Cerebrospinal Fluid (CSF) : Physiology, Classification

Cerebrospinal Fluid (CSF) Physiology

Cerebrospinal fluid (CSF) is a clear liquid that surrounds the brain and spinal cord. It provides a cushion or shock absorbent. The total amount of CSF is about 125 ml. Out of 125 ml, approximately 20 ml of the CSF is present in the ventricles of the brain; 50 ml is present in intracranial subarachnoid space, and 70 ml is present in paraspinal subarachnoid space. The rate of CSF production is 0.3–0.4 ml/min or 300–500 ml/hour, and 50% of CSF is replaced every 5–6 hours. Normal CSF pressure is 10–15 mmHg.

• The CSF is produced from the choroid plexus, which is present in the lateral ventricles.
• From the lateral ventricles, CSF goes into the right and left third ventricles via the foramen of Monro.
• From the third ventricle, it goes to the aqueduct of Sylvius, from where it flows to the fourth ventricle.
• Via the foramen of Luschka and Magendie, it further reaches paraspinal space.

Sites of Cerebrospinal Fluid (CSF) Leak

• Anterior Cranial Fossa
  • Lateral Cribriform Lamella (thinnest) – M.C. site due to F.E.S.S
  • Posterior Fovea Ethmoidalis → Roof of Ethmoid
  • Posterior aspect of the Frontal Recess
  • Sphenoid
• Middle and Posterior cranial fossa
  • Through mastoid cavity and middle ear to ET
  • Lateral Cribriform lamella is the thinnest and most common site of CSF leak.
• Other sites
  • Posterior Fovea Ethmoidalis (roof the ethmoid)
  • The posterior aspect of the frontal recess
  • Sphenoid
• CSF rhinorrhoea can manifest paradoxically.
  • Lesions in the tegmen (temporal bone) can cause CSF to leak into the middle ear, draining into the nasopharynx and nasal cavity via the Eustachian tube.
  • This route is via the middle and posterior cranial fossa.

Classification Of Cerebrospinal Fluid (CSF) Rhinorrhea

• First recognised by Ommaya. 
• 80% of CSF rhinorrhoea causes result from accidental trauma or iatrogenic trauma; only 2-3% of cases are serious head trauma.
• Only 4% are non-traumatic, and 16% occur as a direct result of intracranial and extracranial procedures. 2.6% of head injuries result in CSF rhinorrhoea.

Causes of Cerebrospinal Fluid (CSF) Rhinorrhea

• Idiopathic
  • Cause is unknown
• Trauma: Surgical
  • “Open” surgery for inflammatory sinus disease
  • Endoscopic sinus surgery
  • Skull base surgery
  • Transcranial approaches
  • Transtemporal approaches
• Trauma: Nonsurgical
  • Closed head injuries
  • Open or penetrating injuries
  • Posttraumatic hydrocephalus
• Inflammatory
  • Erosive lesions: Mucoceles, polypoid disease, cystic fibrosis, fungal sinusitis
  • Osteomyelitis of the skull base
  • Postinfectious hydrocephalus
• Congenital
  • Meningocele or meningoencephalocele
  • Congenital skull-based defects
  • Congenital hydrocephalus
• Neoplasm
  • Neoplasms invading the skull base
  • Hydrocephalus

Presentation Of Cerebrospinal fluid (CSF) Rhinorrhea

• Unilateral, watery nasal discharge laterally
• Positional variation (Reservoir sign)
• Rhinorrhoea caused by CSF leak has the typical characteristic of positional variation and occurs only when the patient lowers the head.
• Presence of inflammatory paranasal sinus diseases.
• Headache (in 40%) and visual disturbance (relieved by drainage of CSF)
• History of single or multiple episodes bacterial meningitis.
• CSF cannot be sniffed back.
• There will no stiffening of handkerchief as there is no mucus.
• It increases on bending forward.
• On occlusion of IJV, there will be increased CSF leak.
• When blood-stained nasal discharge is observed in a patient with severe traumatic rhinorrhoea, a halo sign test is performed.
• Halo sign - There is a central red area surrounded by a peripheral halo. 
• Nasendoscopic examination should be performed in the outpatient clinic (30% - 40%)
• An otoscopy should be performed to exclude a middle ear effusion, as a defect in the middle or posterior cranial fossa can be the origin of the CSF rhinorrhoea.
• CSF fluid is detected by beta-2 transferrin test.
• Approximately 2-3 ml of nasal discharge is collected.
•  If beta-2 transferrin is positive, it indicates CSF fluid because it has 100% sensitivity.
• Non-endoscopic examination accounts for only 30-40%.
• An otoscopy is performed to exclude middle ear effusion, which is a defect in the middle and posterior cranial fossa.

Differential Diagnosis Of Cerebrospinal fluid (CSF) Rhinorrhea

• CSF otorrhea presents as CSF rhinorrhea 
• Sinonasal saline irrigations
• Seasonal and perennial allergic rhinitis
• Vasomotor rhinitis

Determination of Cerebrospinal fluid (CSF) Leak

• No mucoprotein component and hence no viscosity.
• CSF fluid cannot be sniffed back to the nasal cavity readily.
• Patients with intermittent leaks report embarrassing dripping that appears suddenly without warning.
• No sneezing, no congestion, and no response to antihistamine.

• Teapot effect: In the normal sitting position, there is a defect in the roof of the sphenoid that causes accumulation of the CSF.
• When this patient bends forward through the ostium of the sphenoid, fluid drains into the nasal cavity.

Investigation In Cerebrospinal fluid (CSF) Rhinorrhea

• Laboratory investigation of rhinorrhoea fluid is done by beta-2 transferrin.
• The most specific protein of beta-2 transferrin is beta trace protein, which is highly specific for CSF.
• Beta 2 transferrin can be present in CSF, aqueous humour, and perilymph.

Also Read: Medical Management Of Chronic Rhinosinusitis

Laboratory Tests In Cerebrospinal fluid (CSF) Rhinorrhea

• Glucose determination is rapid but highly unreliable.
• beta-trace protein known as prostaglandin synthase
• It is primarily synthesized in the arachnoid cells, oligodendrocytes, and choroid plexus in the CNS.
• Beta-2 transferrin is a protein involved in ferrous ion transport produced by neuraminidase activity. Only a few drops of CSF are required for testing.

Radiological Assessment In Cerebrospinal fluid (CSF) Rhinorrhea

• Radiological assessment is an important marker for determining CSF leaks.

• HRCT of the nose and paranasal sinus is used to detect the skull base defect.
• A plain CT scan of a patient shows defects between the bones, but active or inactive CSF leaks cannot be identified.
• An active leak can only be identified by CT-cisternography or lumbar puncture by injecting intrathecal fluorescein.
  • If this fluorescein passes through the defect and enters the nasal cavity, it can record the activity of the cranial cavity.
• CT-cisternography is performed with iohexol and metrizamide for localization.
  • Now, fluorescein is used for the detection of leaks.
• CT-cisternography depicts the precise position of CSF rhinorrhoea with active leaks.
  • Since it is an invasive process, the risk of post-punctual headaches is high, so it is not used frequently.

Conservative Management In Cerebrospinal Fluid (CSF) Rhinorrhea

• A lumbar catheter is used to drain the lumbar fluid.
• It drains the fluid 5-10 ml per hour for 48 hours.
• After 48 hours, the drain is clamped for 6 hours. At this point, the opening pressure is measured.
• If it is > 20 mm Hg, then adjunctive therapy like mannitol or acetazolamide is given to reduce ICP.
• During this treatment, the patient is advised to take strict bed rest, head elevation, stool softeners, etc.
• The patient is advised to avoid coughing, sneezing, nose blowing, and straining

Surgical Treatment In Cerebrospinal fluid (CSF) Leak/ Rhinorrhea

• Intracranial approach
  • Repair of anterior cranial fossa CSF leaks is done by frontal craniotomy
  • Comminuted skull fracture with displacement of fragments requiring reduction, extensive skull base fractures, and fractures associated with intracranial hemorrhages or contusions that ordinarily would require craniotomy for treatment
• Transcranial approach

• Extracranial Approach
  • Detects in the posterior table of the frontal sinus may be approached externally via a coronal incision and osteoplastic flap
  • Cranialization can also be done by elevating the mucosa of frontal sinus and closing the defect.
• Endoscopic approach
• The endoscopic approach is the best approach because it reduces hospital stay, provides good visualization, reduces the morbidity of brain retraction, and detects defects with intrathecal fluorescein.

Postoperative Care

• Postoperative care includes strict bed rest for several days and antistaphylococcal antibiotics.
• Observation in ICU for the first 24 Hours.
• Continue Lumbar Drain for 4 to 5 Days.
• Nasal packing removed after 3-4 Days.
• The operative site may be checked through serial nasal endoscopy.
• Patients are advised to avoid strenuous activity, sneezing, and coughing for 6 weeks after repair.
• Primary cases successful repair: 85%-90%
• Secondary endoscopic repair also has a high likelihood of success.

Failure Of Endoscopic Surgery 

• MC cause is raised ICT.
• Stenosis of aqueduct of sylvius
• Bening intracranial HTN
• The success rate of endoscopic surgery is 76-97%

Complication

• Infections (meningitis and brain abscess)
• Pneumocephalus
• Tension pneumocephalus
• Intracranial hypotension

Also Read: Olfactory Disorders – Pathway, Work up And Causes

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