Anatomical Features and Patient Selection

The posterior inferior cerebellar artery (PICA) has a highly variable origin: in 90% of individuals, it originates from the so-called intradural V4 segment of the vertebral artery (VA), and in about 10% it originates extracranially from the extradural V3 segment of the VA, or from the basilar artery. Anatomically, the PICA runs in a tortuous fashion around the lateral medulla oblongata at the level of the caudal cranial nerves (IX–XII). At the anterior aspect of the cerebellar tonsil, it forms the caudal loop. In its further course, the PICA runs between the dorsal aspect of the medulla oblongata and the tonsils, and forms the cranial loop. The distal portion divides into two main branches to supply the vermis and cerebellar hemisphere. ⁴ Thus, based on its anatomical course in relation to the posterior fossa structure, the PICA can be divided into five segments:

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  Anteromedullary (segment I).

  
  
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  Lateromedullary (segment II).

  
  
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  Tonsillomedullary (segment III).

  
  
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  Telovelotonsillary (segment IV).

  
  
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  The cortical segment (segment V).

The rami perforantes or perforating arteries arise from the first three segments to supply the posterolateral medulla. The first three segments are also referred to as the proximal PICA; segments IV and V are the distal PICA. About 2% of all intracranial aneurysms are associated with the PICA. Clinically, PICA aneurysms may present with rupture, which is virtually always subarachnoid hemorrhage (SAH). Intraventricular hemorrhage is common as well. These aneurysms also may present with signs of ischemia or with mass effect with symptoms such as hiccups, dysphagia, and other paralytic caudal cranial nerve palsies.

In general, initial diagnostic workup should include standard blood laboratory tests and cranial computed tomography (CCT) for confirmation and localization of the hemorrhage and also CT angiography, which helps to distinguish proximal from distal PICA origins, in relation to the cranial base.

Catheter rotational digital subtraction angiography with three-dimensional reconstructions, however, remains the diagnostic method of choice to confirm presence of aneurysms, especially with respect to precise site, size, shape, and projection of the aneurysm, in relation to the perforating arteries. More importantly, catheter angiography gives information about the size of both PICAs and, indirectly, the size of their supply territory, which may be an important factor in decision-making regarding reconstructive versus destructive aneurysm repair. Following angiography, interdisciplinary consultation between the neurosurgeons and/or neuroradiologists should define the optimal mode of repairing the aneurysm, preferably by preserving the parent vessel. The decision will usually include aneurysm-related factors (morphology, size, projection, and location of the aneurysm in relation to the PICA aforementioned segments and of the PICA origin on the VA), patient-related factors (e.g., patient age, clinical status on admission, comorbid disease), and whether surgical evacuation of a hematoma or decompression may be required. Lastly, logistical aspects such as availability of endovascular and neurosurgical competence may play a role during decision-making.

Most PICA-origin aneurysms can be treated by either endovascular or microsurgical route, and there are no robust data to guide decision-making, since in the largest randomized clinical trial comparing clipping and coiling, only 2.6% of all aneurysms included were located in the posterior circulation and only 1.4% of all aneurysms included were PICA aneurysms (a total of 31 cases). ⁵ Additionally, small, single-center and retrospective studies report good outcomes with both treatment modalities, although these studies are not randomized and these types of reports typically underestimate complications ( ▶ Table 15.1). Many PICA-origin aneurysms are relatively broad based and therefore particularly complicated for endovascular therapy. ³ However, the main challenge of microsurgical treatment is the anatomical relation to the caudal cranial nerves and thus the risk of caudal cranial nerve deficits, especially swallowing and airway control.

For PICA-origin aneurysms, the most important factors in the initial interdisciplinary decision-making regarding treatment modality are: (1) the level of PICA origin in relation to the VA and the brainstem and (2) the shape of the aneurysm (i.e., broad- vs. narrow-necked). “High-lying” or prepontine aneurysm locations are difficult to approach microsurgically, and in these situations, the endovascular approach appears more favorable. On the other hand, a broad-based aneurysm—particularly if the PICA arises from the aneurysm—may be challenging for endovascular therapy and may be better repaired by microsurgical means. Access to proximal PICA aneurysms, i.e., segments I–III, is gained via a lateralized suboccipital craniectomy, with removal of the foramen magnum ( ▶ Fig. 15.1). Distal PICA aneurysms, i.e., segments IV–V, of the vermis or of hemispheric branches can be treated surgically with low morbidity, owing to their distal location that requires minimal or no manipulation of the cerebellum. Access is gained via a median suboccipital craniectomy with or without a laminectomy of the atlas ( ▶ Fig. 15.1).

Related posts:

How to Repair the Intracranial Aneurysm: Clipping or Coiling Decision Making Minimally Invasive Approaches for Spontaneous Intracerebral Hemorrhage Pterional Approach Superficial Cavernous Malformations Superficial Temporal Artery to Middle Cerebral Artery Bypass Transsylvian Transclinoidal and Transcavernous Approach for Basilar Bifurcation Aneurysms

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