Aneurysm remnant (AR) after surgical clipping of ruptured aneurysms could be at persistent risk of rebleeding [3-6]. Secondary coiling after incomplete clipping represents a strategy to occlude the AR. According to previous reports, in cases of a complete clipped intracranial aneurysm, the annual risk of recurrence is very low (0.02-0.52%) [2,5-7]. In contrast, in cases of an incomplete clipped intracranial aneurysm with AR, it is substantially higher (0.38 to 7.3%) [4,7,8]. Thus, neurosurgeons have come to a consensus that long-term follow up in patients with AR after incomplete clipping is needed. Generally, blister type aneurysms or dissecting aneurysms have a thin aneurysmal wall, broad-based on the parent artery, and are thus prone to easy premature rupture during the operation and can result in a rapid regrowth of the AR after treatment [1,3,9]. However, risk factors for the regrowth of AR after incomplete obliteration have not been established and also no protocol has been established that defines the timing or optimal modality of follow-up imaging [7].

To the best of my knowledge, cases of the rapid regrowth (within a month) of AR on the on middle cerebral artery (MCA) trunk after unsuccessful surgical clipping in patients with a ruptured cerebral aneurysm have rarely been reported. Here, we report two rare cases of rescue endovascular treatment for AR on the MCA trunk after incomplete surgical clipping.

In past studies, the average duration to regrowth of an AR after clipping was about ten years [1,2,4,12]. Also, according to a recent study, the rate of regrowth was likely slow, less than 0.5 mm per year, which justified following up with DSA 3 to 5 years after microsurgical clipping [13]. Although there is general late recurrence and a slow growth rate of AR after clipping, our two cases described ARs on the MCA trunk that rapidly re-grew within a month as a rapid changing radiologic presentation of dissecting aneurysm. Although no clear mechanism has been known for the rapid regrowth of AR, it was generally thought to be due to arterial wall weakening and/or the influence of hemodynamics stress [11-14]. Similarly, our hypothesis is that a change of focal hemodynamic stress by microsurgical clipping may be one of the main factors that leads to decreasing resistibility and structural fragility of the AR wall. The change of hemodynamic stress not only influences the rapid regrowth of an AR arising from the MCA trunk but also the anatomical location of the proximal MCA.

Generally, compared with the MCA, higher rates of unsuccessful clipping are associated with aneurysms located at the anterior communicating artery complex or posterior circulation [6-9,12]. However, MCA trunk aneurysms are located either on the main trunk, at the origin of an early frontal or temporal branch, or at the origin of the lateral LSA. Although the incidence of aneurysms of the MCA trunk (from its origin up to the bifurcation) is very low (2% to 7% of total aneurysms) [1,7,12-14] this location of an aneurysm usually makes the operation more difficult, since in a high percentage of cases they are broad-necked, and in general, the origin of these branches is firmly incorporated within the neck, and sometimes its visualization is hidden by the aneurysm itself. These features of proximal MCA aneurysms could make microsurgical clipping difficult or unsuccessful.

When the regular shape and small size of the aneurysm remnants after initial clipping was identified via post-clipping imaging (CTA or 3D-DSA), we could not know exactly the need for retreatment (prompt or delayed). Thus we judged that it had a low risk of rebleeding or regrowth. So we determined a short term follow-up and observation. The rapid regrowth of AR is assumed to be due to the decreased resistibility and structural fragility of the AR wall, and it can result in serious and fatal consequences like the natural history of a ruptured blood blister aneurysm [4,5,9]. Against expectation, prompt treatment should be considered when rapid regrowth of AR with putative high re-rupture risk are identified. Before the development of endovascular treatment, re-operation for an AR was the only option for treatment. However, additional clipping or replacement of a clip are usually more difficult due to changes in the anatomical structure or adhesions in the operated area (in particularly for proximal MCA) and this increases morbidity and mortality [1-4]. Lately, endovascular coil embolization has become an alternative treatment modality for AR. Many previous studies have reported that endovascular treatment of AR after surgical clipping has favorable outcomes, suggesting that endovascular coil embolization may be a safe and reliable treatment option [2,13]. In our two cases, fortunately, the neck of the AR was narrow because of neck remodeling as a result of the clipping, and prompt coil embolization was performed as a rescue treatment when the rapid regrowth of the remnant aneurysm was identified.

In summary, many reports have indicated that size, morphology, clip reconstruction technique, flow dynamics, female sex and younger age (less than 45 years) predisposes a patient to regrowth of an AR. Nevertheless, it remains controversial and partially unknown whether aneurysm-specific factors and patient-specific factors can influence the regrowth of AR. When intraoperative assessment is shown to be insufficient to predict clipped AR, we usually perform a routine post-clipping imaging (CTA or 3D-DSA) after aneurysm clipping. The main purpose of post-clipping image was to eliminate the risk of a rebleed from an AR by allowing follow-up and/or retreatment. Like our two patients with subarachonoid hemorrhage, the early detection of an AR was possible because of postoperative image work-up. Unfortunately, magnetic resonance angiography or 3-dimensional CT of the cerebral arteries is less likely to be feasible in patients with a clip artifact. Whether the risk of DSA in follow-up studies is acceptable may be controversial, but a recent report in which meta-analysis was used estimated a low risk of DSA, which seems to justify DSA as an acceptable examination in the detection of re-growing AR [1,6,12]. We thought that higher detection rates of small ARs are now more likely because of postoperative 3D-DSA and prompt endovascular treatment is possible following DSA. Also, special attention to younger patients, anatomical location of aneurysms, unsuccessful clipping and unintended remnants of any size is essential. In such a constellation of risk factors, early postoperative imaging follow-up within a month after clipping may be warranted to rule out a small but rapid regrowth of the AR.

In two rare cases, the location (MCA trunk) of the aneurysm remnant after microsurgical clipping might have been associated with changes of hemodynamic stress and this might have led to their rapid regrowth because of the structural fragility of the aneurysmal wall. To determine when to perform a follow-up angiogram to detect rapid regrowth of a small aneurysm remnant like in our cases is vital.