Episode 26: Spinal Anaesthesia
Author: Lisa Ling
Editor: Dr. Suneet Sood
Narrator(s): Lisa Ling
It was early in the morning. Dr. Anna, the surgeon was on her way to work when she bumped into Dr. Lucy, the junior doctor who appeared to be in deep thoughts.
A: Good morning, Lucy.
B: Hey! Good morning, Dr. Anna.
A: What’s troubling you? You look like you’re thinking hard!
B: Oh! I am sorry, Dr. Anna. I was thinking about an unusual case which happened in the ward recently.
A: What case is that? Tell me about it.
B: There was a postmenopausal 60-year-old lady who presented to the emergency department after falling from a ladder. She had no significant medical history. Examination by Dr Raymond in the emergency department showed that her right hip was flexed, adducted and internally rotated.
A: Fracture neck of femur.
B: Yes. An X-ray of the pelvis confirmed that. So they did a pinning of the femoral neck under spinal anesthesia.
A: Then what happened?
B: She was disoriented and lethargic the next morning. Few hours later, she was found unconscious with dilated pupils before the senior doctor who was busy in the clinic managed to review. She was referred to neurosurgery, who ordered an emergency CT scan. The CT showed herniation of the brain tissues into the foramen magnum. They also found an infratentorial mass, which they had not suspected.
A: That was interesting. The doctor should have taken a proper history preoperatively. A good systems review is important to elicit the possible symptoms which may lead to severe complications perioperatively.
B: You are right. The family members said she has been having headaches for eight months. So the neurosurgeons did an emergency external CSF drainage, and relieved the intracranial pressure, and she became better. They are planning a craniotomy for the mass tomorrow.
A: You know that cerebellar herniation can occur if we do a lumbar puncture in a patient with an intracranial lesion. The herniation was most probably caused by the pressure gradient created by the cerebrospinal fluid leakage during the administration of spinal anaesthesia.
B: Yes, I know. What I’m wondering is, how can a few drops of cerebrospinal fluid cause such a significant shift of the brain?
A: Ah, that’s what you are thinking!
B: Yes. I mean, we just remove about 3-5 ml during a spinal anesthesia. How can the brain move so much? The brain weighs about 1.5 kilos. (1) It’s enclosed in the skull. It must take a lot of force to shift the brain down into the foramen magnum!
A: Okay, so there’s one aspect of physiology, and two principles of physics you need to consider here.
B: Physics! That’s always interesting.
A: Yes it is. First, the physiology. Remember, the CSF is under pressure, normally about 1.5 kilopascals. (1) At the top of the brain, the CSF in the cranial subarachnoid space tries to push the entire brain downwards through the foramen magnum. At the lower surface of the cerebellum, the CSF in the spinal subarachnoid space tries to push the whole thing upwards. I should mention that the area of the upper brain surface is at least 4 times greater than the area of the lower surface of the cerebellum. (2,3)
Second, the physics. The two principles that apply here are Boyle’s law and Pascal’s law. Let’s talk about Boyle’s law first. In any closed compartment, the product of pressure and volume in a fluid remains constant. The mathematical term is P1V1=P2V2. (4) It really applies to gases, but the principle works here. When you remove fluid from the spinal canal, dura mater is not elastic. The volume in the spinal canal cannot drop. Therefore pressure in the spinal subarachnoid space must drop. So we have a pressure gradient. At the top of the brain the pressure is still 1.5 kilopascals. But the spinal canal contains only about 25 ml of fluid. So when we remove 4 ml, we are reducing the volume by 15%. And since the space cannot shrink, the pressure must drop by 15%. So we have a big pressure gradient between the skull and the spine.
Now let’s go to Pascal’s law. It says that the force exerted by a liquid is the pressure multiplied by the area over which the pressure is applied. (5) The fluid in the skull is pushing the brain downwards. The fluid in the spine is pushing the cerebellum upwards. The downward pressure is 1.5 kilopascals, multiplied by about 200 square cm of surface area. The upward pressure is 1.2 kilopascals, multiplied by only about 50 square cm of surface area. (2,3)
B: Ah, that makes sense. But is that enough to move such a heavy brain?
A: Yes it is. Remember, the brain is enclosed in fluid. Because of the buoyancy, so the effective weight of the brain in the cerebrospinal fluid is only about 25 grams, which is about a quarter of an apple! (1) If not for that effect, our brains would be slowly squeezing into our spines every day that we walk! But also for that effect, a disturbance of the pressure gradient can easily move the very light brain into the spine.
B: Of course. When you explain it like that, it’s quite obvious!
A: Of course, this raises a question: by how many centimeters do we expect the brain to move down? Well, obviously 4 cubic centimeters of the cerebellum needs to move into the spine. Enough for the cerebellar tonsils to go partly through the foramen magnum. And a herniation distance of as little as half a cm may be lethal. (6)
B: So what I’m gathering is, because of Pascal’s law, a pressure gradient between the skull and the spine will result in a large downward displacement of the cerebellum.
A: Yes, and don’t forget, it’s important to take a good history. Never cut out the systems review!
B: I agree, and I’ll remember this lesson!
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- Occupational Health & Safety. Study: women’s skulls thicker, men’s wider; might affect protection design [Internet]. Occupational Health & Safety; 2008 Jan 22 [cited 2020 Mar 28]. Available from: https://ohsonline.com/Articles/2008/01/Study-Womens-Skulls-Thicker-Mens-Wider-Might-Affect-Protection-Design.aspx
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- Metterlein T1, Kuenzig H, Bele S, Brawanski A, Graf BM. Coma after spinal anaesthesia in a patient with an unknown intracerebral tumour. Acta Anaesthesiol Scand. 2010 Oct;54(9):1149-51