Cerebral Aneurysm

Introduction​

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The word aneurysm derives from the Greek aneurysma (ana - through, eurys - dilate) and identifies an abnormal dilation of an artery. Cerebral aneurysms can involve both the anterior circulation, in the anterior cerebral, anterior communicating, middle cerebral, and the posterior or vertebrobasilar arteries in the posterior cerebral, posterior communicating, basilar and vertebral arteries.

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Intracranial aneurysms are divided into saccular (berry) or non-saccular on the basis of their shape and etiology, non-sacciform aneurysms include atherosclerotics, spindle-like, traumatic and mycotic; sacciform aneurysms have both etiological and anatomical characteristics that differentiate them from others, for example they typically originate at the level of a vessel bifurcation, moreover they are also those that have the greatest incidence and the greatest clinical interest, for this reason the following treatment is addressed exclusively to them.

Parameters to consider to classify these aneurysms are the following: symptomatic / non-symptomatic, single / multiple, previous history of ESA or not, dimensions of the maximum diameter and of the neck (in mm), location of the artery from which the neck is born in space subarachnoid or outside of it, uni / multilobular form, presence of lobulations, associated arterial anomalies such as asymmetries of the Willis circle, fenestrations, agenesis, or vessel occlusions.

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The natural history of the cerebral aneurysm is given by its rupture, and therefore by the development of a subarachnoid hemorrhage, depending on many variables, such as the size of the aneurysm itself, and any precipitating factors such as high blood pressure. The prognosis of a subarachnoid hemorrhage, despite the significant improvements in medical and neurosurgical treatment, has remained practically unchanged precisely for this reason we can understand what is the importance of identifying and possibly treating the unbroken intracranial aneurysm. The following discussion will therefore deal mainly with the unbroken intracranial aneurysm, that is not yet evolved in its natural history towards subarachnoid hemorrhage.

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Epidemiology

 

Unfortunately many of the studies carried out lack information such as the age of the patients, the measurements and location of the aneurysms, and any associated pathologies. Homogeneous characteristics that can be found in these studies are the dimensions, which tend to be greater for broken aneurysms than for non-broken ones (3.9 versus 9.9 mm).

 

As regards the localization of berry aneurysms, the anterior circulation is certainly the most frequently involved (85%), specifically they are found in the anterior cerebral artery 30% (of which 5% in the distal part of this branch and 25% in the anterior communicant), in the internal carotid artery 30% (of which 4% ophthalmic, 18% posterior communicating, 4% bifurcation with the middle brain, 4% anterior chorioid), and along the course of the middle brain 25%. The remaining 15% located in the posterior circulation can in turn be differentiated into: posterior cerebral artery 2% (including the distal part of the posterior communicator), basilar artery 10% (trunk 3%, bifurcation 7%) and vertebral artery 3% (including the posterior inferior cerebellar artery).

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Multiplicity: intracranial aneurysms are multiple in 10-30% of cases, although the incidence is extremely variable depending on the patient population considered, depending on whether the findings are surgical, radiological or autoptic, and finally depending on whether broken aneurysms are evaluated or not broken. However, about 75% of patients with multiple aneurysms have 2 aneurysms, 15% have 3, and 10% have more than 3. A strong female preference for multiple aneurysms is observable, in fact, while overall the male-female ratio is 1: 5, the ratio rises to 11: 1 in patients with more than 3 aneurysms.

 

 

Mortality and complications: broken intracranial aneurysms are associated with very high mortality rates: approximately 10-20% of patients die before reaching the hospital, about 8% die from progressive deterioration related to initial bleeding. In patients with untreated subarachnoid hemorrhage, the risk of rebleeding is 4.1% on the first day, and 1.5% per day for the following two weeks. At six months the risk is 50% with 70% mortality. Other very serious complications following rupture are hydrocephalus, which can complicate an ESA in 15% of cases, and late cerebral ischemia due to vasospasm (from reflex contraction), which appears between the fifth and twelfth day which it can be more or less serious in relation to the extension limited to the aneurysm site only or interesting a larger area of ​​the vascular tree.

For the "intact" aneurysm, we must instead analyze the risk of rupture related to it.

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Pathogenesis and risk factors

 

Many studies have tried to explain the origin of intracranial aneurysms. Initially it was believed that the origin of these lesions was exclusively genetic, congenital, and therefore an area of ​​weakness of the muscle tunic originated when the embryonic circulatory system transforms into that of the adult through the involution of some blood vessels. However, various histological and experimental studies have failed in an attempt to provide scientific support for this theory. Many studies followed one another with the aim of studying factors such as: the weakness of the aneurysmal vascular tunic, the existence of a locus minoris resistentiae and the consequent histological alterations, vessel fixations, alterations of particular importance at the level of the internal elastic lamina or again the decrease in smooth muscle cells by apoptosis. Another key role in the remodeling of the vessel walls is played by the constituents of the extracellular matrix, which studied under experimental conditions seem to have a greater local enzymatic activity in the aneurysmal vessel.

 

Although the various steps in the pathogenesis of intracranial aneurysm are not yet well delineated, this set of experimental data can schematically be summarized in the combination of a congenital artery wall degeneration area associated with the acquired hemodynamic stress action leads to the formation of the aneurysm.

Many risk factors have been correlated with the development of intracranial aneurysms and with subarachnoid hemorrhage related to their bleeding, these include: arterial hypertension, cigarette smoking, female sex, and genetic predisposition. Patients with connective tissue diseases such as Marfan syndrome, Ehlers-Danlos syndrome, polycystic disease of the adult kidney, coarctation of the aorta or the presence of other intracranial arteriovenous malformations have an increased incidence of intracranial aneurysms. Other pathologies less frequently, but however potentially associated with the presence of intracranial aneurysms are Osler-Weber-Rendu syndrome, Moyamoya syndrome, alpha1-antitrypsin deficiency, systemic lupus erythematosus, bacterial endocarditis, fungal infections, neurofibromatosis type 1 and tuberous sclerosis. Last but not least, a positive family history significantly increases the risk.

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Clinical presentation

 

Ruptured aneurysm: most aneurysms remain asymptomatic and constitute an occasional finding of neuroradiological investigations performed for other reasons. In fact, they remain so until the rupture, a sudden event followed by the formation of a subarachnoid hemorrhage. In this case, the patient typically reports experiencing the worst headache of his or her life, frequently after an effort, this may be associated with signs of meningeal irritation such as rigor nucalis for the antalgic contracture of the para-vertebral muscles, intracranial hypertension syndrome (papillary edema, nausea and vomiting), focal neurological signs for compression of the cranial nerves in the cisterns where the aneurysm is located, the most involved are the cranial nerves II, III and IV, or for compression of the cranial nerves in the cavernous sinus, finally sudden loss of conscience. The most used method to classify the clinical severity of subarachnoid hemorrhage is the Hunt and Hess scale, which has also been seen to correlate well with the patient's prognosis upon entering the emergency room.

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Grade 0: aneurysm not broken

Grade 1: asymptomatic patient or with minimal headache and minimal neck stiffness

Grade 2: moderate to severe headache, neck stiffness, no neurological deficit except cranial nerve palsy

Grade 3: sleepy patient, confused or with moderate focal neurological deficit

Grade 4: stuporous patient, moderate / severe hemiparesis, initial decerebral rigidity, autonomic disorders

Grade 5: deep coma, decerebrate rigidity.

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Unruptured aneurysm: a symptomatology due to an aneurysm that is not associated with subarachnoid hemorrhage is relatively uncommon. Some aneurysms can cause cranial neuropathies, an example of which could be paralysis of the third cranial nerve due to an aneurysm of the posterior communicating artery. Other less common symptoms are vision loss due to an ophthalmic artery aneurysm that compresses the optic nerve, epileptic seizures, headache, continuous migraine.

and transient ischemic attacks or cerebral infarcts secondary to the detachment of an embolus from a partially thrombosed giant aneurysm. The so-called giant aneurysm (maximum diameter over 2.5 cm) very often causes a mass effect symptomatology.

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Specific symptoms have been associated with particular locations of the aneurysm:

• Anterior communicating artery: most common location of subarachnoid hemorrhage due to aneurysm (34%). The aneurysm is generally silent until rupture. Suprachiasmatic pressure can cause visual field defects, abulia, akinetic mutism, anmesic syndromes or hypothalamic dysfunctions. Neurological deficits resulting from rupture result from intraventricular hemorrhage (79%), parenchymal hemorrhage (63%), acute hydrocephalus (25%) or ischemic stroke of the frontal lobes (20%).

• Anterior cerebral artery: excluding the anterior communicant they represent 5% of all cerebral aneurysms. Most are asymptomatic until rupture, although frontal syndromes, anosmia and motor deficits can be noted.

• Middle cerebral artery: they represent about 20% of intracranial aneurysms, generally in the first or second division of the Silvian fissure. Aphasia, hemiparesis, sensory emideficit, anosognosia or visual field defects may be present.

• Posterior communicating artery: aneurysms of the junction between the terminal portion of the internal carotid artery and the posterior communicating artery represent about 23% of brain aneurysms; they can have a lateral, posterior or inferior orientation which is followed by pupillary dilation, ophthalmoplegia, ptosis, mydriasis and hemiparesis.

• Internal carotid artery: these aneurysms represent about 4%. Supraclinoid aneurysms can cause ophthalmoplegia due to compression of the III cranial nerve or various visual defects and optic atrophy due to compression of the optic nerve; compression of the optic chiasm can cause temporal hemianopsia. Hypopituitarism or anosmia can be seen in patients with giant aneurysms. Cavernous-carotid aneurysms can exert a mass effect within the cavernous sinus, resulting in ophthalmoplegia and facial sensory deficit. The rupture of these aneurysms classically produces a carotid-cavernous fistula, subarachnoid hemorrhage or epistaxis.

• Basilar artery: these aneurysms are the most common in the posterior circulation, representing 5% of the total. Clinical signs are present following subarachnoid hemorrhage, although bilateral hemianopsia or paralysis of the oculomotor may occur.

• Vertebral artery and postero-inferior cerebellar artery: ataxia, bulbar dysfunction or spinal involvement are the most frequent signs.

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Diagnosis

 

Advances in neuroimaging techniques over the past twenty years have substantially changed the diagnosis of an intracranial aneurysm. Techniques aimed at visualizing aneurysms add to techniques aimed at studying the parenchyma by providing a quantity of information that helps in planning the intervention.

The three main modalities used today to study the size, localization and morphology of intracranial aneurysms are: thin-layer CT after injection of contrast medium combined with the use of special software (angio-CT), magnetic resonance angiography (angio -RM) and digital subtraction angiography.

 

As the preferred initial method for the evaluation of unruptured intracranial aneurysms in patients with a positive family history for subarachnoid hemorrhages or brain aneurysms or in patients with risk factors for aneurysms, both CT angio and RM angiography can be used almost indifferently. Digital subtraction angiography remains the preferred modality both in patients requiring a more detailed study and in patients with subarachnoid hemorrhage, although CT angiography has also been used in the latter context

 

AngioRM is an excellent method in the case of screening for intracranial aneurysms (example: familiarity). In fact, it is a non-invasive, short-term examination (about 20 minutes), which does not require the use of contrast medium, nor does it require the use of ionizing radiation. The angioRM is able to study intracranial aneurysms, even of small dimensions, with accuracy, allowing a maximum resolution of about 1-2 mm, which can however vary according to the shape, morphology, location and characteristics of the aneurysmal bag itself.

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CT angiography is a non-invasive examination, which requires only venous access to the arm (such as blood sampling), extremely rapid (about 3 minutes), which, by administering iodinated contrast medium, allows a more accurate assessment of vascularization brain, of any aneurysms or vascular malformations, after the use of special image reconstruction software.

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Digital Image Subtraction Angiography (DSA) continues to be the standard diagnostic criterion for showing and outlining the characteristics of a cerebral aneurysm, especially in case of certain diagnostic doubts, or as planning for a possible endovascular treatment. Some advances in instrumental technology, especially 3D rotational angiography, have increased the sensitivity of this method in the study of the anatomy of the aneurysm. The images are acquired at 360 ° and can be rotated in the three spatial dimensions, giving a much more accurate representation of the aneurysm, especially when compared to angiography on two dimensions.

The concept of how, although the diagnostic gold standard remains, it is not possible to take digital angiography as a diagnostic screening test for brain aneurysms, given its invasive characteristics, also remains to be underlined.

 

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Cerebral aneurysm and the risk of rupture

 

The unbroken cerebral aneurysm, especially as regards its clinical management, remains one of the most debated topics in neurosurgery and interventional neuroradiology.

In fact, a vast number of clinical studies have tried to identify "standard" parameters to follow for the treatment of unbroken cerebral aneurysm, trying to provide precise indications regarding the decision to carry out conservative treatment, and therefore follow the evolution in the time of such lesions, or to carry out an invasive treatment to definitively treat the lesion both from a surgical and neurointerventional endovascular point of view.

The decision must always be made by evaluating the relationship between what are the benefits of the treatment and the risks related to it, in other words the relationship between the probability of breakage and the risk of complications associated with the intervention.

 

In principle we can say that:

• Size is a determining factor

• Small aneurysms present a generally low risk of rupture

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Despite this, it is still not clear what the critical dimension is, beyond which the risk increases exponentially, so much so as to favor surgical treatment as soon as possible. To answer this and other questions, the International study of unruptured intracranial aneurysms (ISUIA) was born, with the support of the National Institute of Neurologic Disorder and Stroke (NINDS).

In this respect, the publication of data from the international study on unbroken aneurysms (ISUIA) was an epochal event, in fact this large-scale study, in which 53 different centers from the United States, Canada and Europe participated, unexpectedly demonstrated a very low annual rupture rate for aneurysms below 10 mm in maximum diameter, in patients with no previous history of subarachnoid hemorrhage from it or from associated lesions.

However, the ISUIA study has provoked numerous criticisms in the first place by neurosurgeons, but not only, especially in the face of a common clinical practice in which a large number of patients with subarachnoid hemorrhage who face actually aneurysms of reduced lesions are faced. Indeed other studies report significantly higher bleeding rates even if the number of patients analyzed is always very small compared to the ISUIA study.

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Therapeutic options

 

There are three options for the treatment of intracranial aneurysms: observation, craniotomy with clipping (neurosurgery), and endovascular occlusion with the use of detachable spirals (endovascular intervention). All broken aneurysms (grade Hunt and Hess 1 to 4) are treated early, generally within 72 hours. There is discussion on how to treat patients with the most severe degree (Hunt and Hess 5); the historically high incidence of unfavorable outcomes despite treatment has prompted the suggestion of conservative management unless clinical improvements occur. However, recent evidence supports the use of aggressive therapy for most of these patients, including placement of ventricular drainage and protection of the aneurysm by clipping or coiling.

Endovascular intervention is increasingly common as a less physiologically stressful alternative to classical neurosurgery of the aneurysm in this subset of patients, who already have severe brain injuries.

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Unruptured aneurysms that are discovered by chance are observed or treated in an elective way, depending on the patient, the size and state of the aneurysm, and the risk factors related to this pathology. The observation consists of periodic follow-up imaging and medical visits to review the studies.

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In standard endovascular surgery, a microcatheter is advanced into the aneurysm and controlled-release metal spirals of various sizes and shapes are positioned inside to determine thrombosis of the aneurysmal sac and exclude it from the cerebral circulation. Nowadays, thanks to the evolution of technologies and materials available in interventional neuroradiology, almost all endocranial aneurysms can be treated endovascularly, in a minimally invasive way, avoiding classic neurosurgical intervention and consequent brain manipulations . The use of micro-balloons, stents, flow diversion stents, and other materials allows to treat aneurysms with difficult anatomy, with a large collar or with relative vessels that originate near the bag itself.

In selected cases, however, neurosurgery can find more appropriate indications than endovascular surgery; obviously, the choice must be made on a case-by-case basis, and co-selection in agreement between the patient, the interventional neuroradiologist and the neurosurgeon.

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