Introduction
Aneurysms of the petrous segment of the internal carotid artery (ICA) are rare and
may result from congenital, infectious, or traumatic etiologies,[[1]],[[2]] with the vertical portion most commonly affected.[[3]] Unruptured petrous intracranial aneurysms are frequently asymptomatic, but may
present with cranial neuropathies,[[4]],[[5]],[[6]],[[7]],[[8]],[[9]] Horner's syndrome,[[10]] pulsatile tinnitus,[[11]],[[12]] otalgia,[[11]] traumatic intracranial aneurysms,[[13]] and/or audible bruit.[[14]] Ruptured petrous ICA aneurysms may cause massive epistaxis [[13]],[[15]],[[16]],[[17]],[[18]],[[19]] and hemorrhagic shock or hemorrhagic otorrhagia.[[15]] They pose a unique management dilemma, especially when giant or infectious [[6]],[[16]],[[19]],[[20]],[[21]],[[22]],[[23]],[[24]],[[25]] and are difficult to treat surgically, due to the inherent challenges of microsurgical
access to the carotid canal of the petrous bone.[[26]],[[27]] Endovascular approaches may also prove challenging, typically as the consequence
of therapeutically-unamenable morphology, but occasionally due to size considerations
as well.
Historical background
Historically, petrous ICA aneurysms were managed by parent vessel ligation or surgical
trapping,[[6]],[[11]],[[28]] following balloon test occlusion.[[10]],[[14]],[[29]],[[30]] Endovascular treatment of unruptured petrous ICA aneurysms was first performed
in the 1980s, using detachable balloons [[29]],[[31]],[[32]] and balloon occlusion in conjunction with bypass (superficial temporal artery-middle
cerebral artery and cervical-petrous saphenous vein graft),[[33]],[[34]] a versatile and useful adjunct in the treatment of these and other complex lesions.[[35]],[[36]] Hemorrhagic petrous ICA aneurysms have been successfully treated with balloon parent
vessel occlusion,[[37]] which may also be accomplished using coils. Selective aneurysmal sac obliteration
was also achieved using balloons, coils, or stent-assisted coiling in the 1990s.[[32]],[[38]],[[39]]
Concurrent surgical and endovascular therapy has been used for petrous ICA aneurysms.[[40]] Prior to the advent of flow-diverting stents, covered stents were successfully
used in the treatment of petrous ICA aneurysms, including ruptured lesions, in the
2000s.[[1]],[[41]],[[42]],[[43]] While very effective, these stents are stiff and difficult to navigate through
tortuous anatomy. The advent of flow diversion has offered an exciting new endovascular
approach for these historically challenging aneurysms. The off-label use of pipeline
embolization device (PED) for fusiform petrous ICA aneurysms has proven effective
in several reports.[[44]],[[45]],[[46]],[[47]],[[48]] We propose that petrous ICA aneurysmal morphology and unique anatomy render flow
diversion an excellent therapeutic option for these lesions.
Anatomy of the petrous internal carotid artery: Clinical implications and endovascular
considerations
The cervical segment of the ICA transitions into the petrous segment upon entering
the carotid canal. It averages 5 mm in diameter, is comprised of an initial vertical
portion approximately 10–11 mm in length,[[49]] is related posteriorly and anteriorly to the jugular foramen and Eustachian tube,
respectively, and terminates as the genu, lying anteroinferomedial to the cochlea
and anteromedial (in most cases) to the geniculate ganglion, and turning as the horizontal
segment to run anteromedially for ~20 mm before transitioning into the lacerum segment.
This sharp bend renders use of covered stents challenging (e.g., stenting) and the
frequent fusiform morphology of aneurysms in the petrous ICA makes selective aneurysmal
obliteration/primary coiling not possible. Flow diverters are thus ideal for navigating
the petrous ICA at the genu and bridging the fusiform lesions found here.
Aneurysmal dilation of the petrous segment of the ICA may cause facial paralysis due
to the proximity of the genu to the geniculate ganglion and facial nerve.[[3]],[[50]] The geniculate ganglion is within approximately 6–7 mm of the genu of the petrous
ICA and is found posterolaterally, posteriorly, and laterally to the same in approximately
3/5, 1/3, and 1/6 of individuals, respectively.[[49]] The greater superficial petrosal nerve runs along the anterosuperior margin of
the carotid canal and damage to it may result in alacrima. The horizontal portion
of the petrous segment of the ICA is crossed medially by the abducens nerve and is
located posterior to and below the Gasserian ganglion in Meckel's cave, accounting
for not infrequently observed abducens palsy.[[7]],[[44]] Also in close relation to the petrous ICA is the vestibulocochlear nerve, running
within the internal acoustic meatus, damage to which may underlie tinnitus or hearing
loss in these patients.[[10]],[[11]]
Branch arteries from the petrous ICA are found in approximately two fifths of anatomical
specimens, typically arising from its horizontal aspect. These have historically included
the vidian and caroticotympanic arteries, though the latter were not found in an anatomical
dissection study by Paullus et al.,[[49]] who found the vidian artery most commonly, followed in frequency by periosteal
arteries.[[51]] The otic artery represents an exceedingly rare persistent anastomosis between the
carotid and vertebrobasilar circulations, characteristically anastomosing with the
lower basilar trunk.[[52]] The clinical significance of these branches is that they may fill the ICA in retrograde
fashion in cases of spontaneous or interventional parent vessel occlusion of the cervical
ICA. The clinical inconsequence of compromise of these branches, when they are present,
renders this effect with use of PED unimportant.
The petrous ICA is surrounded by venous and neural plexi. The former is present in
76% of individuals and represents a lateral extension of the cavernous sinus. It exhibits
a variable extent of lateral extension, on average 7.6 mm into the carotid canal.
The peripetrous ICA venous plexus may fistulize with the artery and contribute to
aneurysmal persistence/progression and/or recurrence in untreated/treated cases. The
peripetrous ICA neural plexus is comprised of two trunks from the carotid nerve, variably
sending nerve fibers to accompany arteries and cranial nerves IV-VI, as well as the
deep petrosal nerve, which joins the greater superficial petrosal nerve to form the
vidian nerve. Petrous ICA aneurysms may thus cause a Horner's syndrome by disruption
of these nerve fibers.[[10]]
Treatment of petrous internal carotid artery aneurysms using flow diversion
Flow-diverting stents are microcatheter-delivered self-expanding flexible metallic
stents designed to induce aneurysmal thrombosis and reconstitution of the parent vessel
without occlusion of parent arterial branches via a modest reduction of intra-aneurysmal
flow. PED was approved in April of 2011 for large or giant wide-necked Intracranial
aneurysms arising from petrous segment of the ICA distally as far as the superior
hypophyseal segment of this vessel.
The experience with PED in fusiform aneurysms has been complete or partial occlusion
in most cases,[[52]] with 100% complete occlusion in all lesions involving the vertebrobasilar circulation.[[53]] The occlusion rate with flow diverters is excellent (80%–85% for PED and ~82% for
SILK device;[[54]],[[55]],[[56]],[[57]]). The mortality rate is ~5% (lower for PED and higher for SILK [[55]],[[58]]). Rupture or delayed hemorrhage from PED in a petrous ICA aneurysm might be catastrophic,
especially given the requirement for lifelong antiplatelet therapy, and require treatment
with parent vessel sacrifice. Compromise of side branches, a principal concern with
the use of flow diverters,[[54]],[[59]] is of no clinical consequence in the petrous segment of the ICA. Furthermore, the
use of flow diverters precludes the future ability to coil saccular aneurysms,[[60]] which represent a small fraction of petrous segment ICA aneurysms. Thus, several
of the disadvantages with use of PED are not applicable in the petrous segment of
the ICA. Moreover, use of PED is associated with decreased intervention time and radiation
and contrast exposure in comparison to endovascular coiling.[[45]]
PED has been used in the treatment of petrous ICA aneurysms in 11 patients.[[44]],[[45]],[[46]],[[47]],[[48]],[[61]] Five of these cases were in the context of larger studies and did not provide specific
case data.[[45]],[[61]] Among the other five cases, three patients were treated with a combination of PED
and coiling and two were treated with PED alone, one of whom bore a ruptured pseudoaneurysm
and experienced recurrent bleeding 12 days following intervention requiring parent
vessel occlusion [[Table 1]]. Resolution or improvement in symptoms and aneurysmal obliteration was achieved
in the remaining cases.
Table 1: Petrous internal carotid artery aneurysms treated with Pipeline. Six cases reported
in the literature did not provide demographic or specific diagnostic or outcome data
(Colby et al., 2013; Salhein et al., 2015). *reported by Moon and colleagues in 2014
and in a later series in 2017. HIV, human immunodeficiency virus; ICA, internal carotid
artery; PED, Pipeline endovascular device
Gross et al.[[44]] recently reported on a case series of patients with petrous ICA aneurysms. Of these
lesions, eight were fusiform with only two having an identifiable neck. Three of these
were successfully treated with flow diversion in conjunction with coil embolization.
For two patients having follow-up, both had resolution of cranial nerve palsies and
complete obliteration of the aneurysm. Other patients successfully treated endovascularly
underwent parent vessel occlusion with low-flow bypass, balloon-assisted coiling (complete
occlusion without recurrence >2 years follow-up), and stent-assisted coiling (near-complete
occlusion).[[44]] Lerat et al.[[48]] report on a 64-year-old female with sudden onset left facial paralysis and bilateral
petrous ICA aneurysms treated successfully with PED. Facial paralysis improved and
aneurysmal obliteration occurred completely on the right side with a neck remnant
identified on the left. Moon et al.[[47]] describe the case of a 64-year-old male with visual field cut, facial numbness,
and diplopia secondary to lateral rectus palsy found to have petrous ICA aneurysm
and treated with coiling and PED.
Kadkhodayan et al.[[46]] report on a 50-year-old female with malignant external otitis and bilateral temporal
osteomyelitis and brisk bleeding from the left ear found to have an infectious petrous
ICA pseudoaneurysm. PED was attempted after bleeding transiently ceased, but proved
inadequate with rebleeding occurring after 12 days rendering parent vessel occlusion
necessary. Thus, while PED appears effective and promising for treating petrous ICA
aneurysms, its use may be limited in pseudoaneurysms and ruptured lesions in which
persistent and rebleeding remains a significant risk, requiring more aggressive treatment.[[46]] While previous investigators have successfully used flow-diverting stents in the
treatment of mycotic aneurysms of the ICA,[[62]] those lesions were unruptured.
Conclusion
Flow diversion is an ideal treatment for petrous ICA aneurysms, specifically unruptured
lesions of complex morphology. Other options for treating petrous ICA aneurysms are
challenging, not possible, less effective, and/or carry substantial risks. Furthermore,
several of the disadvantages of PED, occlusion of side vessel branches and preclusion
of future coil embolization, do not apply to the petrous segment of the ICA, lacking
major branches, with aneurysms most commonly fusiform and not amenable to selective
aneurysmal sac embolization to begin with. In addition, flow diversion is the best
option in patients with bilateral petrous ICA aneurysms and morphology unfavorable
to selective aneurysmal sac obliteration. Finally, use of PED in petrous ICA aneurysms
has proven effective in many reports, except for one group's experience with a ruptured
pseudoaneurysm which had initially transiently ceased, but then resumed, to bleed.
While flow diversion appears promising for petrous ICA aneurysms, further experience
with PED alone (i.e., without concurrent coiling) is required to make a stronger assertion.
