With hypercalcemia, hyper -means over and -calc- refers to calcium, and -emia refers
to the blood, so hypercalcemia means higher than normal calcium levels in the blood, generally
over 10.5 mg/dL.
Now, calcium exists as an ion with a double positive charge - Ca2+ - and it’s the most
abundant metal in the human body.
So I know what you’re thinking - yeah, we’re all pretty much cyborgs,- Cool, huh?
So about 99% of that calcium is in our bones in the form of calcium phosphate, also called
hydroxyapatite.
The last 1% is split so that the majority, about 0.99% is extracellular - which means
in the blood and in the interstitial space between cells, and 0.01% is intracellular
or inside cells.
High levels of intracellular calcium causes cells to die.
In fact, that’s exactly what happens during apoptosis, also known as programmed cell death.
For that reason, cells end up spending a lot of energy just keeping their intracellular
calcium levels low.
Now, calcium gets into the cell through two types of channels, or cell doors, within the
cell membrane.
The first type are ligand-gated channels, which are what most cells use to let calcium
in, and are primarily controlled by hormones or neurotransmitters.
The second type are voltage-gated channels, which are mostly found in muscle and nerve
cells and are primarily controlled by changes in the electrical membrane potential.
So calcium flows in through these channels, and to prevent calcium levels from rising
too high, cells kick excess calcium right back out with ATP-dependent calcium pumps
as well as Na+-Ca2+ exchangers.
In addition, most of the intracellular calcium is stored within organelles like the mitochondria
and smooth endoplasmic reticulum and is released selectively just when it's needed.
Now, the majority of the extracellular calcium is split almost equally between two groups
- calcium that is diffusible and calcium that is not diffusible.
Diffusible calcium is separated into two subcategories: free-ionized calcium, which is involved in
all sorts of cellular processes like neuronal action potentials, contraction of skeletal,
smooth, and cardiac muscle, hormone secretion, and blood coagulation, all of which are tightly
regulated by enzymes and hormones.
The other category is complexed calcium, which is where the positively charged calcium is
ionically linked to tiny negatively charged molecules like oxalate, which is a small anions
that’s normally found in our blood in small amounts.
The complexed calcium forms a molecule that’s electrically neutral and small enough to cross
cell membranes, but, unlike free-ionized calcium is not useful for cellular processes.
Finally, though, there’s the non-diffusible calcium which is bound to negatively charged
proteins like albumin and globulin, and the resulting protein-calcium complex is too large
and charged to cross membranes, leaving this calcium also uninvolved in cellular processes.
When the body’s levels of extracellular calcium change, it’s detected by a surface
receptor in parathyroid cells called the calcium-sensing receptor.
This affects the amount of parathyroid hormone that gets released by the parathyroid gland.
The parathyroid hormone gets the bones to release calcium, and gets the kidneys to reabsorb
more calcium so it's not lost in the urine and synthesize calcitriol also known as active
vitamin D. Active vitamin D then goes on to increase calcium absorption in the gastrointestinal
tract.
All together, these effects help to keep the extracellular levels of calcium within a very
narrow range, between 8.5 to 10 mg/dl.
Sometimes, though, total calcium levels in the blood, which includes both diffusible
and non-diffusible - blood can vary a bit, depending on the blood's pH and protein levels.
This happens because albumin has acidic amino acids, like glutamate and aspartate, which
have some carboxyl groups that are in the form of COO- or COOH.
Overall the balance of COOi and COOH changes based on the pH of the blood.
Now, when there’s a low pH, or acidosis, there are plenty of protons or H+ ions floating
around, and a lot of those COO- groups pick up a proton and become COOH.
More COOH groups make albumin more positively charged, and since calcium is positively charged,
these two repel each other, and this decreases bound calcium and increases the proportion
of free ionized calcium in blood.
So as more protons bind albumin, more free ionized calcium builds up in the blood, and
so even though total levels calcium are the same, there’s less bound calcium and more
ionized calcium, which remember is important for cellular processes and can lead to symptoms
of hypercalcemia.
Also, any condition that results in hyperalbuminemia or high albumin levels, causes there to be
a higher concentration of protein-bound calcium, while free ionized calcium concentrations
stay essentially the same due to hormonal regulation.
This is therefore called false hypercalcemia or pseudohypercalcemia, since the concentration
of bound calcium increases but the concentration of free ionized calcium stays the same.
Even though this is rare, it can happen in people with dehydration - where albumin gets
very concentrated.
Now, True hypercalcemia might be caused by increased osteoclastic bone resorption, which
is actually the most common cause, and this is where osteoclasts, which are little bone
eating cells, frantically break down the bone, and release calcium into the blood.
This might happen when the parathyroid gland becomes overgrown and releases more parathyroid
hormone.
Another well-known cause of hypercalcemia are malignant tumors, some of which secrete
parathyroid hormone-related protein or PTHrP, a hormone that mimics the effect of parathyroid
hormone which stimulates the osteoclasts.
Alongside these osteoclast cells, there’re also osteoblast cells, the bone-building cells,
and some tumors cause osteoblasts to die off.
Overstimulated osteoclasts without enough osteoblasts can result in lytic bone lesions
which are commonly seen in some malignancies.
Another cause of hypercalcemia is excess vitamin D either through the diet or through supplements,
which can cause too much calcium being absorbed in the gut.
Finally, there are some medications like thiazide diuretics which increases calcium reabsorption
in the distal tubule of the kidney which contributes to hypercalcemia.
High levels of ionized calcium affect a variety of cellular processes, in particular, electrically
active neurons.
Normally, calcium ions stabilize the resting state of the sodium channels, which prevents
them from spontaneously opening and letting sodium ions enter the cell.
With high levels of extracellular calcium, voltage-gated sodium channels are less likely
to open up, which makes it harder to reach depolarization, and makes the neuron less
excitable.
This is what causes slower or absent reflexes, which is a classic symptom of hypercalcemia.
The sluggish firing of neurons also leads to slower muscle contraction, which causes
constipation and generalized muscle weakness.
In the central nervous system, hypercalcemia causes confusion, hallucinations, and stupor.
In most cases, when there’s too much calcium in the blood, the kidneys try to dump it into
the urine - causing hypercalciuria or excess calcium in the urine.
Hypercalciuria leads to a loss of excess fluid in the kidneys causing an individual to get
dehydrated, and the combination of hypercalciuria and dehydration can lead to calcium oxalate
kidney stones.
Hypercalcemia is diagnosed based on high level of calcium in the blood, generally above 10.5
mg/dL.
An electrocardiogram might have changes like bradycardia, AV block, shortening of the QT
interval, and sometimes in the precordial leads the appearance of an Osborn wave.
To identify the cause, lab tests are typically done, looking at parathyroid hormone, vitamin
D, albumin, phosphorus, and magnesium levels.
In hypercalcemia, the main goal is to lower calcium levels using medications that reduce
calcium in blood.
One approach is to increase urinary excretion of calcium, which can be done by rehydrating
an individual which causes more calcium to get filtered out.
In addition, loop diuretics can be helpful because they inhibit calcium reabsorption
in the loop of Henle, leaving calcium in the lumen of the nephron to get excreted.
Another approach is to increase gastrointestinal excretion, by using glucocorticoids to decrease
intestinal calcium absorption, which allows it to instead simply pass through the gut
without getting absorbed.
Finally, you can prevent bone resorption by using bisphosphonates or calcitonin to inhibit
osteoclasts.
Alright, as a quick recap, hypercalcemia describes a high concentration of free ionized calcium
in the blood, which most commonly results from excess parathyroid hormone as well as
malignancies.
High calcium levels causes excitable cells to be less...excitable, which result in slow
reflexes, muscle weakness, and constipation.