What’s the difference between IPF and UIP?

As a medical student, I found the various interstitial lung diseases (ILDs) to be horribly confusing. The most common idiopathic ILD is idiopathic pulmonary fibrosis, which is often used interchangeably with usual interstitial pneumonia (UIP). Is there a difference?

Well, yes. UIP is a histopathological description of a lung biopsy that has a specific pattern of fibrosis. (It’s a horrible name, but I was recently told that they tried to change it a decade or two ago and couldn’t come up with anything better.) UIP might also be used to refer to specific findings on high-resolution CT that has a very high correlation with UIP on histopathology. High-res CT is now so good that you usually don’t need the biopsy to know that a patient has UIP.

IPF, on the other hand, is what you call someone with UIP in the lungs if you don’t know why they have it. There are many things that cause UIP on CT and biopsy, including chronic hypersensitivity pneumonitis, connective tissue disorders, and drugs. If the patient doesn’t have any of those diseases or exposures, then it’s said to be idiopathic, and you call it idiopathic pulmonary fibrosis.

This distinction is actually very important, because IPF has a very poor prognosis and has no good disease-modifying treatments, whereas some of the other causes of UIP can be treated.

And now that you understand the difference between IPF and UIP, I’d like to add cryptogenic fibrosing alveolitis to your vocabulary—the fancy British way of saying IPF.

Read more: Idiopathic Pulmonary Fibrosis in Chapter 315: Interstitial Lung Diseases, Harrison’s Principles of Internal Medicine 19e.

The four things that cause an anion gap metabolic acidosis

Anion gap metabolic acidosis is one of those classic differentials that we all learn. My favourite approach is not the MUDPILES that I learnt in medical school, but rather a differential based on the four basic things that can cause an increase in unmeasured anions:

  1. Ketoacidosis: this includes your DKA, starvation ketoacidosis, etc.
  2. Lactic acidosis: often from poor perfusion, as in the various causes of shock.
  3. Renal failure: kidney failure alone can raise your anion gap, particularly from decreased clearance of NH4+.
  4. Toxins: ethanol, methanol, ethylene glycol, acetaminophen, and aspirin are some big ones here.

And, for interest’s sake: although the most common form of lactic acidosis is from L-lactate (that’s what the lab reports as “lactate”), there’s also the rare D-lactic acidosis. It’s caused by gut bacteria in patients with jejunoileal bypass, short bowel syndrome, and intestinal obstruction. The bacteria create D-lactate rather than our normal human L-lactate.

Read more: Metabolic Acidosis in Chapter 66: Acidosis and Alkalosis, Harrison’s Principles of Internal Medicine 19e.

Four things that make you nauseous

Your body uses nausea to let you know that you’ve ingested something bad. Or, at least, that it thinks you’ve ingested something bad—it doesn’t always get it right. These are the four major pathways that produce the feeling of nausea:

  1. The cerebral cortex and limbic system: remember that thing that made you really sick, and now even the smell or thought of it makes you nauseous? That’s your body’s first defence against toxic ingestions.
  2. The GI tract: it contains chemoreceptors and mechanoreceptors that try to guess when you’ve eaten something bad or dangerous. It often involves serotonin (5-HT3) receptors in the GI tract.
  3. The vestibular system: disruptions to your inner ear and sense of balance can cause nausea, as anyone who has experienced motion sickness can attest. This pathway uses histamine (H1) and muscarinic acetylcholine (M1) receptors.
  4. Finally, the chemoreceptor trigger zone: this is a specialized bundle of chemoreceptors that sample the blood, responding to drugs, certain metabolites, and bacterial toxins. It mainly uses central dopamine (D2) receptors, but also serotonin (5-HT3) and NK1 receptors.

In order to treat nausea, then, you should target the pathway that’s acting up. For motion sickness, anti-histamines like diphenhydramine; for chemotherapy, serotonin antagonists like ondansetron; for decreased GI motility, prokinetic dopamine antagonists like metoclopramide; and when you don’t know, haloperidol, which mainly blocks dopamine but also affects the other pathways.

Read more: Palliative and End-of-Life Care > Nausea, Harrison’s Principles of Internal Medicine 19e.

Classifying pulmonary hypertension

Pulmonary hypertension (PH) is defined as resting mean pulmonary arterial pressure (mPAP) ≥25 mmHg, compared to a normal value less than 20 mmHg.

Classification of PH is broken into five categories by the WHO:

  1. Pulmonary artery hypertension, the most common category that includes hereditary/idiopathic causes, drugs & toxins, connective tissue disorders like scleroderma/SSc, HIV, and schistosomiasis
  2. Secondary to left heart disease, mostly heart failure with preserved ejection fraction (HFpEF)
  3. Secondary to chronic lung disease and/or hypoxemia, including COPD and OSA as well as interstitial lung disease
  4. Secondary to chronic thromboembolic pulmonary hypertension (CTEPH): fairly self-explanatory, I suppose
  5. Secondary to everything else: weird things that you wouldn’t necessarily think of, including sickle cell disease, other chronic hemolytic anemias, chronic kidney disease, and a grab-bag of other things

A few interesting points:

  • The most common cause worldwide is schistosomiasis (included in Group 1, above)
  • Longstanding PH can lead to cor pulmonale, which is not good

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