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Could someone explain why hypocalcemia causes Chvostek's sign? Seems like it should be the opposite. Thanks.
Hypocalcemia and Chvostek's Sign
- Thread starter TheSeanieB
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Per Wikipedia:
"Low calcium levels in the bloodstream increase the permeability of neuronal membranes to sodium ions, causing a progressive depolarization, which increases the possibility of action potentials. If the plasma Ca2+ decreases to less than 50% of the normal value of 9.4 mg/dl, action potentials may be spontaneously generated, causing contraction of peripheral skeletal muscles.Tetany is also known as hypocalcemia because it is the medical term for tetany [1]"
My assumption is because there is low Ca2+ outside the cell, the Na/Ca antiporter attempts to compensate, thereby allowing Na into the cell.
"Low calcium levels in the bloodstream increase the permeability of neuronal membranes to sodium ions, causing a progressive depolarization, which increases the possibility of action potentials. If the plasma Ca2+ decreases to less than 50% of the normal value of 9.4 mg/dl, action potentials may be spontaneously generated, causing contraction of peripheral skeletal muscles.Tetany is also known as hypocalcemia because it is the medical term for tetany [1]"
My assumption is because there is low Ca2+ outside the cell, the Na/Ca antiporter attempts to compensate, thereby allowing Na into the cell.
Keep in mind that the mechanism of tetanus is more than this:
If muscle is stimulated repeatedly, there is insufficient time for SR to reaccumulate Ca2+ and the intracellular calcium never returns to the low levels that exist during relaxation (time for cross-bridge cycling is extended). Thus muscle doesn't relax (tetany).
Chvostek's sign is due to tetany. Not always due to hypocalcemia. Patiens with normal calcium levels can also have Chvostek's sign.
As far as why does hypocalcemia cause tetany i will go with the above post...
If muscle is stimulated repeatedly, there is insufficient time for SR to reaccumulate Ca2+ and the intracellular calcium never returns to the low levels that exist during relaxation (time for cross-bridge cycling is extended). Thus muscle doesn't relax (tetany).
Chvostek's sign is due to tetany. Not always due to hypocalcemia. Patiens with normal calcium levels can also have Chvostek's sign.
As far as why does hypocalcemia cause tetany i will go with the above post...
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You're probably thinking of calcium's role in cross-bridge cycling in muscle contraction, but that calcium is intracellular. Hypocalcemia refers to extracellular calcium and decreased extracellular calcium causes increased excitability (lowers the threshold potential).
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On the USMLE, they'll assess this by telling you that a med student elicited the sign in a patient who just had thyroid surgery, and then they'll want to know which vessel was damaged (superior/inferior thyroid arteries, because they also supply the parathyroids).
Or someone will elicit the sign, then they'll ask you about the embryology, and you'd have to know the parathyroids are the dorsal wings of the third pouch (inferior) or the dorsal wings of the fourth pouch (superior).
Or someone will elicit the sign, then they'll ask you about the embryology, and you'd have to know the parathyroids are the dorsal wings of the third pouch (inferior) or the dorsal wings of the fourth pouch (superior).
