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The prevalence of an Onodi cell in adult Thai cadavers. (Original Article).


Endoscopic sinus surgery in patients who have an Onodi cell (sphenoethmoid cell) carries a high risk for optic nerve injury. We meticulously dissected 65 embalmed cadaver adult half-heads and attempted to identify an optic canal bulge in each with a nasal endoscope. Our aims were to determine the prevalence of an Onodi cell in adult Thai cadavers, to ascertain the prevalence of an overriding ethmoid cell, and to measure the length of an overriding sphenoethmoid cell's superior and posterior extensions in relation to the anterior sphenoid wall. Moreover, we attempted to determine the minimum amount of bone thickness between an Onodi cell and the optic nerve. We found that an Onodi cell was present in 39 of the 65 specimens (60.0%). We also found that an overriding ethmoid cell was present in 14 specimens, which accounted for 21.5% of the total number of specimens and 36.8% of 38 Onodi cell-positive specimens (the presence or absence of an overriding ethmoid cell was not recorded in one of the 39 Onodi cell-positive specimens). The distance of the overriding ethmoid cell's superior and posterior extensions from the anterior sphenoid wall ranged from 3 to 13 mm (median: 7) and from 4 to 16 mm (median: 9.5), respectively. Measurements of the minimum amount of bone thicknesses between each Onodi cell and optic nerve ranged from 0.03 to 0.54 mm (median: 0.08). Our study demonstrated that the prevalence of an Onodi cell in adult Thai cadavers was as great as the prevalence reported in the only other gross anatomic dissection study performed in Asia and much higher than rates generally reported in Western countries.


Endoscopic sinus surgery is widely used for the management of rhinosinusitis and nasal polyps. With a nasal endoscope, the surgeon can see the operative field, lateral nasal wall, and paranasal sinuses more clearly than is possible with the conventional headlight-and-speculum surgical technique. Nevertheless, major orbital and intracranial complications of endoscopic sinus surgery have been reported. (1-10)

Orbital complications are serious, and they can lead to blindness, particularly when the optic nerve is injured. Maniglia reported three cases of permanent blindness following intranasal ethmoidectomy. (1) Buus et al reported the case of a patient whose optic nerves on both sides were severed during an intranasal ethmoidectomy.(2) The risk of optic nerve injury during sinus surgery is increased when the procedure is performed on the posterior ethmoid and sphenoid sinuses. (11,12) During such surgery, the posteriormost ethmoid air cell can undergo extensive pneumatization and come into close contact with or encroach on the optic canal, thus creating an Onodi cell (sphenoethmoid cell). (13) An Onodi cell is defined as the posteriormost ethmoid cell that features an endoscopically identifiable bulge in the optic canal, even if it is minimal. (11) Patients who have an Onodi cell are also at increased risk for optic nerve injury during endoscopic sinus surgery.

The prevalence of Onodi cells varies greatly. The only previous gross anatomic dissection study of the prevalence of Onodi cells in Asia was conducted in Singapore by Yeoh and Tan. (12) They studied 102 half-heads from 51 cadavers and found an Onodi cell in 52 half-heads (51.0%). An Onodi cell was found bilaterally in 44 half-heads (43.1%) and unilaterally in eight (7.8%). Therefore, at least one Onodi cell was found in 30 of the 51 whole heads, for a prevalence of 58.8%. However, Yeoh and Tan did not mention what selection criteria they used in choosing their specimens.

In this article, we describe our study to determine the prevalence of an Onodi cell in adult Thai cadavers, to ascertain the prevalence of an overriding ethmoid cell, to measure the length of an overriding ethmoid cell's superior and posterior extensions in relation to the anterior sphenoid wall, and to determine the minimum amount of bone thickness between each Onodi cell and optic nerve.

Materials and methods

This cross-sectional descriptive study of embalmed heads was conducted by members of the Department of Otolaryngology and the Department of Anatomy at the Khon Kaen University Faculty of Medicine in Thailand between June 1997 and June 1998. The heads had been prepared to assist in the training of medical students. For our analysis, we selected 65 whole head, which had been obtained from 46 male and 19 female cadavers. One-half of each head was randomly selected for study. The age of the donors at the time of death ranged from 17 to 92 years (mean: 60). None of the donors had had a history of paranasal sinus disease or surgery during their lifetimes, and all had intact posterior ethmoid and sphenoid sinuses.

Each half-head was meticulously dissected for total ethmoidectomy with a drill and other instruments. A 30[degrees] nasal endoscope was used to search for an Onodi cell. All assessments of the presence or absence of an Onodi cell were performed independently by two physicians. We also measured the length of the superior and posterior extensions of the overriding ethmoid cell in relation to the anterior wall of the sphenoid sinus. When we did identify an Onodi cell, we carefully removed the medial wall of the optic canal (figure). We then measured the thickness of the removed bone at five randomly selected locations with a micrometer and recorded the thickness of the bone at its thinnest point.

We estimated that 65 specimens would provide a 90% confidence interval (CI) equal to the prevalence plus or minus 10% when the estimated prevalence was 59%, which was the rate reported by Yeoh and Tan. (12) Descriptive statistics were used for analysis. If the data showed a normal distribution, results were reported as point estimates with a 90% CI; if not, nonparametric statistics were used. Our study was approved by the ethics committee of Khon Kaen University.


We studied 30 right-sided and 35 left-sided half-head specimens. An Onodi cell was found in 39 of the 65 specimens (60.0%; 90% CI: 49 to 70%). There was no clinically or statistically significant difference in prevalence between male and female specimens or right- and left-sided specimens (table 1).

An overriding ethmoid cell was found in only 14 specimens, which accounted for 21.5% of the total number of 65 specimens (90% CI: 14 to 32%) and 36.8% of 38 Onodi cell-positive specimens (90% CI: 24 to 51%) (the presence or absence of an overriding ethmoid cell was not recorded in one of the 39 Onodi cell-positive specimens). There was no overriding ethmoid air cell in any Onodi cell-negative specimen.

The 14 overriding ethmoid cells extended superiorly over the anterior walls of the sphenoid sinuses at a distance of between 3 and 13 mm (median 7; 25th percentile: 5; 75th percentile: 10). They extended posteriorly at a distance of between 4 and 16 mm (median: 9.5; 25th percentile: 7; 75th percentile: 13).

The minimum amount of bone thickness between an Onodi cell and optic nerve was measured in 31 of the 39 Onodi cell-positive specimens (eight measurements were not taken because the bones had broken during removal) (table 2). Minimum thickness ranged between 0.03 and 0.54 mm (median: 0.08; 25th percentile: 0.05; 75th percentile: 0.13). No bony dehiscence was seen.


Sinus surgery patients who have an Onodi cell are at increased risk for optic nerve injury if the surgeon is not aware of the cell's presence. The previously reported prevalence of an Onodi cell varies greatly, ranging from 7 to 51% (table 3). (11,12,14-19) These variations might be attributable to (1) different operational definitions of an Onodi cell, (2) different sampling techniques, (3) racial factors, and (4) differences in examination methods (e.g., observations based on endoscopic or gross anatomic dissection or computed tomography [CT]).

With respect to examination methods, nasal endoscopy should be more sensitive than CT in identifying an Onodi cell because the optic canal can be magnified and visualized directly. (17) In fact, authors who have used an endoscope to look for an Onodi cell have consistently found higher prevalence rates than have those who used CT, even in Asians (table 3). For example, Aibara et al in Japan based their prevalence study on CT of live subjects rather than gross anatomic dissection of cadaver specimens, and they found a prevalence of only 7%. (16)

Although optic nerve injury is the most serious surgical risk in patients with an Onodi cell, there are other risks to vision, as well. Weinberger et al (14) reported two cases of Onodi cell sinusitis, and Ogata et al (20) reported one case of retrobulbar optic neuropathy that occurred in an isolated mucocele in an Onodi cell.

Only one previous study has reported the prevalence of an overriding ethmoid air cell; in the United States, Bansberg et al reported a prevalence of 8% based on their CT examinations. (21) The prevalence in our study was much higher (21.5%). Again, the disparity in these findings might be attributable to our use of anatomic dissection rather than CT and perhaps to racial differences.

Also of note is our finding that an overriding ethmoid cell was present in more than one-third (36.8%) of specimens that had an Onodi cell and in 0% of those that did not. We can conclude from this that any patient who has an overriding ethmoid cell on CT also has an Onodi cell.

Finally, Driben et al reported that the degree of perpendicularity of the face of the sphenoid in large part determines the presence of an Onodi cell and an overriding posterior ethmoid cell. (17)
able 1.

Onodi cell prevalence according to sex and side in 65 cadaver half-head

 Onodi cell Difference in CI *
 (n [%]) 90% CI * (90% CI)

Male (n = 46) 27 (58.7) 46 to 71% 40% (-26 to 17%)
Female (n = 19) 12 (63.2) 42 to 81%
Right side (n = 30) 19 (63.3) 44 to 80% 6% (-14 to 26%)
Left side (n = 35) 20 (57.1) 39 to 74%
Total 39 (60.0) 49 to 70%

* Confidence interval.

able 2.

Minimum width of bony thickness between the Onodi cell and optic nerve
in 31 specimens

Minimum Specimens
thickness (mm) (n [%])

<0.05 4 (12.9)
0.05 to 0.10 16 (51.6)
0.11 to 0.15 5 (16.1)
0.l6 to 0.20 2 (6.5)
0.21 to 0.25 1 (3.2)
>0.25 3 (9.7)

able 3.

Comparison of studies of Onodi cell prevalence

Author Country Type of exam

Kainz and Stammberger, 1992 (11) Austria Endoscopy
Yeoh and Tan, 1994 (12) Singapore Endoscopy
Weinberger et al, 1996 (14) U.S. CT
Jones et al, 1997 (15) U.K. CT
Aibara et al, 1997 (16) Japan CT
Driben et al, 1998 (17) U.S. CT
Arslan et al, 1999 (18) Turkey CT
Basic et al, 1999 (19) Croatia CT
Thanaviratananich et al, 2003 * Thailand Endoscopy

Author Sample size

Kainz and Stammberger, 1992 (11) 52 cadaver half-heads
Yeoh and Tan, 1994 (12) 102 cadaver half-heads
Weinberger et al, 1996 (14) 76 live subjects
 44 cadaver half-heads
Jones et al, 1997 (16) 200 live subjects
Aibara et al, 1997 (16) 200 live subjects
Driben et al, 1998 (17) 41 sinonasal complexes
 from 21 cadaver whole heads
Arslan et al, 1999 (18) 200 live subjects
Basic et al, 1999 (19) 212 live subjects
Thanaviratananich et al, 2003 * 65 cadaver half-heads

Author Prevalence

Kainz and Stammberger, 1992 (11) 42%
Yeoh and Tan, 1994 (12) 51%
Weinberger et al, 1996 (14) 8%
Jones et al, 1997 (16) 8%
Aibara et al, 1997 (16) 7%
Driben et al, 1998 (17) 7%
Arslan et al, 1999 (18) 12%
Basic et al, 1999 (19) 10%
Thanaviratananich et al, 2003 * 60%

* Present study.


The authors thank Dr. Patravoot Vatanasapt of the Department of Otolaryngology of the Faculty of Medicine at Khon Kaen University for his review of the manuscript.


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(17.) Driben IS, Bolger WE, Robles HA, et al. The reliability of computerized tomographic detection of the Onodi (sphenoethmoid) cell. AmJ Rhinot 1998;12:105-11.

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(19.) Basic N, Basic V. Jukic T, et al. Computed tomographic imaging to determine the frequency of anatomical variations in pneumatization of the ethmoid bone. Eur Arch Otorhinolaryngol 1999;256:69-71.

(20.) Ogata Y, Okinaka Y, Takahashi M. Isolated mucocele man Onodi cell. ORL I Otorhinolaryngol Relat Spec 1998;60:349-52.

(21.) Bansberg SF, Harner SG, Forbes G. Relationship of the optic nerve to the paranasal sinuses as shown by computed tomography. Otolaryngol Head Neck Surg 1987;96:331-5.

From the Department of Otolaryngology (Dr. Thanaviratananich, Dr. Kraitrakul, and Dr. Tangsawad) and the Department of Anatomy (Dr. Chaisiwamongkol), Faculty of Medicine, Khon Kaen University, Khon Kaen, Thailand.

Reprint requests: Sanguansak Thanaviratananich, MD, Department of Otolaryngology, Faculty of Medicine, Khon Kaen University, Khon Kaen 40002, Thailand. Phone: 66-43-348-396; fax: 66-43243-336; e-mail:

Originally presented at "The Nose: 2000 and Beyond," a joint meeting of the International Rhinologic Society, the American Rhinologic Society, and the Mayo Clinic; Sept. 20-23, 2000; Washington D.C.
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Title Annotation:sphenoethmoid cell
Author:Tangsawad, Watcharachai
Publication:Ear, Nose and Throat Journal
Geographic Code:9THAI
Date:Mar 1, 2003
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