Cross-Synaptic Learning: A New Approach to USMLE Step 1 (Endocrinology example)

Episode 85: This begins the audio series with the Cross-Synaptic learning approach. USMLE topics are taught while universal principles are pointed out that refer to other topics or physiologic processes. These lectures will be saved online in the MMC and are expected to be completed in 2014.

Cross-Synaptic Learning approach for the USMLE Step 1

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USMLE Step 1 Endocrine Disorders

Primary vs Secondary vs Tertiary:

Cross-Synaptic Principle of “Structural” Learning Process. These correlate to the processes type of information and the graphical content study approach taught in the MMC Exam Prep modules.

* Hashimotos = destruction of the thyroid gland = PRIMARY hypothyroidism (the gland screws up the hormone)

* Hypopituitarism and hypothyroidism = SECONDARY hypothyroidism (no TSH to stimulate)

* Hypothalamic Dz = Sarcoidosis destroying TRH: TERTIARY (no TRH)

* Example: adenoma on parathyroid producing PTH leading to hypercalcemia = primary hyperparathyroidism

* Example: have hypocalcemia/vit D def, and asked the parathyroid to undergo hyperplasia, that is called SECONDARY hyperparathyroidism

* Example: what if after a long time PTH keeps being made = tertiary hyperparathryroidism (rare)

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Overactivity vs underactivity of glands:

Cross-Synaptic Learning Principle of “Interference Tests”. These refer to a physicians alteration of the ‘natural’ (or in this case unnatural) process. Other areas include ECT and the use of adenosine in supra ventricular tachycardia.

Stimulation test: if pt has underactive gland, would use stimulation test to see if the gland is working.

Supression test: if pt has overactive gland, would use suppression test to see if gland will stop working.

Most of the time, things that cause overactivity, we CANNOT suppress them. There are 2 exceptions where we suppress them, and they deal with overactivity in the pituitary gland

1)prolactinoma can be suppressed bc it can prevent the tumor from making prolactin; bromocriptine suppresses it (dopamine analog – normally, women do not have galactorrhea bc they are releasing dopamine, which is inhibiting prolactin (therefore dopamine is an inhibitory substance – bromocriptine is also used for treating parkinson’s because bromocriptine is a dopamine analog (which is what is missing in parkinsons dz)

2) Pituitary Cushings: b9 tumor in the pitiuitary that is making ACTH – you CAN suppress it with a high dose of dexamethasone. These are the only two exceptions for a tumor making too much stuff. (There is no way to suppress a parathyroid adenoma making PTH, or an adrenal ademona making cortisol, or a an adrenal tumor from synthesizing aldosterone – these are AUTONOMOUS).

Example: pt with hypocortisolism – lets do an ACTH stimulation test – will hang up an IV drip and put in some ACTH; collecting urine for 17 hydroxycorticoids (metabolic end product of cortisol) and nothing happens – so what is the hypocortisol due to? Addison dz – gland was destroyed – therefore, even if you keep stimulating it, you will not be making cortisol.

Example: Let’s say after a few days you see in an increase in 17 hydroxycorticoids, then what is the cause of hypocortisolism? Hypopituitarism – in other words, it’s atrophic bc its not being stimulated by ACTH, but when you gave it ACTH over a period of time, it was able to regain its function. So, with that single test, you are able to find cause of hypocortisalism. Can also look at hormonal levels – ie Addison’s causing hypocortisalism, what would ACTH be?