**3. Syntopy**

Understanding of ACP syntopy with surrounding anatomical structures is extremely important for surgical dissection and anterior clinoidectomy during surgery. The base of the process forms the lateral and lower walls of the optic canal, the medial surface forms the ICA canal, and the lateral surface and optic strut are the parts of the upper medial wall of the upper orbit (**Figure 1**). Thus, ACP is located between the canal of the optic nerve, upper orbital fissure, and ICA canal. Extradural resection of the process provides the access to these bony channels and their content. The clinoidal process also separates two leaves of the dura: dura temporalis (DT) and DP. DP represents the lateral wall of the cavernous sinus and extends from the outer to the inner dural rings, where the ICA penetrates the cavernous sinus. Also, it touches the free edge of the tentorium in posterior divisions, which is fixed to the apex of the ACP [17]. Anteriorly it continues to the layers of the upper orbit. It should be remembered that ACP meningiomas usually invade the outer leaf of the dura and very rarely are spread to the DP. Thus, the separation of the dural leaves during surgery allows exposure of the lateral surface of the ACP and provides consequent visualization of the important anatomical structures of the skull base [17, 18].

After performing extradural clinoidectomy, the optic nerve in the dural sheath could be visualized [19, 20]. The lateral wall of the cavernous sinus and the intracavernous part of the ICA that is passing behind are seen as well. The 3rd nerve is located immediately below the projection of the lower clinoidal edge. The 1st branch of the V nerve passes lower.

Variable pathological anatomy of this area due to tumor growth has to be taken into account. Most clinoid meningiomas invade ACP causing its hyperostotic enlargement [21, 22]. Thus, anterior clinoidectomy is considered the key to the radicality of surgery.

However, there is a group of meningiomas that grows from the superior or superolateral surface of the clinoid without invasion into the ACP and hyperostosis does not exist [23–25]. Complete clinoidectomy is not necessary for this type of MAC.

Intradural syntopy in presence of MAC is much more complex and variable in comparison with extradural peculiarities. Primarily, it is due to the nature of meningioma spread, that has two patterns: expansive and invasive. The first type has a small fixation area and the tumor "wraps" around vessels and nerves. Invasive type spreads along the dura and longitudinally ingrowth into the anatomical structures [26]. In practice, a combination of both types with some predominance is usually seen.

The important tip is to follow the olfactory nerve that always leads to the optic nerve if the last is markedly displaced by the tumor.

A1 segment of ACA and all anterior semicircle of Willis are shifted medially and located on the dorsomedial surface of meningioma. M1 segment of MCA "rolls over" through the dome of the tumor on its upper lateral surface. Special attention to perforating and small branches of the anterior circle of Willis should be paid because of their tight inclusion in the tumor [27]. Sharp dissection is the only possible method to separate them from the tumor.

The oculomotor nerve is displaced dorso-medially and could be encased by neoplasm.

The pituitary stalk itself is not commonly involved in MAC, located on the postero-medial portion of meningioma, and could be separated without difficulties. Although, the superior hypophyseal artery has a variable way and has to be saved to prevent postoperative diabetes insipidus.
