Episode 26: Spinal Anaesthesia
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? I have called you several times, but you didn’t answer.
B: Oh! I am sorry, Dr. Anna. I was thinking about an unusual case which happened in the
A: What case is that? Tell me about it.B: There was a postmenopausal 60-year-old lady presented to the emergency department
after falling from the ladder while cleaning the windows for the Chinese New Year
celebration. She was brought in by wheelchair as she was unable to walk due to the severe
pain. She had no other significant medical history. Examination by Dr Raymond in the
emergency department showed that she was alert and conscious. The right hip was flexed,
adducted and internally rotated. Movement of the right lower limb could not be assessed
but the left side was normal. Neurological examination was unremarkable.
A: Any X-ray?
B: Yeah, the pelvic X-ray showed fractured right femoral neck. She was then admitted to the
orthopaedic ward and arranged for a surgical procedure to stabilise her pelvis. Spinal
anaesthesia was given prior to the operation. The procedure was successful, and she was
awake after that.
A: Sounds great! What happened after that?
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. Full blood count, renal profile and ECG results were unremarkable. An
emergency CT scan was performed which showed the herniation of brain tissues into the
foramen magnum. There was a well-defined dural mass identified on the CT scan, suspected
to be a meningioma. She was referred immediately to the neurosurgical department.
Further history taken from her family revealed that she had headache which worsened for
the past 8 months.
A: That was interesting. The doctor should have taken a proper history preoperatively.
B: Exactly. The herniation was most probably caused by the pressure gradient created by
the cerebrospinal fluid leakage during the administration of spinal anaesthesia. But how
does a few drop of cerebrospinal fluid causing such a significant difference?
A: Lucy, do you mean that how could the CSF support the brain tissue which weighs about
1200 g? (1) Hmmm… Do you remember about Pascal’s principle?
B: Yes. It is the principle stating that the input pressure will equal to the output pressure in
an enclosed fluid. (2)
A: Correct. One of the best examples of enclosed fluid in the body is the CSF. The pressure
of a liquid could also be defined as the force per unit area. Therefore, since the pressures
are equal, and F1/A1 = F2/A2, the force exerted will increase with the increase of the area.
Now let’s look at the size of human skull and vertebral canal. Clearly, the vertebral canal is
smaller with an anterior-posterior diameter ranging from 15 to 27 mm. (3) The average
anterior-posterior diameter of a woman’s skull is 171 mm. (4) Can you now calculate how
much force do the CSF need to support the brain?
B: Of course. Wow! We only need 105 g of CSF to support the brain that weighs 1200 g! It is
only about 9% of the brain’s weight!
A: Very good. This is why we should always take a complete history from the patient before
any procedure. But in this case, we could still miss it as it may be a benign tumour which is
slow growing and does not cause any noticeable symptom.
B: I think it will be important for us to watch out for this particular risk of spinal anaesthesia
although it is a rare one.
A: Totally agree!
1. Hartmann P, Ramseier A, Gudat F, Mihatsch MJ, Polasek W. Normal Weight of the
brain in adults in relation to age, sex, body height and weight. Pathologe [Internet].
1994 Jun [cited 2020 Mar 12];15(3)165-70. Available from:
relation-to-age-sex-body-height-and-weight/ doi: 10.1007/s002920050040
2. Fairman JG. Pascal’s principle and hydraulics [Internet]. U.S.A: National Aeronautics
and Space Administration; 1996 [cited 2020 Mar 12]. Available from:
3. Nadalo LA. Spinal stenosis imaging [Internet]. Medscape; 2017 Sep 21 [cited 2020
Mar 12]. Available from: https://emedicine.medscape.com/article/344171-
4. 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 12]. Available from: https://ohsonline.com/Articles/2008/01/Study-