26 Retrosigmoid Approach

26.1 Introduction

The retrosigmoid approach is a lateral approach to the posterior fossa compartment. It is suitable for surgical exposure of the ipsilateral surface of the cerebellar hemisphere, the posterior surface of the petrous bone, as well as the cisternal space

­defned­as­cerebellopontine­angle­(CPA).

Surgical exposure can be further widened caudally toward the magnum foramen and rostrally until the tentorium, which can be cut and opened, providing a transtentorial route to reach the supratentorial space.

The approach is extremely versatile and can be tailored according to the pathology, which must be treated.

The­ approach­ is­ indicated­ for­ lesions­ involving­ the­ CPA­ as­ well as for microvascular decompression in case of trigeminal neuralgia.

26.2 Indications

•The ipsilateral shoulder is slightly displaced inferiorly, with cushions under contralateral armpit and between the knees.

•The­ipsilateral­thigh­is­fexed­and­prepared­for­possible­fat­ and fascia harvest.

26.5 Skin Incision (Fig. 26.1)

•Linear incision

○Landmark:­Mastoid­tip.

○Starting point: Incision starts 2 cm behind the external ear at the level of the pinna.

○Course: Incision line runs inferiorly in a straight line.

○Ending point: It ends 1 cm inferior to the mastoid tip.

26.6 Soft Tissue Dissection (Fig. 26.2)

• Muscles

•Neoplastic and vascular disorders of the cerebello-pontine angle­(CPA).

•Neurovascular decompressions in trigeminal neuralgia.

○Incised according to the skin incision.

○Subperiosteal dissection is carried out laterally, medially, superiorly and inferiorly.

26.3 Neurophysiological

Monitoring

•Somatosensory evoked potentials.

•Facial nerve electromyography.

•Auditory­brain­stem­response.

26.4 Patient Positioning (Fig. 26.1)

•Position:­Patient­is­positioned­in­lateral­decubitus;­the­ contralateral arm is positioned below the surgical table with a­slight­fexion.

•Head: The head is positioned neutral, parallel to the ground, fxed­on­a­Mayfeld­head­holder.

26.6.1 Critical Structures

•Mastoid­emissary­vein.

•Vertebral artery at the atlanto-occipital joint.

26.7 Craniotomy

•Burr holes (Fig. 26.3)

○I: The burr hole is placed on the asterion and performed with a regular cranial perforator or a high-speed drill.

•Craniotomy landmarks (Fig. 26.4)

○Posteriorly:­Craniotomy­runs­about­3­cm­posterior­to­the­ burr hole.

○Inferiorly: It runs 4 cm inferiorly, turning anteriorly, just parallel to the burr hole.

Fig. 26.1 Patient positioning and skin incision.

Abbreviations: E = ear; MT = mastoid tip.

○Anteriorly: The remaining anterior cut is performed with the drill.

○The sigmoid and transverse sinuses have to be exposed by using the drill.

26.7.1 Critical Structures

• Sigmoid and transverse sinus.

26.7.2 Variants

•Extended transmastoid retrosigmoid approach

○Indicated for lesions extending medial to the trigeminal nerve­(V)­but­limited­at­the­infratentorial­compartment.

○Conventional­suboccipital­craniotomy­is­followed­by­partial mastoidectomy and skeletonization of the total length of the sigmoid sinus.

○Dura mater is opened along the sigmoid and transverse sinuses,­which­are­refected­anteriorly­and­superiorly,­ respectively.

26.8 Dural Opening (Fig. 26.4)

•C-shaped­fashion.

•Incision runs just posteriorly to the sigmoid sinus and inferiorly to the transverse sinus.

26.8.1 Critical Structures

•Sigmoid and transverse sinus.

•Dural emissary veins.

Fig. 26.4 Craniotomy.

Abbreviations: AOJ = atlanto-occipital joint; Ast = asterion; DN = digastric notch;

M = mastoid; MEV = mastoid emissary vein; PB = parietal bone; SM = sternocleidomastoid muscle; TB = temporal bone.

26.9 Intradural Exposure (Figs. 26.5–26.7)

•Parenchymal structures:­Petrosal­cerebellar­surface,­focculus, choroid plexus and brain stem.

•Arachnoidal layer: Lateral cerebellomedullary cistern.

•Cranial nerves:­Trigeminal­(V),­abducens­(VI),­facial­(VII),­ vestibulocochlear­(VIII),­glossopharyngeal­(IX),­vagus­(X)­and­ accessory­(XI)­nerves.

•Arteries:­Superior­cerebellar­artery­(SCA)­and­anterior­inferior­cerebellar­artery­(AICA).

•Veins:­Petrosal­vein.

26.10 Pearls

•Bleeding from emissary veins can be controlled with hemostatic agents and bone wax.

•Packing­of­large­amounts­of­bone­wax­must­be­avoided,­due­ the risk of sinus thrombosis.

•Sinus bleeding can be controlled with a patch of muscle or suture, depending on the extension of the tearing.

•Placing­sutures­on­the­anterior­margin­of­the­dural­opening­

and­slightly­refecting­it,­could­ofer­additional­exposure.

•Starting the intradural procedure, by opening the cerebellomedullary­cistern,­to­release­some­cerebrospinal­fuid,­ usually provides a good cerebellar relaxation.

Fig. 26.6 Dural opening.

Abbreviations: CH = cerebellar hemisphere; DM = dura mater; JV = jugular vein; SS = sigmoid sinus; TS = transverse sinus.

Fig. 26.5 Exposure of the dura mater and venous sinuses.

Abbreviations: AOJ = atlanto-occipital joint; DM = dura mater; DN = digastric notch;

M = mastoid; MEV = mastoid emissary vein; SM = sternocleidomastoid muscle;

SS = sigmoid sinus; TS = transverse sinus.

References

1.­ Abolfotoh­ M,­ Dunn­ IF,­ Al-Mefty­ O.­ Transmastoid­ retrosigmoid approach to the cerebellopontine angle: surgical technique.­ Neurosurgery­ 2013;73(1,­ Suppl­ Operative): ons16–ons23,­discussion­ons23­

2.­ Al-Mefty­ O.­ The­ retrosigmoid­ approach­ to­ meningiomas­ of­ the­ cerebellopontine­ angle.­ In:­ Al-Mefty­ O,­ ed.­ Operative­ Atlas­ of­ Meningiomas.­ Philadelphia,­ PA­ :­ Lippincott- Raven;­1988:323–330

Fig. 26.7 Intradural exposure of the cerebello-pontine angle.

Abbreviations: AICA = anterior inferior cerebellar artery; IX = glossopharyngeal nerve; P = pons; PV = petrosal vein; V = trigeminal nerve; VI = abducens nerve; VII/VIII = facial and vestibule-cochlear nerves complex;

X = vagus nerve; XI = accessory nerve.

3.­ Matsushima­ T.­ The­ cerebellopontine­ angle:­ basic­ structures­and­the­“rules­of­three”.­In:­Matsushima­T,­ed.­Microsurgical anatomy and surgery of the posterior cranial fossa.

Tokyo:­Springer;­2006:101–107

4.­ Samii­ M,­ Gerganov­ VM.­ Suboccipital­ lateral­ approaches­(retrosigmoid).­In:­Cappabianca­P,­Califano­L,­Iaconetta­ G,­eds.­Cranial,­Craniofacial­and­Skull­Base­Surgery.­Milan:­ Springer;­2010:143–150