Basics Of Neurosurgery

Monro – Kellie Doctrine

The brain parenchyma, CSF, and blood all have constant volumes within the closed compartment that is the cranial cavity. There are certain restrictions on this: a rise in one of the three causes a fall in the other two.

However, once that particular limit is reached, it results in a rise in intracranial pressure or tension. Only a certain amount of any one of these three can increase to produce a reduction in the other two. An increase in the range of signs and symptoms is a further consequence of the elevation in intracranial tension.

Cerebral Circulation

Cerebral blood flow provides the high levels of oxygen and glucose that the brain need to function as a highly metabolically active organ. Brain blood flow is crucial, and even brief periods of ischemia can have a significant impact on the central nervous system.

The cerebral perfusion pressure is a critical factor in sustaining cerebral blood flow, as it is the primary determinant of cerebral blood flow.

Also Read: Rapid Acquisition Of Key Concepts - Neurosurgery

How to Calculate the Cerebral Perfusion Pressure?         

• Mean Arterial Pressure (MAP) - Intracranial Pressure (ICP) equals cerebral perfusion pressure (CPP).
• A decrease in CPP may result from an increase in ICP. •-Normal value:
• MAP: 90–110 mmHg; CPP: 75–105 mmHg; ICP: 5–20 mmHg or 5–15 cmH2O
• A drop in CPP might result from an increase in ICP or a fall in MAP; this can then cause ischemia and other problems.

Also Read: Types Of Brain Tumors

Intracranial Pressure or Intracranial Tension (ICP/ ICT)

The following are the three ICP measurement waveforms:
The Lundberg waves (MCQ) are the A, B, and C waves. An increase in ICT will cause a range of clinical symptoms, including herniation syndromes.

Herniation Syndromes

There are 5 types of herniation syndromes.

1. Sub-falcine herniation (cingulate)
2. Uncal herniation
3. Central herniation
4. Cerebellar tonsillar herniation
5. Reverse herniation

1.  Sub-falcine herniation

 It is also known as cingulate herniation since it is the herniation of the cingulate gyrus. The opposing hemisphere is forced to the opposite side as a result of the cingulate gyrus's displacement.  This results in Anterior Cerebral Artery Infarcts, the most prevalent symptom.

The anterior cerebral artery rises directly underneath the falx, which explains this.  There will be frontal lobe anterior cerebral artery infarcts.  A central herniation that is about to occur can be detected by a cingulate herniation.

The following are the clinical characteristics of sub-falcine herniation:  The first symptom is loss of consciousness, which might cause a comma.
A central herniation, brainstem compression, and even death, might result from contralateral weakening.

2.  Uncal Herniation    

Because the uncus is the medial portion of the temporal lobe, uncal herniation is also referred to as medial temporal lobe herniation.

Ophthalmoplegia results from the third nerve, the oculomotor nerve, being compressed and dislodged by the medial temporal lobe.  Dilation of the pupils on one side. The compression of the third nerve causes the uncal herniation, which is characterized by its early and consistent indications.

Some late indicators are.
Third nerve palsy; Sustained hyperventilation; Contralateral body weakness (caused by crural compression).

Uncal herniation can occasionally also cause Cheyne-Stokes respiration. Among the really late symptoms include bilateral fixed dilated pupils and quadriparesis. • Abrupt breathing, which is eventually followed by demise.

Kernohan’s Notch

Common scenario: - The opposing cerebral hemisphere controls the body.  It is observed in uncal or trans tentorial herniation.
  When there are symptoms or indicators on the left side of the brain;  When there is damage or injury to the right side of the brain
  Ideally, the compression of the right crus cerebri resulting in left-sided weakness will be caused by the right-sided uncal herniation. The opposite crus cerebri is compressed by the free edge of the falx in Kernohan's notch phenomenon, which results in ipsilateral weakness (palsy on the same side of the third nerve).

3. Central Herniation

The diencephalon or midbrain is pushed down in central herniation; There may be damage to the pituitary stalk;  This may result in diabetes insipidus (DI);  The main cause of the clinical features is the trapping of the posterior cerebral arteries.

Central herniation is also referred to as trans-tentorial or tentorial herniation (tentorial means tentorium cerebelli).
The occipital lobe is primarily supplied by the posterior cerebral arteries, and their involvement can result in occipital infarcts, which can cause blindness.

Do Duret Hemorrhages Exist?

The majority of them are brainstem hemorrhages.

The basilar artery, which supplies blood to the brainstem, is the cause of ischemia that results from compression and shearing of its branches.

Indices of a hernia in the center:
• Early warning indicators:
• Diminished awareness
• Tiny (Pinpoint) but responsive students
• Babinski response on both sides (caused by involvement of the corticospinal tract)
• Delayed indicators:
• Cheyne- Stokes breathing, irregular; • Decorticate stiffness; • Extremely Late sign:
• Very irregular breathing patterns; • Dilated and fixed pupils; • Death

4. Cerebellar Tonsillar Herniation

Tonsillar herniation occurs when the cerebellar tonsils are forced to the foramen magnum. They can occur due to Supra as well as infratentorial mass lesions. It is very dangerous because it can cause medulla compression and death. 

Medulla contains all the vital centres of the body (i.e., respiratory and cardiac centre).       

5. Reverse Herniation

It is herniation of posterior fossa structures (i.e., cerebellum and brainstem). This herniation occurs across the tentorial hiatus upwards.

Where is this type of herniation seen?

• It is usually seen in posterior fossa mass lesions. 
• Whenever they are present with hydrocephalus and a CSF diversion is done, there is sudden decompression.
• This causes upward herniation of the cerebellum and the brainstem. 
• This type of herniation is disastrous and fatal. 
• In the case of hydrocephalus or posterior fossa masses or any lesion leading to raised ICP in the posterior fossa, the release of CSF should be very slow. 

Methods To Reduce Raise ICP

There are 4 types to reduce ICP. 

CSF 

Features 

• It is a clear colourless fluid 
• pH: 7.33 to 7.35
• Specific gravity: 1.007
• CSF volume: 150 ml
• Rate of CSF production: 0.3ml/min (450 ml in 24 hours)
• The entire CSF is circulated 3 times in 24 hrs (MCQ)

Formation

The choroid plexus produces 85% of the CSF.  Dural sleeves of nerve roots (15%) and ependyma are two more locations that generate CSF. Death

Absorption  

It occurs next to the arachnoid villi, which are located on the sinus' superior side.

Also Read: Cerebrovascular Disorders- Clinical Features, Investigations And Treatment

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