Hypocalcaemia

Last updated 1st April 2019 - Tom Heaton
This is another really good video from Osmosis on the topic:
https://www.youtube.com/watch?v=KWZrSYo7xuk

Definition

This is defined as an total serum calcium level below the normal range (this will vary between labs, with 2.1-2.6 mmol/L a guide).
A low ionised calcium level may also be defined as this, given that this is the active state.
However, it is important to note that ‘pseudohypocalcemia’ may exist in states of low albumin.
This is because there is less calcium in the blood overall as nearly half is usually bound to albumin.
As it is the free ionised calcium which has the physiological effects, this is not true hypocalcaemia.
There is often a ‘corrected calcium’ level on blood results to take this into account.

Pathophysiology

A good understanding of the normal physiology of calcium is important to understand the pathology. This is discussed elsewhere.

As calcium is such an important ion for many cellular processes, low levels start to lead to a failure of these.
The keys ones involve:
  • Excitable tissue
  • Coagulation
Calcium acts as a stabiliser for sodium channels in a number of excitable tissues, such as neurons and the muscles.
The reduction in levels therefore tends to lead to over excitability of these cells.
This results in the manifestation of tetany, seizures and cardiac conduction abnormalities which form part of the clinical picture.

Aetiology

The causes of hypocalcaemia can be thought of as:
  1. Decreased entry to the blood
    1. Impaired absorption
    2. Impaired release
  2. Increased loss from the blood
    1. Renal loss
    2. Redistribution/binding

Decreased Entry into Blood

This is most commonly due to loss of parathyroid hormone (hypoparathyroidism):
  • Surgical removal
  • Gland destruction
    • Autoimmune
    • Radiotherapy
    • Metastatic invasion
  • Congenital disease e.g. DiGeorge’s syndrome
  • Hypomagnesaemia
Vitamin D deficiency may also be a cause:
  • Dietary deficiency
  • Inadequate sunlight
  • Malabsorption syndromes
  • CKD
  • Cirrhosis
  • Vitamin D resistance
This may lead to a secondary hyperparathyroidism state.

Increased Removal From Blood

Renal
  • CKD
  • Diuretics
Citrate RRT
Redistribution/binding
  • Pancreatitis
  • Rhabdomyolysis
  • Burns
  • Tumour lysis
  • Massive blood transfusion
  • Hyperventilation
In cases of major tissue trauma e.g. tumour lysis syndrome, the large levels of cell death leads to phosphate release which binds calcium.

Presentation

This may depend of the speed and severity of the decrease.
Rapid changes e.g. with hypoventilation, can produce more notable symptoms.
The history can provide a guide to the possible causes.
The features can be considered on a systems basis.

CNS
  • Tetany
  • Paraesthesias (especially perioral, and peripherally)
  • Muscle cramps
  • Seizures
  • Chvostek’s sign - tapping of the facial nerve (just below the zygomatic arch) provokes facial twitching
  • Trousseau’s sign - inflation of a BP cuff (over SBP) and left for a few minutes triggers spasm in the forearm/hand

CVS
  • QT prolongation
  • Progression to VT

Resp
  • Laryngospasm
  • Bronchospasm

Investigations

Bloods
  • U&Es
  • FBC
  • LFTs
  • Bone profile
  • Mg2+
  • ABG/VBG - ionised calcium
  • PTH/Vitamin D levels

ECG

Urinary calcium - may be persistently high in hypoparathyroidism

Management

This can be differentiated into:
  1. Replacing calcium
  2. Correcting cause

Replacing Calcium

Some patients will need IV replacement:
  • Calcium gluconate 10ml 10% = 2.3 mmol
  • Calcium chloride 10ml 10% = 6.8 mmol
This will include:
  • Symptomatic
  • Ionised Ca2+ < 0.8 mmol/L
  • High CVS support
  • Post bypass
  • Major transfusion
  • Calcium channel blocker overdose

Oral replacement may be suitable in stable patients

Correcting Cause

This may include:
  • Correct magnesium
  • Stop causative drugs
  • Vitamin D - may need activated vitamin D in CKD

Links & References

  1. Hasudungan, A. Endocrinology - calcium and phosphate regulation. Youtube. 2015. https://www.youtube.com/watch?v=EEM0iRJNhU8
  2. Hogan, J. Goldfarb, S. Regulation of calcium and phosphate balance. UpToDate. 2018.
  3. McIndoe, A. Thyroid and parathyroid hormones and calcium homeostasis. e-LFH. 2014.
  4. Power I, Kam P. Principles of Physiology for the Anaesthetist (2nd ed). 2008. Hodder Arnold.
  5. Osmosis. Hypocalcemia - causes, symptoms, diagnosis, treatment, pathology. Youtube. 2017. https://www.youtube.com/watch?v=KWZrSYo7xuk
  6. Tidy, C. Hypocalcaemia. Patient.info. 2015. https://patient.info/doctor/hypocalcaemia
  7. Nickson, C. Hypocalcaemia. LITFL. 2013. https://lifeinthefastlane.com/ccc/hypocalcaemia/​