Chapter 9: Endolymphatic Sac Decompression in Temporal Bone Dissection

Introduction
Endolymphatic sac decompression is a vital surgical procedure designed to relieve endolymphatic pressure, particularly in intractable Menière’s disease with serviceable hearing. It also serves as an essential exercise for temporal bone dissection, offering a thorough understanding of otologic anatomy and surgical techniques (1, 2, 4).

Anatomy and Topography of the Endolymphatic Sac

The endolymphatic sac is a blind-ending structure within the posterior cranial fossa dura, connected to the membranous labyrinth via the endolymphatic duct. It lies medial to the sigmoid sinus, anterior to the posterior semicircular canal, and inferior to Donaldson’s line—a crucial surgical landmark extending from the lateral semicircular canal to the sigmoid sinus (1, 2, 4).

The sac’s location varies but is often identified by its thickened, white dura, contrasting with the bluish-gray surrounding dura. In some cases, the sac extends onto the medial wall of the jugular bulb or beneath the posterior fossa dura (1, 11).

Surgical Landmarks

Key anatomical landmarks include:

1. Sigmoid sinus
2. Donaldson’s line
3. Posterior and lateral semicircular canals
4. Sinodural angle
5. Jugular bulb
6. Bony fallopian canal (1, 2, 4).

Proper identification and preservation of these landmarks are crucial to minimizing risks during surgery, such as facial nerve or vascular injury (1, 11).

Surgical Steps for Endolymphatic Sac Decompression

1. Cortical Mastoidectomy

• Perform a complete mastoidectomy, removing all air cells surrounding the sigmoid sinus and presigmoid dura, extending toward the retrofacial area (1, 4).
• Outline the sigmoid sinus and maintain a thin bony covering for protection.
• Identify the lateral and posterior semicircular canals. Blue-lining of the posterior canal may be necessary but is usually avoided to reduce the risk of hearing loss (1, 2, 4).

2. Exposure of the Sac

• Thin the bone over the dura between the sigmoid sinus and posterior semicircular canal using a diamond burr. Extend dissection medially toward the retrofacial cells (1, 4, 11).
• Expose the posterior fossa dura widely. The sac is located within the dura, often appearing whiter than the surrounding dura due to its thickened structure (2, 4, 11).
• The endolymphatic duct may be visible as a fan-shaped extension beneath the posterior canal, aiding in sac localization (1, 2).

3. Incision and Decompression

• Remove the thin bony covering of the sac. Incise the lateral wall of the sac carefully to avoid penetrating the subarachnoid space (1, 2, 4).
• Under high magnification, make a medial incision along the dural fibers, creating a flap for closure. The medial incision should remain superficial to preserve underlying structures (1, 4).

4. Optional Shunt Placement

• Insert a Silastic tube or catheter into the sac lumen, ensuring it does not penetrate the subarachnoid space. Secure the insertion with a muscle plug and suture to prevent leaks (1, 4, 11).

Hints and Pitfalls

1. Extend bone removal beyond the expected sac location, especially into the retrofacial area, to ensure adequate exposure (1, 4, 11).
2. Exercise caution when operating near the jugular bulb, as the sac may be closely associated with it (1, 2).
3. Be mindful of anatomical variability in the sac’s location and rely on Donaldson’s line and other landmarks for orientation (1, 4, 11).
4. Avoid damaging the facial nerve during dissection of the retrofacial area (1, 4).

Clinical Application

Endolymphatic sac decompression is most commonly indicated for intractable Menière’s disease with preserved hearing. While its clinical efficacy has been debated, it remains a valuable temporal bone exercise, allowing surgeons to master the anatomy and surgical techniques of the posterior fossa dura and retrofacial recess (2, 11).

Conclusion

Endolymphatic sac decompression is a challenging yet rewarding procedure that highlights the intricate anatomy of the temporal bone. Accurate identification of landmarks, careful bone removal, and precise surgical technique are essential to its success. Beyond its clinical application, the exercise is invaluable for developing surgical skills and anatomical understanding in otologic practice.