There are two possible reasons that recurrent post-clipping aneurysms may occur. First, clipping may not completely correct a pre-existing weakness in the parent artery and aneurysm neck, and the aneurysm may therefore continue to grow. Second, clipping may weaken the vascular wall of the aneurysm neck and parent artery and thereby induce de novo aneurysms in these weaker regions (7,26).

These cases have been attributed to incomplete initial clipping or slipping of a clip after complete clipping has been achieved (20). Slipping occurs when an aneurysm neck is wide and calcified; therefore, the clip moves to the distal end of the aneurysm during clipping, causing the residual aneurysm to gradually grow under the impact of blood flow (6,27).

In addition, the risk factors for intracranial aneurysm include smoking, hypertension, dyslipidemia, diabetes, a family history of the condition, multiple aneurysms and sex (a higher incidence is observed in females), all of which are also factors that contribute to PCRRAs (21,28,29).

Ruptured intracranial PCRRAs are mainly characterized by headaches, nausea, vomiting, stiffness, possible limb paralysis, coma and, in severe cases, death (4). These are similar to the symptoms of the initial intracranial aneurysm rupture (34).

Unruptured intracranial PCRRAs are characterized by headaches, progressive vision loss, ocular nerve paralysis, hemiplegia, dysphonia and trigeminal neuralgia (35). These symptoms are related to a variety of factors, including the size, shape and location of the PCRRA (36). However, a number of intracranial PCRRAs exhibit no symptoms or signs (37,38).

At present, DSA is the gold standard for the diagnosis and follow-up of PCRRAs as it can effectively exclude the influence of metallic clip artifacts (41). The diagnosis rate of three-dimensional DSA is much higher than that of two-dimensional DSA (42). Performing intraoperative DSA after aneurysm clipping, particularly in a hybrid operating room, can reduce the incidence of a PCRRA (43,44).

CTA is a novel investigation method that can be used to accurately detect intracranial aneurysms (45). Sun et al (46) found that CTA had a sensitivity of 71% and a specificity of 94% when detecting intracranial PCRRAs. New technology associated with CTA includes image processing with metal artifact reduction software. This process significantly reduces the metal artifacts caused by clipping in PCRRA imaging, which can improve its diagnostic rate of PCRRAs (47). In addition, detection using dual-energy CTA is less affected by clip artifacts and may thus be more effective for the diagnosis of PCRRAs with ≤2 clips (48,49).

Other than intraoperative DSA, practical indocyanine green video angiography (ICG-VA) has become one of the most widely used examination methods. ICG-VA can be used to assess blood flow through the parent artery and to determine whether residual aneurysm remains (50). Özgiray et al (51) treated 109 cases of intracranial aneurysms with clipping and found that ICG-VA could effectively assess the patency of the circulation. However, aneurysm remnants can occur in 6.5% cases after successful clipping (51).

When PCRRA rupture results in bleeding, the PCRRA requires treatment (34).

The size is of particular importance when selecting PCRRA treatment options. Kivelev et al suggested that when a PCRRA aneurysm is ≥3 mm, surgical treatment should be considered. When a PCRRA is 1-2 mm in size, it should be closely monitored (20).

The treatment selected for a PCRRA is associated with its location. Jabbarli et al (52) examined 112 PCRRA cases and noted that the location (e.g., anterior cerebral artery > internal carotid artery > posterior circulation > middle cerebral artery) was an important risk factor for PCRRAs. Therefore, treatment should be selected when an aneurysm is in the anterior communicating artery (52).

When PCRRAs become giant aneurysms due to thrombosis or when the rupture of the PCRRA produces an intracranial hematoma resulting in space occupying effect, craniotomy should be seriously considered (53).

Re-clipping remains the main method used for the treatment of PCRRAs. This procedure is much more difficult to perform than the initial clipping, mainly as the scarred and adhered brain tissue renders the exposure of the operative field and the parent artery difficult, and the previously placed clip interferes with the ability to expose the aneurysm neck. Additionally, intraoperative rebleeding can occur while the existing clip is being moved (7,20,54). The re-clipping of an intracranial PCRRA should proceed according to the following sequence: Dissection toward the aneurysm, bypass assistance if necessary, mobilization of the existing clip and placement of the new clip(s) (7,20).

Among the events mentioned above, whether to move the existing clip during re-clipping is a key decision that must be made; in addition, previous studies have proposed that it is beneficial to move an existing aneurysm clip in order to allow sufficient space in which to operate (55,56). However, another study did not suggest the intraoperative removal of an existing aneurysm clip as this may cause a tear in the aneurysm (57). Therefore, whether an existing clip is moved should be determined based on the needs of the procedure.

In addition, if the PCRRA is large in size or contains a thrombosis, it can be cut after the PCRRA is trapped. The presence of a thick and atherosclerotic aneurysm wall may necessitate the suturing of the edges of the incised sac to facilitate clip placement at the neck (20).

Currently, EVT is the main effective treatment method for PCRRAs (58). It also has a higher success rate for blocking PCRRAs. Gross et al (59) described 43 cases of intracranial PCRRAs in which EVT was used, and they found that 79% of the PCRRAs were completely occluded, 14% had residual neck tissue and 7% had stable small dome residues. A number of EVT methods are available for the treatment of intracranial PCRRAs, including coiling (or stent- or balloon-assisted methods) and flow-diverting stents (FDSs) (or coil embolism-assisted) (60-63).

Single coiling is the most practical method to treat an intracranial PCRRA, particularly for PCRRAs with a narrow neck (Fig. 3). Gross et al (59) used single coiling in 18 cases of narrow-neck PCRRAs and observed no recurrence during an average follow-up period of 3.9 years. However, in wide-necked, large, complex PCRRAs, stent- or balloon-assisted EVT is required (61,64,65). The recanalization rate is high in complex PCRRAs (66).

FDSs are a new type of stent that has emerged in recent years that can effectively treat intracranial PCRRAs. An FDS is a flexible, low-porosity, endoluminal stent that is capable of altering the hemodynamics of the parent artery and aneurysm, resulting in the formation of a thrombosis in the aneurysm. FDSs can also guarantee blood flow through the normal para-aneurysm branch and are therefore especially suitable for large, wide-neck PCRRAs (67,68). For instance, in a previous study, seven cases of PCRRA were treated by Adeeb et al (8), and all were completely embolized without sequelae following the implantation of FDSs.

However, the treatment of a PCRRA using FDSs often requires a longer time to achieve complete occlusion. For example, Dornbos et al (69) performed FDS implantation in four cases of intracranial PCRRAs, and post-operative DSA revealed that 80% of the PCRRAs had embolized at six months post-operatively, while 100% had embolized at 12 months post-operatively.

Bypass surgery is considered as a ‘last resort’ for the treatment of complex PCRRAs that are difficult to treat (20). Bypass surgery can be divided into three categories according to its purpose, as follows: i) To provide permanent and adequate blood flow for the distal parent artery of the PCRRA; ii) to prevent cerebral ischemia caused by the temporary occlusion of the parent artery; and iii) to isolate the PCRRA and reconstruct the parent artery (53).

The selection of bypass surgery that is most appropriate depends on the individual case. Kivelev et al (20) described 25 cases of intracranial PCRRA in which bypass treatment was applied, including clipping of PCRRAs with bypass treatment, PCRRA trapping with bypass treatment and proximal occlusion of PCRRAs with bypass treatment. Over an average post-operative follow-up period of 3.5 years, 23 patients exhibited a good prognosis, and their modified Rankin scale score was <2 points (20).

During bypass surgery for intracranial PCRRAs, the most commonly used supply arteries include the following: i) The superficial temporal artery (STA) and the occipital artery, both of which are suitable for middle- and low-flow bypass surgery; and ii) the radial artery and the great saphenous vein (required to connect the external carotid system), which are ideal interposition grafts for high-flow bypass surgery (20,70-72).