Definition and Epidemiology of Intracranial Aneurysms
A cerebral aneurysm is an abnormal dilation of a local area of artery wall in the central nervous system (CNS). The term is commonly used interchangeably with intracranial aneurysm. Most occur at junction points of the major arteries of the brain. This is usually around the circle of willis. They can either compress adjacent structures or rupture and cause hemorrhagic stroke.
Prevalence of intracranial aneurysms
Studies done at autopsy show that 1 – 5% of the population suffer from unruptured intracranial aneurysms.
The majority of aneurysms occur around the circle of willis, which makes up the major blood supply to the brainstem and supplies most of the cortex.
Around 40% of aneurysms arise at the junction between the anterior cerebral artery and the anterior communication artery. 20% occur at the junction of the posterior communicating artery and the middle cerebral artery; 34% occur at branches off the middle cerebral artery and 4% occur at the junction between the two posterior cerebral arteries and the basilar artery.
Incidence of hemorrhagic stroke from intracranial aneurysms
Incidence for ruptured aneurysms is around 10 in 100,000 people per year. Adults and women are at an increased risk of developing cerebral aneurysms, as are those with a family history of them.
Etiology of Intracranial Aneurysms
Cerebral aneurysms are an acquired defect typically resulting from hemodynamic stressors. However, they can arise as a result of infections, tumors and trauma.
A genetic component is likely as a family history is a risk factor for the development of an intracranial aneurysm. Patients with inherited connective tissue disorders like polycystic kidney disease, neurofibromatosis type 1, Ehlers Danlos syndrome type IV and Marfan’s syndrome have also been shown to have an increased risk of intracranial aneurysm development, and this should be taken into account during history in a patient with a suspected hemorrhagic stroke.
Classification of Intracranial Aneurysms
Intracranial aneurysms are classified by size and shape. Their shape is split into three groups:
- Saccular aneurysms
- Typically referred to as Berry aneurysms, these present as an outpouching of an artery and are named as such due to their resemblance with berries hanging off a branch.
- Fusiform aneurysms
- A fusiform aneurysm causes widening of the entire blood vessel – rather than just one localized segment. These rupture less often.
- Charcot Bouchard aneurysms (microaneurysms)
- These are associated with hypertension and form in small arteries, typically those in the lenticulostriate vessels of the basal ganglia. These commonly cause hemorrhage.
Pathophysiology of Intracranial Aneurysms
Pathophysiologically, aneurysms form when there are weaknesses in the walls of the arteries supplying the CNS. However, it is less easy to ascertain how these form and the etiology of different classifications and sizes of aneurysms may be different.
Many hypotheses have been suggested for the link between smoking and saccular aneurysms. It has been suggested that inhibition of proteases (by smoking) causes the loss of connective tissues in the arterial walls. The pressure at these points along the arteries causes outpouching and development of aneurysms.
The obvious progression to a cerebral aneurysm is rupture.
Symptoms of Intracranial Aneurysms
Many patients will have no symptoms associated with an unruptured aneurysm. However, an outpouching of the vessel can press on surrounding structures and cause various symptoms. Patients can experience:
- Loss of vision
- Double vision
- Pain around the eye
- Numbness/weakness of one side of the face
- Difficulty speaking
- Loss of balance (vertigo)
Aneurysms can rupture, typically causing a subarachnoid hemorrhage. This is often described in patients as “the worst headache of their life” and its onset will be sudden. Patients will often also experience:
- Loss of consciousness
- A third nerve cranial palsy (this causes deviation of the eye out and downwards, as the third cranial nerve controls a number of muscles in the eye. Patients can also present with full ptosis. This occurs when a posterior communicating artery aneurysm compresses the ipsilateral third cranial nerve).
- Unilateral or bilateral sixth cranial nerve palsies (this presents as double due to loss of lateral rectus. It indicates a raised intracranial pressure often caused by a bleed).
- Focal neurological deficits.
Diagnosis of Intracranial Aneurysms
Diagnosis of an unruptured cerebral aneurysm
Diagnosis of an unruptured cerebral aneurysm may be difficult as patients may present with vague and unspecific symptoms. A full history should be taken whereby risk factors for cerebral aneurysms should be obtained. Key risk factors are:
- Previous diagnosis of a subarachnoid hemorrhage
- A family history of subarachnoid hemorrhage
- Heritable connective tissue disease Marfan’s, Ehlers Danlos syndrome, pseudoxanthoma elasticum and neurofibromatosis type 1.
Other weaker risk factors can include:
- Head trauma
- Infection (intracranially)
- Drug abuse
Common symptoms were described above. The patient will typically give a history of a headache. They may have had seizures or have some nuchal rigidity. A decreased level of consciousness can be found in some patients.
The examination is typically unremarkable. In patients with strong risk factors and unexplained symptomatology, it may be prudent to order a number of diagnostic investigations. Investigations that may aid diagnosis are:
- CT head scan can show blood in the subarachnoid space from leaking or a ruptured aneurysm. If the aneurysm has calcified, it may also be seen.
- MRI can show unruptured aneurysms.
- Lumbar puncture can show elevated red blood cell count with xanthochromia
- CT angiography to locate aneurysm location and size
- One can also use magnetic resonance angiography
Diagnosis of a ruptured cerebral aneurysm
Patients whose aneurysm has ruptured will have a different presentation from those who have unruptured aneurysms. If a patient is suffering from a sudden onset severe headache, a subarachnoid haemorrhage (SAH) should be ruled out. Typically, these patients will also suffer from vomiting and loss of consciousness – coma and death can occur quickly and so suspected SAH is considered a medical emergency.
Other symptoms will include neck stiffness (nuchal rigidity) and Kernig’s sign.
CT head scan is your first port of call with any patient with severe onset headache. SAH is usually seen on a CT and has a sensitivity of 95% within 24 hours. If a CT confirms a subarachnoid hemorrhage, then lumbar puncture is not necessary, although in some cases where CT is inconclusive this may be indicated.
On lumbar puncture, you typically find xanthochromic CSF (yellowing of the CSF). Visual inspection of the CSF is usually sufficient for diagnosis.
Differential Diagnosis for an unruptured cerebral aneurysm
- Arteriovenous malformation
- Usually has a subacute presentation
- Can also present with seizures
- Cerebral angiography or MRI
- Hypertensive intracerebral hemorrhage
- This typically occurs in patients above 55 years old. They will have a history of hypertension. Hemorrhage causes focal neurological signs.
- A CT head scan should be ordered to rule out a bleed
- Cerebral venous sinus thrombosis
- This typically presents in middle-aged women. They will have papilloedema on examination and describe a subacute history (hours to days).
- Traumatic subarachnoid hemorrhage
- Usually seen in older patients who had a fall. A CT head scan can rule this out.
- Haemorrhagic tumour
- The patient usually has a history of some malignancy.
Differential Diagnosis for subarachnoid hemorrhage
- Arterial dissection
- Patients will not describe as excruciating pain. Pain is often localized behind the eye or to the neck.
- Non-aneurysmal perimesencephalic SAH
- Very difficult to differentiate from a SAH.
- No aneurysms are found with angiography.
- Cerebral arteriovenous malformation
- Can be seen with cerebral angiography (or CT).
- A headache is usually less severe. The neurological symptoms are recurrent.
- Saccular aneurysms of spinal arteries.
- Cardiac myxoma
- Septic aneurysm
- Pituitary apoplexy
Therapy of Intracranial Aneurysms
Management of unruptured aneurysms
Patients can either be treated conservatively and be observed, or surgery can be performed. Treatment is either by “clipping” or endovascular obliteration. This decision is made on a case by case basis.
When making the decision, a number of things should be considered. These include the patient’s age (older patients with less life expectancy are more likely to be given observation), the size and location of the aneurysm (some aneurysms are very unlikely to rupture e.g. cavernous carotid artery aneurysm and observation may trump treatment due to the risk associated with clipping, etc).
The risks of treatment should also be considered: rupturing the aneurysm during surgery can lead to hemorrhage and ultimately death.
Management of ruptured aneurysms
Patients with a ruptured aneurysm are considered a medical emergency. Hunt and Hess grading should be done as this impacts treatment severity and swiftness. Hunt and Hess grades are used to assess how severe a subarachnoid hemorrhage is and are as follows:
- Asymptomatic, the patient has a mild headache with some nuchal rigidity.
- Moderate to a severe headache with nuchal rigidity.
- Drowsiness, mild focal neurological deficit, patients are often confused.
- Patient is in a stupid coma, decerebrate posture.
Give calcium channel blockers (to reduce vasospasm) and maintain a systolic blood pressure press 135 mmHg. Clipping or endovascular therapy should then be considered for any unruptured aneurysms. Endovascular therapy is preferable.
Complications of Intracranial Aneurysms
Patients with an unruptured cerebral aneurysm are at significant risk of rupture and death. Patients can also develop hyponatremia, experience seizures and hydrocephalus. In the longer term, aneurysms may recur.
Prevention of Intracranial Aneurysms
Prevention is done via screening for unruptured aneurysms. Magnetic resonance angiography (MRA) is indicated in patients with two+ family members who have been diagnosed with cerebral aneurysms. Screening is also performed for patients with heritable connective tissue disorders like Ehlers Danlos syndrome. Patients who have had a previous history of aneurysms should also be screening periodically (every few years).