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Modified retrolabyrinthine approach with partial labyrinthectomy: Anatomic study
Modified retrolabyrinthine approach with partial labyrinthectomy: Anatomic study GIUSEPPE MAGLIULO, MD, Rome, Italy
This study was undertaken to evaluate the feasability of the modified retrolabyrinthine approach (traditional retrolabyrinthine approach plus resection of the posterior semicircular canal) to expose the entire fundus of the internal auditory canal (IAC). This approach is advocated by its proponents to manage acoustic neuromas reaching the lateral IAC and with the preservation of hearing as the goal. Little anatomic data directly estimate the limitations of this exposure. Measurements were recorded from 25 cadaver temporal bones dissected with this modified approach. The distances were taken between the porus acousticus (inferior and superior portions), the dome of the jugular bulb, the midportion of the sigmoid sinus, and the fundus of the IAC (inferior and superior portions). All of the measurements were then compared with those of the translabyrithine approach. The current study shows that despite the sacrifice of the posterior semicircular canal, the superior lateral fundus cannot be completely visualized. There is a distance (on average 1.1 mm) that differentiates the superior area of the IAC accessible with translabyrithine and modified retrosigmoid techniques. This value is smaller than that observed in the classic retrosigmoid approach indicating that the modified technique affords a more adequate, even if not ideal, exposure to minimize the risk of recurrence. The modified retrolabyrinthine approach provided an optimal exposure of the inferior half of the IAC. A superior blind area, smaller than that of the traditional retrolabyrinthine technique, cannot be completely approached via this route. We believe that this approach can be considered as an alternative technique in selected cases especially for tumors involving the inferior vestibular nerve. (Otolaryngol Head Neck Surg 2001;124:287-91.)
Hearing preservation after surgery for acoustic neuroma is an accepted possibility. Both approach routes through the middle cranial fossa as well as the retrosigmoid approach are useful to reach this goal. These techniques have strict indications, specific advantages, and well-defined limitations. Anatomic studies have demonstrated that both techniques fail to completely expose the fundus of the internal auditory canal (IAC) with a potential risk of tumor recurrence.1-7 Some authors8,9 proposed to remove the structures of the labyrinth that limited the IAC exposure at the fundus. The theoretical and fundamental basis on this topic was first posed by the work of Parnes and McClure8 who demonstrated the possibility of occluding the posterior semicircular canal and, at same time, maintaining baseline hearing. They devised this type of procedure to treat intractable positional vertigo due to cupololithiasis. The concept that the isolation of the endolymphatic membranous canal from the vestibule does not provoke adverse effects on hearing function has opened the field of application to the acoustic neuroma surgery. Molony et al10 described the sacrifice of the posterior canal to improve the exposure of the retrolabyrinthine approach with successful outcomes. Arriaga and Gorum11 proposed to combine the retrosigmoid approach with the ablation of the posterior semicircular canal through a transmastoid approach. Their preliminary results were encouraging. However, no anatomic data from either of these modified techniques were given; this left some doubts about the real possibility reaching the fundus of the internal canal. The present study was undertaken in an effort to estimate anatomically the depth into the IAC obtainable with the removal of the posterior semicircular canal through an anterosigmoid dissection. METHODS AND MATERIAL
From the ENT Department, University “La Sapienza” Roma, Italy. Reprint requests: Giuseppe Magliulo, MD, via Gregorio VII 80, 00165 Roma, Italy; e-mail, [email protected]. Copyright © 2001 by the American Academy of Otolaryngology–Head and Neck Surgery Foundation, Inc. 0194-5998/2001/$35.00 + 0 23/77/113137 doi:10.1067/mhn.2001.113137
Twenty-five human temporal bones preserved in formalin were dissected with the use of a binocular microscope and an electric drill. Each bone was prepared injecting the venous system (sigmoid sinus, superior petrosal sinus, and jugular vein) with blue-colored silicon. After the removal of the periosteal flap, the cortical mastoid was exposed, and a complete mastoidectomy was performed. All of the mastoid cells from the middle fossa tegmen, sinodu287
288
Otolaryngology– Head and Neck Surgery March 2001
MAGLIULO
Fig 1. Modified retrosigmoid dissection shown to illustrate the measurements taken (right ear). Measurements are shown in Table 1.
Fig 2. Translabyrinthine dissection shown to illustrate the measurements taken (right ear). Measurements are shown in Table 1.
ral angle, and the posterior fossa anterior, and 3 cm posterior to the sigmoid sinus were exenterated. The sigmoid sinus was skeletonized until the jugular bulb was identifed. The exposure of the 3 semicircular canals was accomplished next. This was performed using the horizontal canal as the initial landmark. The posterior semicircular canal was then resected. The extradural identification of the internal auditory canal was achieved removing all bone (180 degrees) between the jugular bulb and the area of the resected posterior canal. This maneuver required sacrificing the endolymphatic duct in order to enlarge the area of the dissection. It should be pointed out that it is facilitated positioning the back of the suction tube on the totally skeletonized surfaces of the sigmoid sinus and retrosigmoid dura and applying a slight pressure. In some temporal bones, a high jugular bulb hindered extradural removal of bone from around the internal auditory canal. In these cases, the exposure of the internal canal was obtained by totally removing the bone from the jugular bulb and compressing it in order to increase its distance from the internal canal. After the dissection, several measurements were estimated. In detail, the distance between the dome of the jugular bulb and the inferior portion of the porus acousticus (A) and inferior limit of the exposed internal auditory canal (A1) together with that between the anterior edge of the sigmoid sinus (in front of the porus) and the superior portion of the porus (B) and the superior limit of the exposed internal canal (B1) were measured using a millimeter rule with a 2-point needle caliper (Fig 1). The dissection was then converted to a translabyrinthine approach in order to compare the differences concerning the exposure of the fundus of the internal canal. Further measurements were taken between the inferior and superior portion of the internal canal and the jugular bulb dome (A2) and the sig-
moid sinus (B2) (Fig 2). Statistical significance of the data was calculated using the mean, the standard deviation, and multiple t tests. RESULTS
The measurements taken from the temporal bones dissected using the partial labyrinthectomy are summarized in Table 1. In all of the bones, the approach allowed the complete exposure of the porus acousticus. The distance between the porus and the dome of the jugular bulb showed a range between 2 and 12.5 mm. This datum confirms the variable anatomic position of the jugular bulb (Figs 3 and 4) previously observed in other studies.12-14 However, it does not influence the possibility to safely and completely expose the porus. The inferior portion of the fundus of the internal auditory canal was reached in all of the temporal bones. The relationship between the sigmoid sinus and the porus had less variability (see Table 1). The mean distance of the superior limit of the canal from the sigmoid sinus resulted in 18.4 mm ranging from 12 to 23.5 mm. Table 2 focuses on the measurements taken in relation to the translabyrinthine approach. The comparison with the previous data showed the same values of the measurements taken in the infralabyrinthine compartment (inferior portion of the porus and of the fundus) and in the superior portion of the porus. The difference between the translabyrinthine and partial labyrinthectomy approaches is delineated by the numbers regarding the measurements between the sigmoid sinus and the superior limit of the canal. Its mean value was 19.5 mm with a difference of 1.1 mm between the 2 approaches.
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MAGLIULO
289
Fig 3. Modified retrosigmoid dissection (left ear). Jugular bulb is quite far from the IAC.
Fig 4. Modified retrosigmoid dissection (left ear). Note jugular bulb rising the level of the IAC.
It represents an area, even if limited, in the fundus of the internal canal not accessible despite the sacrifice of the posterior semicircular canal.
these potential disadvantages. Their anatomic study demonstrated that the falciform crest creates an obscure recess of 1.82 × 2.33 mm. This does not allow complete visualization of the internal canal. Its clinical significance regarding potential recurrences is debated and not well defined. A review of the literature indicates that only a few series2,4 have evaluated the incidence of recurrence following the middle fossa approach with variable and not comparable findings due to the heterogeneous population of patients examined (small and large acoustic neuromas). However, the existence of this blind area undoubtedly represents an increased risk to leave tumor especially when it arises from the inferior vestibular nerve. In an attempt to overcome the limitations of the retrosigmoid and middle fossa approaches, many suggestions have been proposed. For the retrosigmoid approach, these include the drilling of the limb of the internal canal up to the level of the vestibule, the use of specific landmarks (singular canal, vestibular aqueduct, cochlear aqueduct), the preoperative CT evaluation of the tumor extension in the IAC as well as of the IAC length, and the use of endoscopes,3,16,17 while the exposure of the middle fossa approach is enlarged through the complete skeletization of the superior semicircular canal, of the geniculate ganglion, and of the horizontal and meatal facial nerve segments.15 All of these procedures minimize the risk of recurrences, but they are not able to completely prevent them. Thus, other authors have advocated combining the traditional techniques with a partial labyrinthectomy. There are numerous reports in the literature demonstrat-
DISCUSSION
The ideal surgical approach devoted to hearing preservation in acoustic neuroma surgery should provide a thorough exposure of the intrameatal tumor extension as well as of that of the cerebellopontine angle. Furthermore, the procedure should be achieved with minimal disturbance of the anatomic structures of the internal canal and should be easily reproduced by other surgeons with minimal technical difficulty and limited operating time. Traditional techniques have met these objectives with varying degrees of success. The retrosigmoid approach fails to reach the distal portion of the IAC when hearing preservation is attempted. It has been described as a blind recess approximately 2 to 3 cm deep that impedes the exposure of the entire length of the canal unless the labyrinth is opened. This anatomic limitation suggested contraindications to the use of this route if the acoustic neuroma involves the fundus due to high risk of tumor recurrence. The recurrence rate, as reported in large series, varies from 1.8% to 2.4%.1,3,5,7 The middle fossa approach is considered an alternative route. This procedure has been recommended for its ability to accomplish the exposure of the entire IAC. However, this assumption has been contradicted by the clinical observations of Sanna et al2 who noted some difficulty in visualizing the medial aspect of the fundus in most cases. Furthermore, Haberkamp et al15 confirmed
290
Otolaryngology– Head and Neck Surgery March 2001
MAGLIULO
Table 1. Distance taken between the inferior limit of the porus acousticus (A), the inferior limit of the fundus of the IAC (A1) and the dome of the jugular bulb. B and B1 indicate the measurements between the sigmoid sinus and the superior limit of the porus acousticus and the superior limit of the fundus of the IAC respectively; modified retrolabyrinthine approach Mean
A A1 B B1
5.8 mm 9.2 mm 14.9 mm 18.4 mm
SD (±)
Range
2.9 3.2 3.1 3.8
2-12 4.3-16 14.5- 19 12-23.5
Table 2. Distance taken between the inferior limit of the porus acousticus (A), the inferior limit of the fundus of the IAC (A2) and the dome of the jugular bulb. B and B2 indicate the measurements between the sigmoid sinus and the superior limit of the porus acousticus and the superior limit of the fundus of the IAC respectively; translabyrinthine approach
A A2 B B2
Mean
SD (±)
Range
5.8 mm 9.2 mm 14.9 mm 19.5 mm
2.9 3.2 3.1 4.2
2-12 4.3-12 14.4-19 17-24.7
ing that the ablation of the posterior or superior semicircular canals is compatable with preservation of hearing. McElveen et al9 reported that a modified translabyrinthine approach with the removal of all 3 semicircular canals can afford hearing preservation if the vestibule is filled with fluid and plugged. Molony et al10 described the sacrifice of the posterior canal to improve the exposure of the retrolabyrinthine approach and that of the superior canal to extend the middle fossa approach allowing the removal of lesions of the internal canal directed toward the cerebellopontine angle for more than 0.5 cm. Arriaga and Gorum11 proved the feasibility of combining the standard retrosigmoid approach with the removal of the posterior semicircular canal. They offered these techniques to patients with tumors situated in the fundus of the internal canal and extended more than 0.6 cm into the cerebellopontine angle. The combined retrosigmoid-partial labyrinthectomy approach embraced the advantages of both middle fossa and standard retrosigmoid approaches. Although the preliminary clinical outcomes were successful, no anatomic data were available about the possibility that a blind area still remains.
The present study was specifically designed to investigate this topic. Our findings showed that there is a pocket at the fundus of the IAC that cannot be approached with a retrolabyrinthine modified technique. This recess is situated only in the superior portion of the IAC, and its mean width is 1.1 mm. This value is smaller than that observed in the classic retrosigmoid approach and in the middle fossa approach indicating an exposure of the lateral extent of the canal almost similar to that of the translabyrinthine approach. In detail, no differences were detected between these 2 approaches for the inferior portion of the IAC. Despite some temporal bones exhibited a high jugular bulb in proximity to the internal meatus, the extradural dissection was always accomplished. The risk of leaving tumor in the fundus, thus, relates to acoustic neuromas of the superior vestibular nerve. However, it should be emphasized that the intradural resection affords a cleavage plane between the facial nerve located superiorly in the canal. In our experience, a round knife, positioned over the facial nerve and directed superiorly, encountered the bony area of the transverse crest allowing the removal of the tumor with a reasonable degree of safety.18 Clinically, previous studies9-11,18 confirm that partial labyrinthectomy does not mean additional morbidity and allowed to preserve a useful hearing. It is important to point out that this combined approach is relatively easily reproduced providing similar successful rates of hearing preservation. Moreover, it did not significantly increase the operating time. The adjunctive time necessary for partial labyrinthectomy did not exceed 1 hour, which is acceptable from an economic viewpoint as well. An advantage due to extradural dissection is represented by prevention of the potential aseptic meningitis and postoperative headaches. It is well known that these complications can be linked to the presence of bone dust in the cerebellopontine angle following the drilling of the posterior limb of the internal canal. The major disadvantage was the need to transect the endolymphatic duct with the potential risk of a postoperative hydrops. Previous studies on an experimental primate (macacus rhesus) model19 proved that no longterm loss of cochlear function developed after resection of the endolymphatic duct. These findings cannot be directly applied to humans so that many questions remain on this particular aspect. In summary, the modified retrolabyrinthine approach provided an adequate inferior exposure of the IAC. A superior blind area, smaller than that of the traditional retrosigmoid and middle fossa techniques, could not be completely approached via this route. We believe that this approach can be considered as an alternative technique in tailored cases especially for tumors involving
Otolaryngology– Head and Neck Surgery Volume 124 Number 3
the inferior vestibular nerve and reaching the fundus of the internal auditory canal. The ideal size of the tumor should not exceed 2 cm. Further research should be done to evaluate the hearing results in large series of patients with longer follow-up. REFERENCES 1. Domb GH, Chole RA. Anatomical studies of the posterior petrous apex with regard to hearing preservation in acoustic neuroma removal. Part I. Laryngoscope 1980;90:1769-76. 2. Sanna M, Zini C, Mazzoni A, et al. Hearing preservation in acoustic neuroma surgery: middle fossa versus suboccipital approach. Am J Otol 1987;8:500-6. 3. Silverstein H, Norrell H, Smouha E, et al. The singular canal: a valuable landmark in surgery of the internal auditory canal Otolaryngol Head Neck Surg 1988;98:138-43. 4. Shelton C, Brackmann DE, House WF, et al. Middle fossa acoustic tumor surgery: results in 106 cases. Laryngoscope 1989;99:405-8. 5. Thedinger BS, Whittaker CK, Luetje CM. Recurrent acoustic tumor after a suboccipital removal. Neurosurgery 1991;29:681-7. 6. Kanzaki J, Ogawa K, Yamamoto M, et al. Results of acoustic neuroma surgery by extended middle cranial fossa approach. Acta Otolaryngol Suppl 1991;487:17-21. 7. Roberson JB, Brackmann DE, Hitselberger WE. Acoustic neuroma recurrence after suboccipital resection: management with translabyrinthine resection. Am J Otol 1996;17:307-11. 8. Parnes LS, McClure JA. Posterior semicircular canal occlusion for intractable benign paroxysmal positional vertigo. Ann Otol Rhinol Laryngol 1990;99:330-4. 9. McElveen JT, Wilkins RH, Erwin AC, et al. Modifying the translabyrinthine approach to preserve hearing during acoustic tumor surgery. J Laryngol Otol 1991;105:34-7.
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10. Molony TB, Kwartler JA, House WF, et al. Extended middle fossa and retrolabyrinthine approaches in acoustic neuroma surgery: case reports. Am J Otol 1992;13:360-3. 11. Arriaga M, Gorum MM. Enhanced retrosigmoid exposure with posterior semicircular canal resection. Otolaryngol Head Neck Surg 1996;115:46-8. 12. Rauch SD, Xu WZ, Nadol JB. High jugular bulb: implications for posterior fossa neurotologic and cranial base surgery. Ann Otol Rhinol Laryngol 1993;102:100-7. 13. Balyan FR, Caylan R, Aslan A, et al. Morphometric evaluation of the infralabyrinthine approach to the internal auditory canal. ORL 1997;59:18-22. 14. Aslan A, Falcioni M, Russo A, et al. Anatomical considerations of high jugular bulb in lateral skull base surgery. J Laryngol Otol 1997;111:333-6. 15. Haberkamp TJ, Meyer GA, Fox M. Surgical exposure of the fundus of the internal auditory canal: anatomic limits of the middle fossa versus the retrosigmoid transcanal approach. Laryngoscope 1998;108:1190-4. 16. McKennan KX. Endoscopy of the internal auditory canal during hearing conservation tumor surgery. Am J Otol 1993;14: 259-62. 17. Magnan J, Chayas A, Caces F, et al. Contribution of endoscopy of the cerebellopontine angle by retrosigmoid approach: Neuroma and vasculo-nervous compression. Ann Otolaryngol Chir Cervicofac 1993;110:259-65. 18. Magliulo G, Varacalli S, D’Amico R, et al. Retrosigmoid approach combined with partial labyrinthectomy and hearing preservation in acoustic neuroma surgery. In: Filipo R, Barbara M, editors. Acoustic neuroma: trends and controversies. The Hague, The Netherlands: Kugler Publications; 1999. p. 137-44. 19. Swart JG, Schuknecht HF. Long-term effect of destruction of the endolymphatic sac in a primate species. Laryngoscope 1988;98: 1183-9.
This study was undertaken to evaluate the feasability of the modified retrolabyrinthine approach (traditional retrolabyrinthine approach plus resection of the posterior semicircular canal) to expose the entire fundus of the internal auditory canal (IAC). This approach is advocated by its proponents to manage acoustic neuromas reaching the lateral IAC and with the preservation of hearing as the goal. Little anatomic data directly estimate the limitations of this exposure. Measurements were recorded from 25 cadaver temporal bones dissected with this modified approach. The distances were taken between the porus acousticus (inferior and superior portions), the dome of the jugular bulb, the midportion of the sigmoid sinus, and the fundus of the IAC (inferior and superior portions). All of the measurements were then compared with those of the translabyrithine approach. The current study shows that despite the sacrifice of the posterior semicircular canal, the superior lateral fundus cannot be completely visualized. There is a distance (on average 1.1 mm) that differentiates the superior area of the IAC accessible with translabyrithine and modified retrosigmoid techniques. This value is smaller than that observed in the classic retrosigmoid approach indicating that the modified technique affords a more adequate, even if not ideal, exposure to minimize the risk of recurrence. The modified retrolabyrinthine approach provided an optimal exposure of the inferior half of the IAC. A superior blind area, smaller than that of the traditional retrolabyrinthine technique, cannot be completely approached via this route. We believe that this approach can be considered as an alternative technique in selected cases especially for tumors involving the inferior vestibular nerve. (Otolaryngol Head Neck Surg 2001;124:287-91.)
Hearing preservation after surgery for acoustic neuroma is an accepted possibility. Both approach routes through the middle cranial fossa as well as the retrosigmoid approach are useful to reach this goal. These techniques have strict indications, specific advantages, and well-defined limitations. Anatomic studies have demonstrated that both techniques fail to completely expose the fundus of the internal auditory canal (IAC) with a potential risk of tumor recurrence.1-7 Some authors8,9 proposed to remove the structures of the labyrinth that limited the IAC exposure at the fundus. The theoretical and fundamental basis on this topic was first posed by the work of Parnes and McClure8 who demonstrated the possibility of occluding the posterior semicircular canal and, at same time, maintaining baseline hearing. They devised this type of procedure to treat intractable positional vertigo due to cupololithiasis. The concept that the isolation of the endolymphatic membranous canal from the vestibule does not provoke adverse effects on hearing function has opened the field of application to the acoustic neuroma surgery. Molony et al10 described the sacrifice of the posterior canal to improve the exposure of the retrolabyrinthine approach with successful outcomes. Arriaga and Gorum11 proposed to combine the retrosigmoid approach with the ablation of the posterior semicircular canal through a transmastoid approach. Their preliminary results were encouraging. However, no anatomic data from either of these modified techniques were given; this left some doubts about the real possibility reaching the fundus of the internal canal. The present study was undertaken in an effort to estimate anatomically the depth into the IAC obtainable with the removal of the posterior semicircular canal through an anterosigmoid dissection. METHODS AND MATERIAL
From the ENT Department, University “La Sapienza” Roma, Italy. Reprint requests: Giuseppe Magliulo, MD, via Gregorio VII 80, 00165 Roma, Italy; e-mail, [email protected]. Copyright © 2001 by the American Academy of Otolaryngology–Head and Neck Surgery Foundation, Inc. 0194-5998/2001/$35.00 + 0 23/77/113137 doi:10.1067/mhn.2001.113137
Twenty-five human temporal bones preserved in formalin were dissected with the use of a binocular microscope and an electric drill. Each bone was prepared injecting the venous system (sigmoid sinus, superior petrosal sinus, and jugular vein) with blue-colored silicon. After the removal of the periosteal flap, the cortical mastoid was exposed, and a complete mastoidectomy was performed. All of the mastoid cells from the middle fossa tegmen, sinodu287
288
Otolaryngology– Head and Neck Surgery March 2001
MAGLIULO
Fig 1. Modified retrosigmoid dissection shown to illustrate the measurements taken (right ear). Measurements are shown in Table 1.
Fig 2. Translabyrinthine dissection shown to illustrate the measurements taken (right ear). Measurements are shown in Table 1.
ral angle, and the posterior fossa anterior, and 3 cm posterior to the sigmoid sinus were exenterated. The sigmoid sinus was skeletonized until the jugular bulb was identifed. The exposure of the 3 semicircular canals was accomplished next. This was performed using the horizontal canal as the initial landmark. The posterior semicircular canal was then resected. The extradural identification of the internal auditory canal was achieved removing all bone (180 degrees) between the jugular bulb and the area of the resected posterior canal. This maneuver required sacrificing the endolymphatic duct in order to enlarge the area of the dissection. It should be pointed out that it is facilitated positioning the back of the suction tube on the totally skeletonized surfaces of the sigmoid sinus and retrosigmoid dura and applying a slight pressure. In some temporal bones, a high jugular bulb hindered extradural removal of bone from around the internal auditory canal. In these cases, the exposure of the internal canal was obtained by totally removing the bone from the jugular bulb and compressing it in order to increase its distance from the internal canal. After the dissection, several measurements were estimated. In detail, the distance between the dome of the jugular bulb and the inferior portion of the porus acousticus (A) and inferior limit of the exposed internal auditory canal (A1) together with that between the anterior edge of the sigmoid sinus (in front of the porus) and the superior portion of the porus (B) and the superior limit of the exposed internal canal (B1) were measured using a millimeter rule with a 2-point needle caliper (Fig 1). The dissection was then converted to a translabyrinthine approach in order to compare the differences concerning the exposure of the fundus of the internal canal. Further measurements were taken between the inferior and superior portion of the internal canal and the jugular bulb dome (A2) and the sig-
moid sinus (B2) (Fig 2). Statistical significance of the data was calculated using the mean, the standard deviation, and multiple t tests. RESULTS
The measurements taken from the temporal bones dissected using the partial labyrinthectomy are summarized in Table 1. In all of the bones, the approach allowed the complete exposure of the porus acousticus. The distance between the porus and the dome of the jugular bulb showed a range between 2 and 12.5 mm. This datum confirms the variable anatomic position of the jugular bulb (Figs 3 and 4) previously observed in other studies.12-14 However, it does not influence the possibility to safely and completely expose the porus. The inferior portion of the fundus of the internal auditory canal was reached in all of the temporal bones. The relationship between the sigmoid sinus and the porus had less variability (see Table 1). The mean distance of the superior limit of the canal from the sigmoid sinus resulted in 18.4 mm ranging from 12 to 23.5 mm. Table 2 focuses on the measurements taken in relation to the translabyrinthine approach. The comparison with the previous data showed the same values of the measurements taken in the infralabyrinthine compartment (inferior portion of the porus and of the fundus) and in the superior portion of the porus. The difference between the translabyrinthine and partial labyrinthectomy approaches is delineated by the numbers regarding the measurements between the sigmoid sinus and the superior limit of the canal. Its mean value was 19.5 mm with a difference of 1.1 mm between the 2 approaches.
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Fig 3. Modified retrosigmoid dissection (left ear). Jugular bulb is quite far from the IAC.
Fig 4. Modified retrosigmoid dissection (left ear). Note jugular bulb rising the level of the IAC.
It represents an area, even if limited, in the fundus of the internal canal not accessible despite the sacrifice of the posterior semicircular canal.
these potential disadvantages. Their anatomic study demonstrated that the falciform crest creates an obscure recess of 1.82 × 2.33 mm. This does not allow complete visualization of the internal canal. Its clinical significance regarding potential recurrences is debated and not well defined. A review of the literature indicates that only a few series2,4 have evaluated the incidence of recurrence following the middle fossa approach with variable and not comparable findings due to the heterogeneous population of patients examined (small and large acoustic neuromas). However, the existence of this blind area undoubtedly represents an increased risk to leave tumor especially when it arises from the inferior vestibular nerve. In an attempt to overcome the limitations of the retrosigmoid and middle fossa approaches, many suggestions have been proposed. For the retrosigmoid approach, these include the drilling of the limb of the internal canal up to the level of the vestibule, the use of specific landmarks (singular canal, vestibular aqueduct, cochlear aqueduct), the preoperative CT evaluation of the tumor extension in the IAC as well as of the IAC length, and the use of endoscopes,3,16,17 while the exposure of the middle fossa approach is enlarged through the complete skeletization of the superior semicircular canal, of the geniculate ganglion, and of the horizontal and meatal facial nerve segments.15 All of these procedures minimize the risk of recurrences, but they are not able to completely prevent them. Thus, other authors have advocated combining the traditional techniques with a partial labyrinthectomy. There are numerous reports in the literature demonstrat-
DISCUSSION
The ideal surgical approach devoted to hearing preservation in acoustic neuroma surgery should provide a thorough exposure of the intrameatal tumor extension as well as of that of the cerebellopontine angle. Furthermore, the procedure should be achieved with minimal disturbance of the anatomic structures of the internal canal and should be easily reproduced by other surgeons with minimal technical difficulty and limited operating time. Traditional techniques have met these objectives with varying degrees of success. The retrosigmoid approach fails to reach the distal portion of the IAC when hearing preservation is attempted. It has been described as a blind recess approximately 2 to 3 cm deep that impedes the exposure of the entire length of the canal unless the labyrinth is opened. This anatomic limitation suggested contraindications to the use of this route if the acoustic neuroma involves the fundus due to high risk of tumor recurrence. The recurrence rate, as reported in large series, varies from 1.8% to 2.4%.1,3,5,7 The middle fossa approach is considered an alternative route. This procedure has been recommended for its ability to accomplish the exposure of the entire IAC. However, this assumption has been contradicted by the clinical observations of Sanna et al2 who noted some difficulty in visualizing the medial aspect of the fundus in most cases. Furthermore, Haberkamp et al15 confirmed
290
Otolaryngology– Head and Neck Surgery March 2001
MAGLIULO
Table 1. Distance taken between the inferior limit of the porus acousticus (A), the inferior limit of the fundus of the IAC (A1) and the dome of the jugular bulb. B and B1 indicate the measurements between the sigmoid sinus and the superior limit of the porus acousticus and the superior limit of the fundus of the IAC respectively; modified retrolabyrinthine approach Mean
A A1 B B1
5.8 mm 9.2 mm 14.9 mm 18.4 mm
SD (±)
Range
2.9 3.2 3.1 3.8
2-12 4.3-16 14.5- 19 12-23.5
Table 2. Distance taken between the inferior limit of the porus acousticus (A), the inferior limit of the fundus of the IAC (A2) and the dome of the jugular bulb. B and B2 indicate the measurements between the sigmoid sinus and the superior limit of the porus acousticus and the superior limit of the fundus of the IAC respectively; translabyrinthine approach
A A2 B B2
Mean
SD (±)
Range
5.8 mm 9.2 mm 14.9 mm 19.5 mm
2.9 3.2 3.1 4.2
2-12 4.3-12 14.4-19 17-24.7
ing that the ablation of the posterior or superior semicircular canals is compatable with preservation of hearing. McElveen et al9 reported that a modified translabyrinthine approach with the removal of all 3 semicircular canals can afford hearing preservation if the vestibule is filled with fluid and plugged. Molony et al10 described the sacrifice of the posterior canal to improve the exposure of the retrolabyrinthine approach and that of the superior canal to extend the middle fossa approach allowing the removal of lesions of the internal canal directed toward the cerebellopontine angle for more than 0.5 cm. Arriaga and Gorum11 proved the feasibility of combining the standard retrosigmoid approach with the removal of the posterior semicircular canal. They offered these techniques to patients with tumors situated in the fundus of the internal canal and extended more than 0.6 cm into the cerebellopontine angle. The combined retrosigmoid-partial labyrinthectomy approach embraced the advantages of both middle fossa and standard retrosigmoid approaches. Although the preliminary clinical outcomes were successful, no anatomic data were available about the possibility that a blind area still remains.
The present study was specifically designed to investigate this topic. Our findings showed that there is a pocket at the fundus of the IAC that cannot be approached with a retrolabyrinthine modified technique. This recess is situated only in the superior portion of the IAC, and its mean width is 1.1 mm. This value is smaller than that observed in the classic retrosigmoid approach and in the middle fossa approach indicating an exposure of the lateral extent of the canal almost similar to that of the translabyrinthine approach. In detail, no differences were detected between these 2 approaches for the inferior portion of the IAC. Despite some temporal bones exhibited a high jugular bulb in proximity to the internal meatus, the extradural dissection was always accomplished. The risk of leaving tumor in the fundus, thus, relates to acoustic neuromas of the superior vestibular nerve. However, it should be emphasized that the intradural resection affords a cleavage plane between the facial nerve located superiorly in the canal. In our experience, a round knife, positioned over the facial nerve and directed superiorly, encountered the bony area of the transverse crest allowing the removal of the tumor with a reasonable degree of safety.18 Clinically, previous studies9-11,18 confirm that partial labyrinthectomy does not mean additional morbidity and allowed to preserve a useful hearing. It is important to point out that this combined approach is relatively easily reproduced providing similar successful rates of hearing preservation. Moreover, it did not significantly increase the operating time. The adjunctive time necessary for partial labyrinthectomy did not exceed 1 hour, which is acceptable from an economic viewpoint as well. An advantage due to extradural dissection is represented by prevention of the potential aseptic meningitis and postoperative headaches. It is well known that these complications can be linked to the presence of bone dust in the cerebellopontine angle following the drilling of the posterior limb of the internal canal. The major disadvantage was the need to transect the endolymphatic duct with the potential risk of a postoperative hydrops. Previous studies on an experimental primate (macacus rhesus) model19 proved that no longterm loss of cochlear function developed after resection of the endolymphatic duct. These findings cannot be directly applied to humans so that many questions remain on this particular aspect. In summary, the modified retrolabyrinthine approach provided an adequate inferior exposure of the IAC. A superior blind area, smaller than that of the traditional retrosigmoid and middle fossa techniques, could not be completely approached via this route. We believe that this approach can be considered as an alternative technique in tailored cases especially for tumors involving
Otolaryngology– Head and Neck Surgery Volume 124 Number 3
the inferior vestibular nerve and reaching the fundus of the internal auditory canal. The ideal size of the tumor should not exceed 2 cm. Further research should be done to evaluate the hearing results in large series of patients with longer follow-up. REFERENCES 1. Domb GH, Chole RA. Anatomical studies of the posterior petrous apex with regard to hearing preservation in acoustic neuroma removal. Part I. Laryngoscope 1980;90:1769-76. 2. Sanna M, Zini C, Mazzoni A, et al. Hearing preservation in acoustic neuroma surgery: middle fossa versus suboccipital approach. Am J Otol 1987;8:500-6. 3. Silverstein H, Norrell H, Smouha E, et al. The singular canal: a valuable landmark in surgery of the internal auditory canal Otolaryngol Head Neck Surg 1988;98:138-43. 4. Shelton C, Brackmann DE, House WF, et al. Middle fossa acoustic tumor surgery: results in 106 cases. Laryngoscope 1989;99:405-8. 5. Thedinger BS, Whittaker CK, Luetje CM. Recurrent acoustic tumor after a suboccipital removal. Neurosurgery 1991;29:681-7. 6. Kanzaki J, Ogawa K, Yamamoto M, et al. Results of acoustic neuroma surgery by extended middle cranial fossa approach. Acta Otolaryngol Suppl 1991;487:17-21. 7. Roberson JB, Brackmann DE, Hitselberger WE. Acoustic neuroma recurrence after suboccipital resection: management with translabyrinthine resection. Am J Otol 1996;17:307-11. 8. Parnes LS, McClure JA. Posterior semicircular canal occlusion for intractable benign paroxysmal positional vertigo. Ann Otol Rhinol Laryngol 1990;99:330-4. 9. McElveen JT, Wilkins RH, Erwin AC, et al. Modifying the translabyrinthine approach to preserve hearing during acoustic tumor surgery. J Laryngol Otol 1991;105:34-7.
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