Pathophysiology
Formation
- Unclear pathophysiology: Contemporary insight into the pathophysiology of intracranial aneurysms in general contradicts the classical concept of aneurysms being truly congenital lesions. Laboratory research (with specimens from adults) revealed an average age of collagen within aneurysms of less than 5 years, indicating a high turnover (72).
- Abnormal repair processes: An alternative hypothesis is the concept of an underlying vessel wall dysfunction with transient or permanent failure to repair insults. Abluminal and luminal factors are thought to play a role (73). In children, given the rarity of the risk factors associated with intracranial aneurysms in adults (including hypertension, advanced age, smoking, and drug abuse), it is likely that these faulty “defense” mechanisms explain the formation of aneurysms not associated with “offensive” factors such as head trauma, infection, or inflammation (74).
Etiology and classification
Although certain overlaps exist with regard to their etiology and configuration, pediatric aneurysms are most commonly classified into the following categories (49,74):
- Saccular (“berry”) aneurysms:Â Between 32% and 70% of pediatric intracranial aneurysms are of this type (see examples in Relationships to Other Disease States and Syndromes) (75). Given that hemodynamic factors may play only a limited role in the formation of pediatric aneurysms, their etiology is controversial (49).
- Nontraumatic dissecting aneurysms: These are four times more frequent in children than adults and often localize to the posterior circulation (see example in Evaluation of Intracranial Aneurysms in Children) (49). Their appearance is typically fusiform, resulting from intimal damage and the subsequent entry of blood into the subintimal space (74). These aneurysms may or may not present with subarachnoid hemorrhage, and focal arterial stenosis may contribute to an ischemic presentation (75).
- Giant aneurysms: A subset of dissecting aneurysms, giant aneurysms are also four times more frequent in children and form due to recurrent intramural hemorrhage (49). An “onionskin” appearance may be observed; these aneurysms most often present with mass effect rather than subarachnoid hemorrhage (74).
- Traumatic aneurysms: Representing 5% to 39% of pediatric aneurysms, traumatic aneurysms most often result from blunt head injury but can also be due to penetrating injury (49). These usually present 2 to 4 weeks after injury and result from the formation of a false lumen or pseudoaneurysm (74).
- Infectious (mycotic) aneurysms: These are caused by microbial infiltration of the vessel wall, which is subsequently weakened and prone to formation of a pseudoaneurysm (75). The most common causative organisms are Staphylococcus aureus, viridans streptococci, and various gram-negative bacteria (49,74). Fungal organisms are rarely encountered, although immunosuppression from treatment of hematologic malignancies may induce a particular susceptibility to the development of infectious aneurysms (49,74). The source of the infection may be septic emboli from bacterial endocarditis or contiguous spread from sinusitis, osteomyelitis, or cavernous sinus thrombophlebitis (74,75).
- Iatrogenic aneurysms: Another rare form of pediatric aneurysm, iatrogenic aneurysms occur after surgical intervention, such as brain tumor resection or radiotherapy (76,77).
Location
In a literature review of 480 pediatric aneurysms, Beez et al. found the vascular distribution illustrated below (78). Whereas in adults the most common location for an aneurysm is the anterior communicating artery, the most common aneurysm site in children is the ICA, as determined by a review of 671 pediatric aneurysms of nontraumatic origin (42). Roughly half of these ICA aneurysms were located at the ICA bifurcation.
Molecular/Genetic Pathology
- Predisposition: Genetic conditions predisposing for intracranial aneurysms in children are mainly sickle-cell anemia, phakomatoses (e.g., tuberous sclerosis complex and NF1), and heritable connective tissue disorders (e.g., Ehlers-Danlos and Marfan syndromes) (43,49,55,62,63).
- Manifestation: Important implications of underlying genetic pathology are higher rates of multiple aneurysms (60% of cases) and presentation at younger age (younger than 40 years), most evident for sickle-cell anemia (79,80). Heritable connective tissue disorders may affect the integrity of the arterial wall, thereby predisposing patients with these disorders to aneurysm formation and/or rupture (22).
Histopathology
- Abnormality of internal elastic membrane: Absence or irregular disruption of the internal elastic membrane is a common histopathological observation (78).
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