Biopsy of lesions in Meckel's cave area
Biopsy of Meckel cave area can be achieved via two minimally invasive routes. The current imaging technology does not identify the pathology of these lesions accurately. This makes the role of biopsy rather crucial in managing these lesions.
Percutaneous biopsy of Meckel’s cave is commonly practiced. The entry point for the needle is 2.5 cm lateral to the labial commissure. Using fluoroscopic guidance, the needle is advanced through the cheek, medial to the ramus of the mandible, all the way to the inferior skull base where the foramen ovale is penetrated. This route is also known as Hartel’s route.
The importance of fluoroscopic guidance should be stressed at this point. It helps in prevention of injury to structures at inferior orbital fissure / jugular foramen. This technique is suitable for all tumors that involve Meckel cave / third division of trigeminal nerve.
Sindou biopsy needle is used for this purpose. It has an outer needle to penetrate and inner cannula to aspirate. The cannula is slightly smaller than the outer needle so that it will stop short of its tip. This route was first described by Hartel as transjugal-transoval route in 1912. He used this route initially to inject alcohol at the level of Gasserian ganglion to treat trigeminal neuralgia. The first attempt at biopsy using this approach was done by Stechison and Bernstein in 1989. Dresel et al popularized this approach in 1991.
The anatomy of the region crossed by the biopsy needle makes an interesting study. This area can be considered as an inverted, three-sided pyramid. The apex of the pyramid is situated at the cheek 3 cm lateral to the labial commissure. The base which is triangular is located at the skull base. This can be delineated by the following landmarks:
Superolateral – This cutaneous point is the orbito-meatal line along the inferior border of zygoma, 3 cm anterior to the tragus.
Superomedial – Corresponds to the pupil of the eye
Deep seated – This is the foramen ovale
This inverted pyramid can be subdivided into three segments for better understanding.
Inferior segment – This is about 13 mm long on an average. This comprises of the portion between the apex of the pyramid (where the needle enters the cheek) to the point where the probe contacts the parotid duct.
Middle segment – This is 29 mm long on an average. This consists of the portion of the pyramid from the parotid duct to the lateral pterygoid muscle. This portion is filled with fatty tissue and contains the lingual, the chorda tympani, the buccal and inferior alveolar nerves. In this portion the needle could encounter branches of the maxillary artery or its trunk if it is tortuous.
Superior segment – This segment starts from the lateral pterygoid muscle and ends at the foramen ovale. In this segment maxillary artery runs posterior to the lateral pterygoid muscle and may come into contact with the inserted needle. Pterygoid venous plexus is also present in this area and may even be penetrated. After passing through foramen ovale the needle enters the trigeminal cave.
At the skull base (base of the pyramid) the following structures may be endangered. The internal jugular vein located 27 mm postero lateral to the needle trajectory, the internal carotid artery at its entry into the petrous carotid canal (25 mm posterior to the needle axis), internal carotid artery at foramen lacerum if the needle deviates 10° medially from its correct pathway, the membranous portion of Eustachian tube if the trajectory of the needle deviates by 9° in anteromedial direction. Superior and inferior orbital fissure should be avoided. Injury to optic nerve could occur if the needle is placed 17° too anterior to its correct trajectory.
Patient is placed in supine position. The head is under lateral fluoroscopic control. Surgery is performed under light and short acting general anesthesia with intravenous propofol. Intubation is not needed. Infiltration of local anesthetic agent 10 ml of 1 % xylocaine is given at the site of skin puncture of the cheek up to the pterygopalatine fossa. Sindou biopsy needle is pushed along the Hartel’s route through the pterygomaxillary space up to the foramen ovale. The needle tip location is checked with lateral x-ray. The needle is connected to a 20 ml syringe. Negative pressure is applied to the needle till tissue for cytology is obtained.
This method can be considered only in patients with central skull base lesions located in the Meckel’s cave, posterior portion of the cavernous sinus or the upper part of Petroclival region. This is indicated only in patients whom imaging does not provide reliable pathological diagnosis.
The following imaging studies need to be performed before embarking on percutaneous needle biopsy:
1. MR imaging with both T1 and T2 weighted image studies 2. HRCT 3. Selective intracranial DSA. Shows up the detailed vasculature of the tumor. 4. CT scan with 3 D reconstruction. This provides information regarding central skull base and middle fossa bony structures, especially the integrity of the foramen ovale 5. Pre-operative neuro-otological examination and ophthalmic examination should also be performed.
Biopsy can be attempted via an expanded endonasal approach. Compared to the percutaneous route, this is more time-consuming procedure, but the exposure obtained is rather wide providing greater access to the lesion. All branches of the nerve can be exposed endoscopically. The biopsy procedure can be converted to complete tumor resection if need be.