How is a Decubitus Ulcer (bed sore) - a subdural Haemorrhage; a spinal cord injury and glaucoma all similar?

They are obviously different, but the one pathophysiology they have at there core, is pressure.

Let’s talk capillary pressure: 

Capillary pressure (Pc) is the pressure difference across the interface between two immiscible (un-mixable) fluids arising from the capillary forces.  At the end of your arteries the arterial blood supplying oxygen and nutrient to ALL tissues enters the smallest of blood vessels - the capillaries.  A single cell thick and full of tiny holes called fenistrations, capilliaries are the end of blood’s journey to perfuse tissues.  Unlike the high pulsing pressure in arteries, capillary pressure, ranges from 10 to 22 mmHg , and here is the crux. 

If external or internal compartment pressures from compression, oedema or other internal or external forces exceed these forces, then the tiny capilliaries are squished (real word), and oxygen doesn’t reach its destination tissue/organ. 

Now think the patient lying on their buttocks, or heels lying in one place on the bed.  There is too much compression over these bony prominence, this exceeds the capillaries pressure, the tissue becomes ischaemic (lacking oxygen), and ultimately tissue dies.  This is exactly how pressure ulcers start. 

In closed head injury, there maybe collections of blood (haematoma), and as the tissues of the brain start to become inflamed and swell, along with the collection of haematoma, intracranial pressure rises.  When the rise in intracranial pressure exceeds maximum capillaries pressure, then brain tissue becomes ischaemic and dies. 

Spinal-cord injuries that leave permanent damage are rarely a result of a severed spinal-cord.

The vast majority of spinal cord injury occurs as a result of bleeding or swelling into the bony canal where the spinal-cord sits.  As the swelling or bleeding increases into this area, it places compression or pressure on the spinal-cord, this exceeds the capillaries pressure resulting in, you guessed it, ischaemia and cell death. 

Glaucoma is a collection of disorders disorders that result in a blockage of aqueous humour, A fluid that is constantly being manufactured in the anterior chamber of each eye.

As the pressure builds up inside the anterior chamber, this exerts compression across the entire globe of the eye.  When this compression rises higher than capillaries pressure, then diffusion to the optic nerve is diminished causing optic nerve ischaemia and death.  This results in permanent blindness if it’s not caught and treated.  

So there you have it.  The common denominator in all of these injuries and so so many other conditions that we try this nurses is compression or pressure that exceeds a capillary’s ability to perfuse its target tissue. 

More on capillaries? Watch this tutorial: