

The treatment of pneumonia depends on the type and severity of pneumonia. Since bacteria are the most likely cause, antibiotics are often prescribed.
In addition, cough suppressants and pain medications any supplemental oxygenation are often used to help with symptoms. Now, let’s go through some of the antibiotics that are commonly tested.
For typical pneumonia, the most commonly used antibiotics are penicillin or cephalosporin in infection with Streptococcus pneumoniae, amoxicillin with or without clavulanate for Haemophilus influenzae, and nafcillin or vancomycin for Staphylococcus aureus.
In atypical pneumonia with Mycoplasma, Legionella, and Chlamydia, the best choice is either macrolides, doxycycline, or a fluoroquinolone.
In aspiration pneumonia, clindamycin seems to be the best option as it covers common anaerobes. For pneumocystis pneumonia, trimethoprim/sulfamethoxazole is usually used for treatment and dapsone as a prophylaxis for HIV+ people with a CD4+ count under 200 cells per microliter.
Allright, as a quick recap… Pneumonia is an infection of the lungs that results in consolidation or infiltrates where the tissue is filled with exudate.

Review16:33–17:20

Based on the clinical picture, it can be either typical or atypical. The disease can also be classified by being either community-acquired or hospital acquired with some of those being ventilator associated pneumonias.
Aspiration pneumonia is associated with food or gastric content aspiration. Pneumonia can also be characterized by where the infection is in the lungs.
Bronchopneumonia spread throughout the lungs, atypical or interstitial pneumonia happens in the interstitium around the alveoli, and lobar pneumonia usually infects an entire lobe of the lung.

Summary17:20–18:30

Now back to our cases. Mariah was admitted with crackles at the left lung base, productive cough with yellow sputum, chest pain, and systemic signs of inflammation like fever, tachycardia, and tachypnea.
X-ray revealed a left lower lobe infiltrate, which, given her suggestive symptoms and history, are enough to diagnose community-acquired pneumonia.
in this case, a sputum sample is taken for culture and staining and she was started on antibiotics. The other individual, Jeremy, who was already hospitalized two weeks ago for an acute stroke, developed a cough and right-sided chest pain, tachycardia, and a high fever.
Examination revealed fremitus, decreased breath sounds, and dullness to percussion in the right lower lung field. Chest x-ray revealed a right lower lobe infiltrate.
Because his rather severe symptoms developed after being hospitalized, we can diagnose Jeremy with hospital-acquired pneumonia.
sputum culture alongside Gram stain and culture were done and he was also started on antibiotics.
Pneumonia: Pathology Review: Video and Causes | Osmosis