00:02
A 28 year old woman presents to a physician with
repeated muscle cramps for the last 2 weeks.
00:08
She mentions she commonly has these
in the lower limbs and in her back.
00:12
She also feels that she has
become irritable over last month.
00:16
On physical examination, her vitals are stable.
00:19
The Trousseau sign and Chvostek sign are present,
and deep tendon reflexes are exaggerated.
00:25
Her serum ionized calcium level is 0.9 mmol/L.
00:29
Which of the following electrophysiolgical mechanisms
best explains clinical features of the woman?
Answer choice (A) - Stimulation of GABA receptors
Answer choice (B) - increased firing
threshold for action potentials
Answer choice (C) - Reduction
of afterhyperpolarization
Answer choice (D) - Inhibition of Sodium and calcium
currents through the cyclic nucleotide-gated (CNG) channels
or answer choice (E) - inhibition of sodium
current through the sodium leak channels (NALCN)
Now take a moment to come to an answer by
yourself before we go through it together.
01:09
Okay let's jump right in to this question.
01:11
First let's talk about the question characteristics.
01:13
Now this is a physiology question.
01:15
What we're looking at a patient in a state
in which she has abnormal electrolyte levels
and looking about how
depolarization of muscles occur.
01:25
This is a 2-step question because we have
to find out the cause of her symptoms.
01:29
In here, it's going to be the low calcium
and then determine how that correlates physiologically
to how her membrane action potential works.
01:37
And the stem is absolutely required because
we have to figure out the clinical situation
and understand the outright abnormality
through the question stem.
01:45
So let's walk through this question.
01:47
First we have to determine
the cause of her symptoms.
01:49
She's got muscle cramps, she's irritable and she has a
positive Trousseau sign and Chvostek's sign on the exam.
01:55
So we can see that the patient has a serum ionized
calcium of 0.9 and this is marked hypocalcemia.
02:02
Now a normal serum ionized calcium level should range
from 2.2 to 2.7, so this patient's calcium is very low.
02:09
Now hypocalcemia leads to
increased neuronal excitability.
02:14
The Trousseau sign is a reliable clinical sign
of latent tetany in patients with hypocalcemia.
02:21
It is often acompanied by exaggerated
reflexes, also called hyperreflexia.
02:26
So the cause of this patient's symptoms, even
her muscle cramps is due to low calcium levels.
02:33
Now we need to know and determine the mechanisms by
which hypocalcemia increases neuronal excitability.
02:41
Now hypocalcemia increases neuronal excitability
through a number of mechanisms including
increasing the resting membrane potential
and decreasing the firing threshold.
02:53
Next it can also inhibit GABA receptors,
it can reduce afterhyperpolarization
and it can increase currents through the cyclic nucleotide-gated
(CNG) and also the sodium leak channels (NALCN).
03:09
Now let's refer to our image to
help better understand this.
03:13
Now first we see that we have a threshold.
03:16
We have a resting state of -70 millivolts, and
the threshold is after the stimulus occurs
and our voltage goes to our threshold limit, then we'll have
depolarization-repolarization which is our action potential
which will then be followed by refractory
period and then back to resting state.
03:34
Now, hypocalcemia can cause as we
said through various mechanisms,
one of them is what's called
'reducing hyperpolarization'.
03:45
That means you have a decrease
in the refractory period.
03:50
So you have less time of resting between action
potentials and you actually have increased firing
and that's why you have the positive clinical signs - the
Trousseau sign and the Chvostek's sign and these tetany
because we have smaller refractory period or
also called 'reduced afterhyperpolarization'
which causes the increased clinical symptoms.
04:11
So in this case, the answer choice is answer choice (C)
which is correct, which is reduction of hyperpolarization.
04:18
Now let's go through some high-yield facts
regarding hypercalcemia and neuronal excitability.
04:24
Now hypocalcemia can cause increased neuronal excitability
which may present with muscle stiffness and cramps,
or in severe cases, general tetany.
04:34
And the Trousseau sign is a reliable
clinical sign of latent tetany.
04:38
Now hypocalcemia increases neuronal excitability
through a number of mechanisms including
increasing the resting membrane potential and decreasing
the firing threshold, inhibiting GABA receptors,
reducing afterhyperpolarization and increasing
currents through the cyclic-gated (CNG)
and sodium leak channels (NALCN).