Inferior Cochlear Vein and Cochleostomy

Background

Cochleostomy is used here to mean an enduring opening into the cochlea through which a CI electrode is inserted. There are currently three leading categories of cochleostomy:

• The original round window cochleostomy used with short, single channel implants by Wm. House and currently used with certain electrodes to conserve residual hearing,
• Traditional bony cochleostomy, suggested by Graeme Clark to avoid misdirection of long multichannel electrodes by the crista fenestra,
• Extended RW cochleostomy, consisting of extension of the RW by drilling labyrinthine bone antero-inferiorly in cases where the RWM is too small or angled too inferiorly to use effectively.

Helge Rask-Andersen and his colleagues at Uppsala University continue their remarkable work on the anatomy of the hook portion of the cochlea that is relevant to hearing conservation during cochlear implantation. In their detailed description of the hook region in 2014, they conclude that there may be anatomic reasons to prefer RW insertions in hearing conservation cases (1).

New Knowledge

In their most recent work, the Uppsala group focuses on the surgical anatomy of the inferior cochlear vein (ICV) using micro dissection of venous injection molded corrosion casts (2). The ICV is the major venous drainage from the scalae and modiolus of the cochlea and runs in a bony channel, the accessory canal of the cochlear aqueduct (CA). The scalar aperture is located 0.67 mm to 0.81 mm from the RWM in the floor of scala tympani and it terminates in the posterior fossa. The ICV is closer to the RWM than the CA and thus more vulnerable. The authors speculate that little attention is paid to this structure due to the “assumption that venous drainage occurs in analogy with the arterial supply.” However, as the major drainage of the modiolus, surgical damage to the ICV may result in loss of residual ganglion cells and residual hearing.

Gary Wright and Peter Roland previously described venules in the lateral wall and floor of scala tympani that converge on the ICV (3). They also suggested that drilling of a bony cochleostomy may impact the ICV causing loss of residual hearing. The Rask-Andersen lab suggests that risk to the ICV is minimal during RWM cochleostomy, small during standard cochleostomy but may be greater in RWM enlargement techniques that extend the cochleostomy inferiorly.

Some recent studies show no significant functional outcome difference between RWM and traditional bony cochleostomy (4,5). However a study by George Wanna and colleagues at Vanderbilt demonstrated that RWM and extended RW insertions were both associated with better speech reception scores than bony cochleostomy on CNC words (p<0.045) but not on AzBio sentences or HINT. This appeared to be due to a high incidence of bony cochleostomy insertions ending up in scala vestibuli (p<0.001)(6).

Take Home

The ICV is the major vein of the cochlea and is susceptible to injury during drilling of an extended RW cochleostomy. This poses a theoretical risk of damaging residual hearing during cochlear implantation. However, clinical outcome studies to date do not support the notion that extended round window cochleostomy results in poorer CI function or more loss of residual hearing. This may be due to the dozens of clinical variables, each of which may influence conservation of neural elements as well as function. Nonetheless, surgeons should be aware of this relevant anatomy and consider it in cochlear implantation.

References

1. Francesca Atturo, Maurizio Barbara, Helge Rask-Andersen. On the Anatomy of the 'Hook' Region of the Human Cochlea and How It Relates to Cochlear Implantation. Audiology and Neurotology 2014; 19(6): 378-385.
2. Guo R, Zhang H, Chen W, Zhu X, Liu W, Rask-Andersen H. The inferior cochlear vein: surgical aspects in cochlear implantation. Eur Arch Otorhinolaryngol 201; Feb 21.
3. Wright CG, Roland PS. Vascular trauma during cochlear implantation: a contributor to residual hearing loss? Otol Neurotol 2014; 34:402-407.
4. Sun CH, Hsu CJ, Chen PR, Wu HP. Residual hearing preservation after cochlear implantation via round window or cochleostomy approach. Laryngoscope 2015; doi: 10.1002/lary.25122. (Epub ahead of print).
5. Hassepass F, Aschendorff A, Bulla S, et al. Radiologic results and hearing preservation with a straight harrow electrode via round window versus cochleostomy approach at initial activation. Otol Neurotol 2015 (Epub ahead of print).
6. Wanna GB, Noble JH, Carlson ML, et al. Impact of electrode design and surgical approach on scalar location and cochlear implant outcomes. Laryngoscope 2014. 124: Sup 6; S1-S7.

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