'Hyper-' means 'over' and '-magnes-' refers to magnesium, and -emia refers to the
blood, so hypermagnesemia means higher than normal magnesium levels in the blood, and
symptoms typically develop at a level over 4 mEq/L.
An average adult has about 25 grams of magnesium in their body.
About half is stored in the bones, and most of the other half is found within cells.
In fact, magnesium is a really common positively charged ion found within the cell, second
only to king potassium.
A very tiny fraction, roughly 1% of the total magnesium in the body, is in the extracellular
space which includes both the intravascular space - the blood and lymphatic vessels, and
the interstitial space - the space between cells.
About 20% of the magnesium in the extracellular space, which would be about 0.2% of the total
magnesium, is bound to negatively charged proteins like albumin, but the other 80% or
0.8% of the total magnesium, can be filtered into the kidneys.
So in the kidney, that magnesium gets filtered into the nephron, andi about 30% gets reabsorbed
at the proximal convoluted tubule, 60% gets reabsorbed in the ascending loop of Henle,
and 5% get reabsorbed at the distal convoluted tubule.
That leaves only 5% to get excreted by the kidneys.
So, in order for there to be too much magnesium in the blood, this normal balance has to be
disturbed.
The most common reason is when those nephrons in the kidneys can't excrete magnesium properly
- which can happen in renal failure, when the kidneys typically aren't able to excrete
anything properly.
Another cause of hypermagnesemia is ingesting more magnesium then the kidneys can excrete.
Sometimes this can be due to an intravenous infusion of magnesium that isn't prepared
correctly.
Other times it can be due to a magnesium containing medication like magnesium hydroxide which
can be used to treat symptoms like constipation and heartburn.
If these medications are taken in excess over a long period of time, it can lead to hypermagnesemia.There
are some less common causes of hypermagnesemia.
One of these is tumor lysis syndrome which results from a rapid number of cancer cells
dying in unison releasing their contents - which includes magnesium.
Hypermagnesemia can cause some serious complications.
At the neuromuscular junction, there are voltage-gated calcium channels on presynaptic neurons which
need to open and let calcium in to cause the neuron to release neurotransmitter and cause
muscle contraction.
Under normal circumstances, magnesium seems to inhibit calcium influx a little bit, which
actually helps stabilize the axon.
But with too much magnesium floating around, it tends to inhibit calcium influx even more,
which interferes with neurotransmitter release and ultimately slows muscle contraction.
Also, normally the parathyroid hormone from the parathyroid gland usually stimulates calcium
release into the blood.
High levels of magnesium in the blood seems to inhibit release of parathyroid hormone
from the parathyroid gland, which causes calcium levels to fall, leading to hypocalcemia.
This affects smooth, skeletal, and cardiac muscle.
It can lead to vasodilation, muscular weakness, diminished reflexes, respiratory failure,
and even coma.
Really high levels of magnesium also alter the electrical potential across the cardiac
cell membrane, which can lead to cardiac arrhythmias like heart block and even asystole.
Anytime hypermagnesemia is causing symptoms, the level of free magnesium ions in the blood
is usually over 4 meq/L. Treatment depends on the cause, and if its due to excessive
intake, then interrupting the source cures the condition.
Because calcium and magnesium compete for binding, some of the symptoms of hypermagnesemia
can be countered by injecting calcium, usually in the form of calcium gluconate, which acts
as a physiological antagonist.
Sometimes furosemide can be used to promote kidney excretion of magnesium, and hemodialysis
can also be used in severe cases.
Alright, as a quick recap - Hypermagnesemia refers to an elevated free magnesium ion level
which usually causes symptoms once its over 4 meq/L. This can be caused by impaired excretion
by the kidneys or an increased intake of magnesium.
The magnesium antagonizes calcium and can lead to a neuromuscular blockade causing symptoms
like vasodilation, muscle weakness, and even cardiac arrhythmias.
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