Pathophysiology of Disease: An Introduction to Clinical Medicine (2nd edition) (II)

My first post about the book can be found here. In this post I’ll talk a little bit the chapters 4-5, which deal with infectious diseases and neoplasia.

When reading chapter 4 – on infectious diseases – it was a great help to have read chapter 3 first – that chapter had a lot of stuff on how the body defends itself against the kind of stuff they talk about in chapter 4, and even though they recap a bit of that stuff in chapter 4 it’s probably smart to read those two chapters in the order they appear in the book. I did not find chapter 4 particularly hard to read, in part probably because this stuff is closely related to the microbiology stuff I read in the past, which dealt in much more detail with the microorganisms causing these diseases. There’s a lot of important concepts covered: transmission mechanisms; factors impacting disease susceptibility; the normal microbial flora and how it relates to this topic at hand; constitutive defences the body (‘defence mechanisms which do not require prior contact with the microorganism – such as physical and chemical barriers to colonization, inflammatory response, the complement system, and phagocytosis); disease progression (to cause disease all microorganisms must go through four stages: they must encounter the host, gain entry, multiply and spread, and cause host tissue injury. The course of an infection may vary from asymptomatic to life threatening; an important distinction is between acute (and sub-acute) and chronic infections). Chronic infection is not the only outcome of a long-term colonization; an individual may also enter a carrier state, or the infection may become latent until reactivation.).

After the ‘general stuff’ has been covered in chapter 4, they deal with the pathophysiology of some examples of infectious diseases; infective endocarditis (bacterial or fungal infection of the interior of the heart), meningitis, pneumonia, infectious diarrhea, and sepsis/sepsis syndrome/septic shock. I’ll not go into much detail about these diseases, but I should probably note here that the names we use to describe infectious diseases like these may cause people to misunderstand how they work: To be clear, there isn’t just one ‘endocarditis bacterium’ or a specific ‘meningitis bacterium’. When specific sites/organ systems are invaded by microorganisms which spread and cause tissue damage (damage which can be caused both by the toxins released by the invading microorganisms and by the host response to the invasion), we have a name for that – but many different microorganisms may cause symptoms by invading the specific site or organ system in question, though some are more likely to affect specific sites than others. Sometimes the names of the microorganisms may even add to this confusion; for example one of the most common causes of bacterial meningitis (infection of the meninges) in children aged 2 months to 15 years is H Influenzae (which kind of sounds like, well…).

Chapter 5 deals with neoplasia. If you dont’ feel like reading this stuff, at least read Mukherjee. The chapter deals briefly with colon carcinoma as an example of an epithelial neoplasia; this stuff from Khan Academy is another great resource on this subject – it also deals a bit with cancer development more generally, and it’s a lot more accessible than is this chapter.

Anyway, the chapter… It starts out with a bit of a downer: “The recognition of overt malignancy by physical examination or imaging requires the presence in the body of about 1 billion malignant cells.” It goes on to note that: “A preclinical phase may sometimes be recognized” but even so, “More commonly, the preclinical phase goes undetected until invasive cancer, occasionally with regional or distant metastases, is already present.” That’s the way it is. The chapter then goes on to talk about many of the same things Mukherjee covers in the latter half of his book, like the role of tumor suppressor genes and oncogenes, the role of environmental triggers (carcinogens), inheritance, … In the field of oncology there seems to be a big focus on the role of genetic changes taking place in the cell(s), and: “A paradigm for sequential genetic alterations has been proposed as a necessary set of events leading to tumorigenesis.” Mukherjee also has more on this, if you’re interested. There are a lot of oncogenes and tumor suppressor genes that play a role in human cancers. In terms of the phenotypic changes they cause, this table is relevant (click to view full size):

A good related quote from the first part of the chapter:

“Molecular and cellular changes in tumor cells are, in a sense, a modification of normal physiology that benefits their growth and spread. The initial alterations may be “preprogrammed” in rare inherited malignancies, or they may be acquired as a consequence of mutations brought about by environmental exposure or occuring by chance during normal cell division. In a process akin to evolution, albeit in a fast time frame, additional genetic changes occur that favor further growth, invasion, and spread. Evasion of the host’s immune system, enhanced proliferative and invasive potential, and resistance to therapy are examples of early, middle, and late changes in the progression of neoplasia.”

The simple way to think about cancer is this: Cancer cells outcompete the surrounding cells because they’re better at growing and spreading, and they cause disease because the reason why they’re better at growing and spreading is that they’re no longer doing what they’re supposed to be doing, and because they’re taking up space and nutrients from the cells that still do their jobs.

The chapter also has some stuff on breast cancer and talks a bit about the BRCA mutations. After that they talk about mesenchymal, neuroendocrine and germ cell neoplasias, which are types of neoplasias the pathophysiology of which “can be described in terms of the embryonic tissue of origin.” The examples they include are carcinoid tumors, testicular cancer, and sarcomas. Again the naming of these diseases may be a bit confusing – tissue will often migrate during development and you can actually end up with, say, a testicular neoplasm which is not located anywhere near your testes (for example, the testicular tissue may have migrated to your chest..). Then they cover hematologic neoplasms (blood, bone marrow, or lymph nodes), and here they’ve included some stuff on lymphomas (“uncontrolled proliferation and potential dissemination of lymphocytes“) and AML, a type of leukemia (Mukherjee has a lot more stuff on that). I found it surprising that they did not spend more time on the last part, the systemic effects of neoplasia – basically they only spend one page on that stuff, though they do also include a few tables to illustrate these aspects of the diseases. I’ve included one of the figures below (click to view in a higher resolution):

Indirect systemic effects of cancer which are not caused by the local presence of cancer cells are what’s called pareneoplastic syndromes.