Module 5

Deriving the initial hypothesis

Part 1

In this module, you will learn what to do after you have analyzed the chief complaint, filled in information for the history of the present illness and looked for connections and relationships between the chief complaint and the medical and dental histories. When you get into a real situation with a patient, or when you work up one of the cases that follow these exercises, periodically review the information you have. Write down the three categories on a piece of paper and fill in what you know under each:

Chief Complaint (abbreviation CC) - under this category write down the quote from the patient as to what their problem is in their own words. Underline the WHO,WHAT, WHERE, WHEN, HOW information and label it. Note where there is missing information.

History of Present Illness (abbreviation HPI) - Write down the words WHO,WHAT, WHERE, WHEN, HOW. Fill in the information you know. Make a list of the specific information you need (missing or supplementary) and write questions to elicit that information

Connections with Medical and Dental History (abbreviations MH, DH) -write down any specific information from the medical or dental history that is or may be relevant to the chief complaint. Make a list of missing information and specific inconsistencies to check out. Think of questions you can ask to clear up the information.

Amazing how much you already know about the patient and their problem.

Before you propose a hypothesis, organize your information in such a way as to delineate what you want to propose a hypothesis about. Using all the relevant information you have learned from the CC, HPI, MH and DH, construct a succinct one or two sentence summary statement that clearly describes your patient and their problem.

This statement is called a Pre-hypothesis Summary Statement. When this statement is firmly in mind, you can propose a hypothesis.


Mr. Norwood, a caucasian male comes into your office.

CC: I've got this lump here on my lip, can you help me with it.

Lump on Lip

From your conversation, physical examination, questioning and history taking you have constructed the following HPI, MH and DH.

HPI: Mr. Norwood is a 45 year old pastry chef who has noticed a lump on his right lower lip for the past year. It has been growing in size and now measures 4X5 mm. You examine it and find it a freely movable, pink, firm, submucosal lump on the right aspect of his lower lip, mucosal surface.

MH: Medical history is entirely normal and Mr. Norwood has had no diseases, allergies or hospitalizations and he is taking no medications at the present time.

DH: Regular dental care with check-ups and cleanings every 6-12 mos. No malocclusion of any significance. The patient has all permanent teeth with several conservative fillings.


Your Pre-hypothesis Summary Statement should contain the following specific data.

  1. The patient's name and a demographic description (age, sex, race)
  2. Facts from the MH that appear pertinent to the CC (not all elements of the medical history are pertinent to the CC)
  3. Basic facts from the HPI that do not overlap information from the CC.
  4. A complete description of the lesion including findings from the physical exam:
    1. Site or Location - for how to describe specific locations, see the unit on Describing Oral Soft Tissue Lesions.
    2. Color for specific color descriptions see unit on Describing Oral Soft Tissue Lesions
    3. Morphology for how to describe morphology, see the unit on Describing Oral Soft Tissue Lesions
    4. Radiographic characteristics (if a radiograph is indicated) for how to describe radiographic appearance, see unit on Describing a Radiograph and Interpreting What You See

Pre-hypothesis Summary Statement for Mr. Norwood:

Mr. Norwood is a 45 year old caucasian male pastry chef who has had a 4X5 mm firm, pink, freely movable, submucosal lump in his lower right lip for the past year. His medical and dental history are non-contributory.


NOTEThe pre-hypothesis summary statement is primarily a learning device. Usually you will not make a "formal" pre-hypothesis summary statement in practice. We use it in this instance to get you in the habit of stopping to summarize as a check to know where you are. Before you jump into your initial hypothesis, you should stop briefly and summarize what you know. This summarization is effectively the pre-hypothesis summary statement. So we ask you to "formalize" it here so you can see the benefit of summarization.

Take the quiz

Go try out the quiz: Writing the Pre-hypothesis Summary Statement.

Part 2

Now you can proceed with the Hypothesis:

Of course you know what a hypothesis is. Just in case you don't, Webster defines Hypothesis as: a tentative assumption made in order to draw out and test its empirical consequences.

In many situations in life you have to propose a hypothesis. You are planning a trip to Fargo, North Dakota in January. You look at the map, note the geographic location of your destination, note the time of year, look at the weather channel and see what the weather's been like for the past week and make your hypothesis on which you will base your decision as to what kind of clothes to pack. The time of year, geographic location and weather for the past week allow you to make the assumption the weather will be cold. Thus, yourhypothesis is:"if the weather is cold, then I will pack a heavy jacket, hat, boots and gloves." You operate on that hypothesis, given all the information you can gather at the time. On the chance the weather turns out to be in the 70s by some freak climatic change, you may be the type who would pack a bathing suit just in case.

You use the same hypothesis generation technique to discover the causes of patient problems in your practice of dentistry.

  • Hear the chief complaint which contains some information (rarely complete).
  • Ask some questions to flesh out and supplement information in the chief complaint, and develop thehistory of the present illness.
  • Obtain the medical and dental history with which you make connections and look for relationships between the historical data, CC and HPI.
  • Perform a physical examination, take radiographs if needed and carry out any other investigative measures.
  • Compose your pre-hypothesis summarization.
  • Propose your initial hypothesis.

INITIAL HYPOTHESIS - the possible diseases or conditions you think the patient could have, given the information you've collected up to that point. It is rare to have only one initial hypothesis. Usually you have several diseases or conditions in mind.

This list, at any one moment in the diagnostic process, is your list of initial hypotheses or differential diagnoses. Actually each disease on the list is a separate hypothesis. So if you have a list of 8 disease possibilities after gathering all your initial data, then you have 8 hypotheses and your differential diagnosis consists of 8 diseases.

Initial hypothesis = Differential diagnosis

Take the example above: a 45 year old pastry chef with a submucosal lump in his lower right lip. If after your CC, HPI, initial physical examination, MH and DH you believe a mucocele, a fibroma or a lipoma could be the cause of the lump, then:

Your initial hypothesis is: If Mr. Norwood, a dentally and medically healthy 45 year old caucasian male pastry chef, has had a 4X5 mm firm, pink, freely movable, submucosal lump in his lower right lip for the past year, then he has either a mucocele or a fibroma or a lipoma.

You then look up Mucocele, Fibroma, and lipoma in your text and read about their characteristics. An easy way to do this at first is to write mucocele, fibroma and lipoma across the top of a sheet of paper and under each, list its characteristics. Compare the three lists with the characteristics of the lesion with which your patient presents. Don't look for an exact fit because you rarely find one. The information in texts is distilled from many cases and case series reported in journals, therefore it tends to describe the "average" case, and often includes more characteristics than one would see in any individual case.

You may be thinking:Donkey

"With all of the diseases I have seen in my book, how do I know which ones to choose at first?" How do I know that the situation of a 45 year old pastry chef with a submucosal lump in his lower right lip suggests mucocele, fibroma or a lipoma?"

GOOD QUESTION! The answer is

  • using good resources and
  • learning from experience

Clinicians with experience can sometimes propose an initial hypothesis without using resources. You will gain this experience over time.

But until you gain experience.....

There is a valuable tool you can use to help you gain experienceREMEMBER: IT IS ALWAYS BETTER, EVEN FOR EXPERIENCED CLINICIANS, TO USE RESOURCES for the initial hypothesis.

Turn to The Appendix p. 663 in your Oral Pathology text.

This appendix, located in the last 34 pages of the book, is designed to organize and direct you toward the differential diagnosis of patient's problems beginning with the clinical manifestations of the disease.

Diseases always present as a set of clinical manifestations. No one ever saw a patient with a lesion labeled "fibroma" so it would be easy to look it up in a text. The chapters of your book are organized, for the most part, by name of entity or disease, not by problem. So to use your book efficiently you have to know the disease your patient has before you can look it up. Patients just don't present that way. Why don't authors organize their books according to problems? We have no idea, but it would seem a logical way to go. How the book is organized doesn't change anything though, you still have to learn how to use the resources at hand. Take a hint, get to know this appendix very well..

The information in the appendix is not complete. You still have to read about each disease or condition on the page(s) listed in the far right hand column. In the 5 parts of the appendix, you can find an initial hypothesis (differential diagnosis) list for most soft and hard tissue abnormalities you encounter.

Thus in the example of the 45 year old pastry chef, you have a soft tissue mass in the lower lip. On p. 678 you will find a section entitled "Soft Tissue Masses (lumps and Bumps): Lower Lip." In this section there are 7 entities listed along with the page numbers where you will find complete information in your text. This list is a good STARTING POINT for your initial hypothesis.

The Following Paragraph is Very Important

It is critical to your mastery of the reasoning processes associated with diagnosis that you understand that the Appendix of your book is NOT a static crib sheet for hypothesis generation and differential diagnosis. The divisions and presentation of information in this section are general, limited and full of overlaps. Thus, the rest of this diagnostic skills process depends completely on your commitment and ability to evaluate critically the list of entities under whatever part you are working and decide for yourself whether entities should be added or dropped.




What happens if I don't have

the appendix of my book to go to;

how do I make an initial hypothesis then?

GOOD QUESTION. There will be times when you don't have your book with you, or when none of the offerings in the appendix to your book seem to fit. At times like this you have to go back to basics. What basics? Anatomy. The simplest technique to get an initial list of possible diagnoses (your initial hypotheses) is to list the anatomic tissues found in the area of the lesion. In our example of the 45 year old pastry chef with a submucosal lump on his lower right lip, the tissues in the area would be:

  • 1. Stratified squamous epithelium
  • 2. Connective tissue
  • 3. Blood and lymph vessels
  • 4. Peripheral nerves
  • 5. Minor salivary glands
  • 6. Striated muscle

Given this list, what lesions originating from these tissues could present as a submucosal lump on the lower lip? Think critically and selectively.

For example: What lesions of stratified squamous epithelium present as a submucosal mass on the lip?

Squamous cell carcinoma?

If you look up squamous cell carcinoma in your book, you will find the clinical appearance of squamous cell carcinoma is a rough, ulcerated, or white plaque-like mass that involves the surface epithelium. The surface epithelium in this case is apparently normal. Squamous cell carcinoma does not present as a submucosal mass. This is what we mean by "think critically and selectively."

Anyone can "shotgun" all the possible lesions of the six tissues above and feel fairly certain that the differential diagnosis is in there somewhere. This doesn't make things easier. We are not interested in every possible lesion of connective tissue, only the ones that truly could present as a "firm, pink, freely movable, submucosal lump." You must be selective. Being selective is "thinking critically." What allows you to be selective is to have complete information available and to have a good working knowledge of normal anatomy, physiology etc. This good working knowledge of basic science is absolutely essential. You can't succeed without it. Working knowledge does not mean knowing a lot of facts. It means being able to integrate the basic sciences into the living, functioning processes of a human and being able to picture what is going on.

As complete a clinical description as possible is essential from the start.

Example: Staying with our 45 year old pastry chef, let's look at the following:

1. If you describe his problem as a submucosal lump, then

  • fibroma, lipoma, neuroma, mucocele, rhabdomyoma, rhabdomyosarcoma,
  • leiomyoma, leiomyosarcoma, lymphangioma, lymphoma, fibrosarcoma,
  • neurofibroma, neurilemmoma, hemangioma

All fit this incomplete description. The more incomplete and general your description, the greater number of possibilities you must consider for the differential diagnosis.

2. If you just palpate the lesion, you will find it to be smooth, round, firm and freely movable. Now you have asmooth, round, firm and freely movable submucosal lump. Simply with the added description, look how many of the above choices you have removed (in bold type).

  • fibroma, lipoma, neuroma, mucocele, rhabdomyoma, rhabdomyosarcoma,
  • leiomyoma, leiomyosarcoma, lymphangioma, lymphoma, fibrosarcoma,
  • neurofibroma, neurilemmoma, hemangioma

You have eliminated all the malignant tumors because malignant tumors are not generally smooth, freely movable and round (remember that from pathology?). That eliminates 4 from your list.

3. If you describe color, and consider consistency, you can eliminate even more choices. Thus, complete information allows you to be selective even in your initial hypothesis. The ability to be selective comes from two sources:

  • complete, accurate information gathering and
  • careful reading and understanding what you read.

So, even if you have trouble making an initial hypothesis from the Appendix of your book, you have theanatomic (tissue) approach to hypothesis generation to fall back on.

Ruling out and ruling in

To rule in or rule out choices, compare what you read (the clinical and radiographic characteristics for each choice) to information you have discovered about your patient's problem. The disease that matches most closely will be your diagnosis (ruled in), and the diseases that do not match will be the ones discarded(ruled out).

Pooh Bear

"After I've read about each disease on my Hypothesis List (the Differential Diagnosis list), how do I decide which one it is?"

GOOD QUESTION: If your initial hypothesis list comprises only 3 alternatives, as in this example, you attempt to exclude two choices (rule them out) and include the remaining choice (rule it in). When you get your list narrowed down to one choice, that is your diagnosis.

If the situation arises where you rule out all of your choices, then you start over again with other hypotheses. The initial gathering information from the CC, HPI, MH/DH, XRAYS and clinical description and formulating a clinical diagnosis is the first level in the diagnostic process. Lab tests, biopsies and other diagnostic tests that may take time and require patient reappointment are considered second level examinations.

The process you are learning is:

  • gather data
  • propose hypotheses to explain the data
  • look at known information about the hypotheses
  • compare that known information with what you know about your case
  • include or exclude hypothetical explanations on the basis of this comparison

Pooh Bear

Comparing diseases this way often uncovers information you haven't found out about your patient's problem. If so, then you devise a way of finding out that information. Sometimes it is as simple as asking the patient another question. Other times you may have to do a lab test, take a radiograph or perform a biopsy. The results can help you make your comparison more accurately and perhaps rule in or rule out one of your hypotheses.

If, as usually happens, you have a list of several choices (6 or 8 or so) in your initial list of hypotheses, you place the disease that matches most closely at the top of your list and the next closest match will be second and so on. This is called prioritization.

In some clinical situations your initial hypothesis list will consist of, say, 8 choices before your first round of comparisons and exclusions. After this round you will have whittled your original list to 5 choices and you might have added an additional hypothesis through reading and comparing, then your hypothesis list for the second round will be 6 choices.

At the second round, you have to do more reading, more testing, perhaps more asking questions and data gathering to see if you can rule out or rule in any more choices. And this is how it goes, round after round of gathering data, proposing hypotheses, comparing and testing, ruling in or ruling out until you have exhausted all of the hard, objective investigative measures at your disposal. At this point you may just have to make an educated guess and treat, or try a more invasive investigative procedure to narrow your choices further.

What can I expect in the usual situation with a patient? In most cases, you will arrive at a clinical diagnosis after your first or second round of hypothesis generation and comparison with what you find in your reference book. At that point you will either be able to treat the patient based on the clinical diagnosis or you will have to obtain a biopsy to ascertain the diagnosis before treatment.

Usually a biopsy is a good second level step to obtain a diagnosis. It is not so important that you be concerned with how long or how complicated the process is, rather you need to discipline yourself to be orderly, methodical, critical and thorough. You should keep careful records of your steps, what you "rule in" and "rule out" and always keep thinking ahead to anticipate problems or sources of confusion that may arise.

Take the quiz

Now go and try the quiz 2 for Module 5: Deriving the Initial Hypothesis