I happened on the striking video above in a recent New York Times story about Alzheimer's disease.

For non-anesthesiologists, I'll briefly deconstruct it.

1. The clear plastic hub of the needle (whose business end is three inches inside the patient) marks a significant advance over those I worked with back in the last quarter of the 20th century: mine were stainless steel, so we had to lean down and peer into the tiny hole where the needle abuts the hub to see the first drops of cerebrospinal fluid (CSF).

2. Ideally the CSF is crystal clear as pictured; if it's bloody one of two things will happen: the blood will clear or it won't. If it clears we assume the needle hit a small blood vessel on the way in, which we call a "traumatic tap"; if it remains bloody as long as the needle is in place there is pathology present, most likely from a bleed somewhere in the central nervous system or trauma.

3. CSF that's tinged with any color indicates pathology. 

• Yellow CSF (xanthochromia): This commonly indicates the presence of bilirubin, which results from the breakdown of red blood cells in the CSF. Xanthochromia is most often used to diagnose subarachnoid hemorrhage, especially if a head CT scan is normal. It can also be seen in cases of previous bleeding (more than 3 days duration), increased CSF protein, or severe jaundice. 

• Brown, Orange, or Pink CSF: May be associated with elevated protein, past bleeding events, or increased breakdown products in the fluid.

• Green CSF: This may indicate infection or the presence of bilirubin from hemolysis (breakdown of red blood cells).