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Current Neurobiology

Variations of sphenoid sinus and their impact on related neurovascular structures

Author(s): H. Mamatha, G. Saraswathi, L.C. Prasanna

Vol. 1, No. 2 (2010-10 - 2011-03)

H. Mamatha1, G. Saraswathi2, L.C. Prasanna3

(1)Department of Anatomy, Kasturba Medical College, Manipal, Karnataka, India
(2)Department of Anatomy, J.S.S. Medical College, Mysore. Karnataka, India
(3)Department of Anatomy, J.J.M.M.C., Davangere, Karnataka, India

Abstract

The sphenoid sinus is deeply seated in the skull and is the most inaccessible paranasal sinus intimately related to numerous vital neural and vascular structures. This work determines the incidence of the different anatomical variations of sphenoid sinus as detected by CT scan and their impact on related neurovascular structures, for the safe removal of inter sphenoid and pituitary lesions. The CT scan of 20 patients were reviewed regarding the dif-ferent anatomical variations of the sphenoid sinus: degree of pneumatisation, protrusion of internal carotid artery (ICA), optic nerve (ON), and dehiscence of the walls of ICN and ON, and the septation pattern. There were 10 cases with protrusion and 9 cases of dehiscence of the bony wall on ICA, 13 cases of optic nerve protrusion and 10 cases of dehiscence. Sellar pneumatisation was present in majority of the patients (55%), with 5 patients having presel-lar (25%) and 4 patients having post sellar pneumatisation (20%). Different anatomical configurations of the sphenoid sinus can seriously affect the access to the sella via the nose. The surgeon should be aware of these findings preoperatively to reach the sella safely and effectively.

Key words: Sphenoid sinus, skull. Neural, vascular, carotid artery, optic nerve, pituitary lesion
Accepted June 24 2010

Introduction

Sphenoid sinuses are the most inaccessible paranasal sinuses and are surrounded by significant anatomical structures such as the orbit and its contents, cavernous sinus and ICA and the anterior cranial fossa. Only thin plates of bones separate these structures from the sphenoid sinus [1].

Pneumatisation of these irregular cavities ranged from their absence to extensive. According to the extent of sinus pneumatisation, the bone covering the carotid arteries and optic nerves can be thin or even absent, making these structures susceptible to iatrogenic injury[2].

Injury to internal carotid artery or optic nerve is a serious complication of transphenoidal surgery[3]. The different routes to the sella include transethmoidal, transnasal, transseptal, whether microscopic or endoscopic, ulti-mately pass through the sphenoid sinus to reach the sella.

Therefore the anatomical variations of the sphenoid sinus have major impact on the surgical access and the possibility of complications [4].

Computed tomography is the most precise imaging technique to demonstrate paranasal sinuses. Axial and coronal views may be useful for delineating the anatomical landmarks of the sinonasal cavity, but coronal scans shows progressively deeper structures as they are encountered by the surgeon during the operation[2].

The aim of the study is to evaluate the incidence of the different anatomical variations of the sphenoid sinus that are relevant to trans-sphenoid pituitary surgery.

Materials and Methods

This prospective study comprised 20 paranasal computer-ized tomography scan of South Indian patients attending E.N.T.Department of our medical college between May 2008 to May 2009.

Patients aged between 16 to 50 years, who are subjected to radiological investigations after a clinical examination and diagnosis is sinusitis were included for the study. Patients with prior sinus surgery, sinonasal tumors, facial trauma and patients younger than 16 years were excluded because according to Gray, the extension of the nasal cavity into the body of the sphenoid bone to form the sphenoid sinus is present before birth but does not reach its full extension until adolescence[8].

For the tomographic studies, systemic studies of the nasal sinus region were performed in coronal scans of all cases. In all the patients, the protrusion of internal carotid artery (ICA) and optic nerve (ON), dehiscence of the bony wall of ICA and ON, degree of pneumatisation,presence or absence of septa and or accessory septa were noted.

In coronal sections, protrusion of ICA and ON was determined by finding any degree of protrusion of the structures into the sinus cavity and dehiscence is defined as absence of visible bone density separating the sinus from the concerned structure. Whenever a clear decision between very thin wall and total dehiscence was not feasible the results were accepted as dehiscence.

Results

Internal carotid artery ICA produced a definite bulge in the supero-lateral wall of the sinus in 10 (50 ) patients, of which 3 (15) were bilateral, 6 (30%) were left sided, and one on right side. A definite dehiscence of the bony wall was seen in 9 (45 ) cases, of which 3 (15) were bilateral, 6 (30%) were on the left and one on the right.

Optic nerve

The protrusion of the optic nerve was observed in 13 (65%) patients. Protrusion were bilateral in 9 (45%) pa-tients,while in 2 (10%)patients on the right and 2 (10%) on the left. Dehiscence of the bony wall of the optic canal was observed in 10 (50%) patients, of which and (40%) were bilateral and 2 (10%) on the left side.

Regarding the degree of pneumatisation of the sphenoid sinus, there were 6 cases with presellar pneumatisation (25%), 11 patients with sellar type (55%), and 4 patients with post sellar pneumatisation (20%).

Table 1. Anatomic variants of sphenoid sinus in the present study

Characteristics Bilateral No. (%) Unilateral Total No. (%)
Rt Side No. (%) Lt Side No. (%)
1.Protrusion ICA ON 3 (15%) 9 (45%) 1 (5%) 2 (10%) 6 (30%) 2 (10%) 10 (50%) 13 (65%)
2. Dehiscence ICA ON 3 (15%) 8 (40%) 0 0 6 (30%) 2 (10%) 9 (45%) 10 (50%)

Coronal CT image

Figure 1. Coronal CT image showing multiple septation with dehiscence of internal carotid artery (ICA)

Coronal CT image

Figure 2. Coronal CT image showing Dehiscence of optic nerve (ON) iImpact of sphenoid sinus variation on neurovascular structures.

Coronal CT image

Figure 3. Coronal CT image showing Extensive sinus Pneumatisation with protruding internal carotid artery (ICA)and optic nerve (ON)

Discussion

Sphenoid sinus is extremely variable in size, shape and relation to the sella. A detailed description of all possible variations of the sphenoid sinus were presented by Renn and Rhoton [5].

Internal carotid artery

Hewadi et al., reported that the internal carotid artery was protruded into the sinus cavity in 41% of patients, and dehiscence of the artery in 30%[2]. Sirikei et al., encountered least incidence of 26.1% protrusion of ICA and Sareen et al found dehiscence of the carotid artery in 5%. Whereas higher incidence of protrusion of ICA was re-ported by Sethi et al., 93% and dehiscence by Davoodi et al.,41.95%[2,3]. In this study, we found that protrusion of internal carotid artery into the sphenoid in 50% of patients, and dehiscence of the artery in 45%. The discrep-ancy between these prevalence rates may be due to different techniques or else it may reflect ethnic differences between the populations[2,3].

If the surgeon is unaware of dehiscence or protrusion of the artery, fatal hemorrhage can occur because it is hardly possible to control the bleeding from a ruptured ICA within the sphenoid sinus.

Optic nerve

Previous studies reported a wide range of protrusion rates of 8 to 70%. Bademci et al., reported 34.4% of protrusion of the ON into sinus cavity and Fuji et al., found only 4% of optic nerves were dehiscent if bone in the lateral sphe-noid[2]. Higher rates of protrusion of ON was reported by Davoodi et al., 36.45%[3] . In the present study, we found that the protrusion of the optic nerve was seen in 65% and dehiscence of the optic nerve in 50% of the pa-tients. These were obviously very high and most likely explained by our criteria for defining protrusion and dehiscence.

The optic canal is the place where optic nerve is least nourished, which makes it very susceptible to injury. Hence compression of the optic nerve can cause ischemia and venous congestion of the nerve. Optic nerve injury can occur in case of protrusion or dehiscence either due to surgical trauma or as a complication of sinus disease. If the surgeon damages the nerve within the sinus, the risk of blindness is high. Moreover, visual deficits may result from sphenoid sinus infection or from a mucocele com-pressing the optic canal or nerve.

The degree of pneumatisation of the sphenoid was the prime concern for accessing the sella[4]. Type of sphenoid sinus pneumatisation depends on the position of the sinus in relation to the sella turcica. In the present study, the most common type of pneumatisation of the sphenoid sinus was the sellar type 55%, followed by presellar and the least encountered was post sellar with 20%. Similar observations were noted by Hamid O. et al [4]. It is comparatively lesser than the earlier observations of Ro-mano A. et al., Odowy OE et al., Banna M et al.,[5,6,7].

The septa of the sphenoid sinus were found to be variable. In most cases, the septum deviates quite laterally and terminates on the carotid artery [4]. Multiple septa were found in half of the cases, these could have been transverse, or vertical and some were difficult to remove. In this situation it is wise to use extreme caution while r-moving the septum in order to prevent accidental and dis-astrous injury to the carotid artery.

Conclusion

In order to avoid morbid consequences during surgery, it is imperative that clinicians determine the location and extent of the walls of the sphenoid sinus and its relationship into adjacent vital structures whenever transsphenoid pituitary surgery is contemplated.

References

  1. Bademci G, Unal B. Surgical importance of neurovascular relationships of paranasal sinus region. Turkish Neurosurgery 2005;15(2):93-6.
  2. Hewaidi GH,Omami GM.Anatomic variation of sphenoid sinus and related structures. In Libyan population Hewaidi GH, Omami GM. Anatomic variation of sphenoid sinus and related: CT scan study. Libyan J Med, AOP : 080307. www//ljm.org.ly page 128-133.
  3. Davoodi M, Saki N, Saki G, Rahim F. Anatomical variations of neurovascular structures adjacent sphe-noid sinus by using CT scan. Pak J Biol Sci 2008;1296):522-5.
  4. Hamid O, Filky LE, Hassan O, Kotb A, Fiky SE. Anatomic variations of the sphenoid sinus and their impact on trans-sphenoid pituitary surgery. Skull Base 2008;18:9-16.
  5. Romano A, Zuccarello M, Loveren HR, Keller JT. Expanding the boundaries of the transphenoidal approach : A microanatonmic study. Clin Anat 2001;14:1-9.
  6. Idowu OE, Balogun BO, Okoli CA. Dimensions, septation, and pattern of pneumatisation of the sphenoid si-nus. Folia Morphol 2009;68(4):228-32.
  7. Banna M, Olutola PS. Patterns of pneumatisation and septation of the sphenoid sinus. J Can Assoc Radiol 1983;34(4):291-3.
  8. William PL. Gray’s anatomy 38th edn., London :ELBS with Churchill Livingstone 1995:585-88.

Correspondence:
L.C. Prasanna

Department of Anatomy, J.J.M. Medical College
Davangere, Karnataka, India
Phone: +91-9611109065
E-mail: anatomylcp(at)yahoo.com

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