Procedure – LP

Master answer

Formation / Production of CSF

• 60-70% of the CSF is formed by the choroid plexuses
• 30-40% by the cerebral vessels lining the ventricular walls
• Normal rate is 20mL/hour
• Formation independent of ventricular pressure
• Mechanism
  • From Coroid Plexus by net transport of Na+, K+, Cl-, HCO3- and water, from plasma to ventricles
  • Na down Conc grad
  • Others down electro chem grads

Distribution / Circulation of CSF

Absorption of CSF

• Absorbed through the arachnoid villi into the cerebral venous sinuses
  • Absorption by bulk flow, is proportional to ventricular pressure
  • If pressure < 7 cmH2O, CSF absorption ceases
  • Above 7cmCSF absorption is linear
  • At approximately 11 cmH2O, CSF Absorption = Formation

Composition of CSF

IDENTICAL to brain ECF, but differs in several manners from plasma

Compared with plasma:

• ↑ pCO2 (50 mmHg)
  • ↓ pH (7.33)
• ↓ protein content
  • 0.5% of plasma; 20 mg/dL
  • poor acid-base buffering capacity
• ↓ content of glucose (by 60%) and cholesterol
• ↑ [Cl-] (by 7-14%) and ↑ [Mg2+] (by 40%)
• ↓ [K+] (by 40%), ↓ [Ca2+] (by 50%) and ↓ [Pi] (by 20-30%)
• ↑ creatinine (by 25%) but ↓ urea
• IDENTICAL osmolality (295), [Na+] (145), and [HCO3] (25)

Role / Functions of CSF

• Protective role (main function)
  • Water bath effect
    • Attributed to the low specific gravity of CSF (1.007)
    • Causes brain to be buoyant
    • ↓ its effective net weight from 1400 g to 50 g
    • Mechanical cushion against acceleration/deceleration forces
  • Buffer ↑ ICP by CSF translocation to extracranial subarachnoid space
    • Abrupt ↑ ICP buffered by translocation of CSF within the vault to extracranial compartments
• Maintains constant ionic environment conducive to neuronal electrical activity CSF
• Supply role of nutrients (Eg simple sugars, amino acids) and O2 to brain
• Excretion of toxic substances, metabolic by-products, and CO2 from brain
• “Lymph-type” function → interstitial proteins in brain ECF return to circulation by
• CSF absorption across arachnoid villi
• Acid-base regulation → due to content, CSF allows for tight respiratory control
• Endocrine transport function → transports hormones to other brain regions

Gladwin / JC 2020

Anatomy:

• Position in sitting or lateral decubitus position
• Level:
  • Any of the interspaces between between L2-L5
  • L4/5 interspace: Tuffier’s Line: line between iliac crests
  • (or) L3/4 interspace: Line joining PSISs (Posterior superior iliac spines)
  • Discrepancy between identified and actual intervertebral space in 50% of cases
  • Conus medullaris ends at L1 in about 94% of patients

Tissues and target for LP:

• In the subarachnoid space between the arachnoid mater and the pia mater.
• The tissues pierced are (in order):
  • skin
  • subcutaneous tissue
  • supraspinal ligament,
  • interspinal ligament,
  • ligamentum flavum,
  • dura mater,
  • the arachnoid mater into the subarachnoid space.

• Lateral/paraspinal approach: Skin – subcutaneous tissue – erector spinae muscles – ligamentum flavum – dura – arachnoid – subarachnoid space

Epidural Space

• Posterolateral Epidural Space
  • Posterolateral epidural space extends vertically down the spinal canal and contains arteries, venous plexus, and fat
  • Posterolateral epidural space is larger than the anterior epidural space
  • Posterolateral epidural space is larger in the sacral region than it is in the cervical region
• Anterior Epidural Space
  • Anterior epidural space is a virtual space under normal circumstances (due to adherence of dura to bone of vertebral bodies from the foramen magnum down to L1)

Gladwin / JC 2020

Answer by Gladwin, JC