Raised intracranial pressure

Raised intracranial pressure (ICP), also referred to as intracranial hypertension, refers to an elevation in the ICP above ≥20mmHg. The normal ICP is between 7-15mmHg in adults (it generally lower in children). However, there is no clear ICP range as such as it can vary and also because it is difficult to measure, especially in certain positions which greatly affect the ICP. Nevertheless, having an ICP of ≥20 for longer than 5 minutes along with signs or symptoms is considered the threshold for management.

🏃‍♀️ Physiology

Our skull is rigid framework of bones that encloses the cranium (also known as the neurocranium). As it is a solid structure that is unable to expand, the volume which it is able to accommodate is fixed.

The intracranial compartment is made up of 3 components:

1. Brain parenchyma (80%)
2. Cerebrospinal fluid (CSF) (10%)
3. Blood (both venous and arterial blood) (10%)

The volume that the intracranial compartment can accommodate is 1400-1700mL on average. An increase in the volume of one of the intracranial components leads to a decrease in the volume of the other compartments in an attempt to maintain the fixed volume. This is referred to as the Monro-Kellie doctrine.

Let’s look at an example:

The presence of a mass (such as a tumour) in the brain will lead to a raise in the ICP, however, there is a compensation through decreasing the volume of the other components:

• CSF volume - decreased through downward displacement of CSF via the ventricular system - from the cranium → dural sac.
• Venous blood volume - venoconstriction leads to increased drainage of the dural venous sinuses → internal jugular vein.

💡 This doctrine only applies to adults and older children as infants do not have a rigid skill. The fontanelles and unfused sutures allow for an increase in the intracranial volume to an extent.

This compensatory mechanism will occur to maintain the normal ICP. However, if the pathology continues to occupy more intracranial space then we will observe intracranial decompensation and the ICP will increase. Once decompensation occurs then a small increase in intracranial volume leads to an exponentially larger increase in ICP.

Pathophysiology

The cerebral perfusion pressure (CPP) is the total pressure gradient that drives oxygen to the cerebral tissue. It is measured by the difference between the mean arterial pressure (MAP) and the ICP.

CPP is calculated as such:

Cerebral blood flow (CBF) is equal to CPP ÷ cerebrovascular resistance (CVR). Cerebral autoregulation aims to keep the CBF constant across a variety of blood pressures. This can be done by increasing the CVR (through vasoconstriction) when the MAP (and CPP) are increased, or alternatively by decreasing the CVR (through vasodilation) when the MAP (and CPP) are decreased.

• When the CPP drops below the lower limit of autoregulation → reduced CBF and ischaemic injury.
• When the CPP is raised above the upper limit of autoregulation, the CBF will become too high. This breaks down the blood-brain-barrier → seizures, headaches, encephalopathies, stroke.

Other factors involved in the control of CBF are:

1. Partial pressure of arterial oxygen (PaO2) - hypoxia leads to cerebral vasodilation, which drops the CVR and increases the CBF to maintain oxygen supply to the brain.
2. Partial pressure of arterial carbon dioxide (PaCO2) - hypercapnia also leads to cerebral vasodilation to increase the CBF. Hypocapnia reduces the CBF as well.

So in summary if the ICP is raised, the CPP will be lower and this also results in the CBF being reduced which can lead to focal or global ischaemia to the brain.

So what causes a raised ICP?

• Arteriovenous malformation
• Infections - such as brain abscesses, encephalitis, meningitis.
• Tumours
• Traumatic brain injuries - leading to haemorrhage.
• Central nervous system vasculitis - commonly in the on text of autoimmune conditions such as SLE, dermatomyositis, or systemic vasculitIdes.
• Idiopathic intracranial hypertension

Let’s quickly discuss hydrocephalus, cerebral oedema and idiopathic intracranial hypertension before we discuss the presentation, investigations and management of raised ICP.

Hydrocephalus:

Hydrocephalus refers to the accumulation of CSF within the ventricular system of the brain. An increase in the CSF will also lead to an increase in ICP as mentioned earlier.

CSF is predominantly produced in the choroid plexus found in the lateral ventricles. From there it flows through the interventricular foramina to the third ventricle before flowing through the cerebral aqueduct to enter the fourth ventricle. From the fourth ventricle it may then enter the subarachnoid space through the foramen of Magendie (medial aperture) or the foramina of Luschka (lateral apertures). It is then resorbed through the arachnoid granulations in the subarachnoid space back into the venous system. This system helps us maintain a normal ICP.

If the aforementioned system fails we then may get a hydrocephalus and raised ICP.

🔢 Classification

1. Communicating hydrocephalus - this is when the ventricles and subarachnoid space are communicating with each other (i.e. there is no obstruction in between them). This means the blockage is occurring in the subarachnoid space or there is poor resorption of the CSF by the arachnoid villi.

Causes of communicating hydrocephalus:

1. Subarachnoid haemorrhage
2. Meningitis
2. Non-communicating hydrocephalus - this is when the ventricles and subarachnoid space are no longer in communication due to a blockage between the two.

Causes of non-communicating hydrocephalus:

1. Congenital malformations - such as:
• Aqueductal stenosis
• Chiari malformation - when the cerebellum herniates downwards through the foramen magnum, blocking the CSF outflow.
• Arachnoid cysts
2. Tumour or vascular malformation - these often block the interventricular foramen.
3. Intraventricular haemorrhage - especially in premature infants as their blood vessels are more fragile and their cerebral blood pressure fluctuates due to underdeveloped autoregulatory mechanisms.

😷 Presentation

• Infants:
• Bulging fontanelles
• Sunset eye sign
• Poor feeding and vomiting
• Macrocephaly

• Older children and adults:
• Headaches
• Vomiting
• Visual disturbances - such as diplopia, unilateral ptosis or vision loss.
• Parinaud syndrome - a combination of vertical gaze palsy (similar to the sunset sign in children), upper eyelid retraction (Collier’s sign) and light-near dissociation (when the pupillary light reflex does not cause constriction but accommodation reflex does)
• Papilloedema
• Ataxic gait

🔍 Investigations

• Cranial ultrasound - in infants.
• 🥇 CT scan - shows ventricular enlargement.
• 🏆 MRI - gold-standard as it may show more detail and help determine the cause of the hydrocephalus.

🧰 Management

• Ventriculoperitoneal (VP) shunt - this is when a flexible catheter is used to redirect the CSF from the brain to the peritoneal cavity where it is more easily reabsorbed. It requires regular follow-up to monitor for complications such as shunt malfunction or infection.
• Endoscopic third ventriculostomy (ETV) - this involves the creation of a hole in the bottom of the third ventricle to allow CSF to flow out of the brain.

Idiopathic intracranial hypertension (IIH):

IIIH, also known as pseudotumor cerebri, is a condition commonly seen in young, obese females. The cause of raised ICP is not confirmed as there is no intracranial mass or hydrocephalus visible and the CSF composition itself is normal. The theory is that there is an obstruction in cerebral venous outflow.

The female predisposition suggests a possible role for sex hormones and factors released by adipose tissue (such as adipokines).

⚠️ Risk factors

• Pregnancy
• Combined oral contraceptive pill
• Steroids

• Tetracyclines
• Retinoids
• Lithium

😷 Presentation

• Headache - non-pulsatile, bilateral headaches, typically worse in the morning or after bending forwards. Some patients may also experience morning vomiting.
• Visual loss and blurred vision - likely due to optic nerve ischaemia.
• Papilloedema
• Enlarged blind spot
• Pulsatile tinnitus
• Back and neck pain
• Abducens nerve palsy - causing an adducted eye.

🔍 Investigations

• Ophthalmoscopy - shows bilateral papilloedema.
• CT/MRI - shows signs of raised ICP.
• ⭐️ Lumbar puncture - reveals an opening pressure of >20. If it is normal then a different diagnosis should be suggested.

🧰 Management

• 🥇 Encourage weight loss
• Pharmacological options - used to treat visual loss and prevent migraines.
• Acetazolamide - a carbonic anhydrase inhibitor. May help with visual loss.
• Topiramate - helps in prevention of migraines and may also help with weight loss.
• Candesartan - a migraine preventative.
• Therapeutic lumbar puncture, CSF shunting, or optic nerve sheath decompression and fenestration - reserved for cases that are resistant to progressive visual loss.

Cerebral oedema:

There are 3 types of cerebral oedema:

1. Vasogenic oedema

This is caused by the breakdown of the blood-brain-barrier. As the plasma proteins proceed to leak into the brain, the brain interstitial fluid osmotic pressure rises, thus drawing water from the CSF into the brain interstitium → an increase in brain parenchymal volume.

Causes include: head injury, haemorrhage, infarct, tumours, meningitis, abscesses, etc.

The oedema can be localised or generalised.

2. Interstitial oedema

This is usually caused by obstructions in passages between brain ventricles (obstructive hydrocephalus). The excess CSF accumulation leads to an expansion of the ventricles, accompanied by trans-ependymal movement of CSF from ventricles into the brain.

3. Cytotoxic oedema

This can be caused by metabolic poisoning, hypoxia or anoxia, and ischaemia. This type of oedema usually accompanies one of the above types. The root cause is a loss of neuronal energy stores (depletion of ATP) leading to a failure of the Na/K-ATPase. Loss of Na/K-ATPase activity impairs the ability of cells to maintain normal osmotic balance. As the osmotic pressure within the neurons increases, water moves from the extracellular fluid to the intracellular fluid, causing the cells to swell (which may injure or kill the cells).

TO COMPLETE: Presentation, Investigations, Managment