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Transnasal approach to the vidian nerve : anatomical considerations.
Vasomotor rhinitis is a non-infective, non-allergic condition characterised by profuse rhinorrhoea and sneezing, with or without nasal obstruction, occurring in attacks which may be either paroxysmal or parennial. The running of the nose and sneezing may be so severe as to even disable the patient. Malcomson[4] showed conclusively that stimulation of the parasympathetic or interruption of the sympathetic nerve supply to the nasal mucous membrane caused vasodilatation, hypersecretion and sneezing, and hence it is reasonable to assume that under normal conditions there exists a balance between the two systems. The symptoms of vasomotor rhinitis seem to be due to an imbalance between these two systems. While the causes of vasomotor rhinitis could be psychogenic or due to drugs like methyldopa or rauwolfia, or endocrine, the logical step in the treatment of idiopathic nasal vasomotor imbalance is vidian neurectomy, when it is found that all other available methods of treatment have failed and the symptoms are severe enough to justify surgery. The principle underlying vidian neurectomy is to sever the preganglionic fibres that reach the sphenopalatine ganglion through the vidian nerve. Several routes have been described to approach the deep seated vidian nerve: (1) Transantral (a) Classic,[2] (b) Subperiosteal,[7] (2) Transeptal,[5] (3) Transpalatal,[1],[6] (4) Transethmoidal,[9] and (5) Transnasal.[3],[8] The multiplicity of these routes suggests that none of them is entirely satisfactory. This is because the vidian nerve is deep seated, at the base of the skull, behind the pterygopalatine fossa and lateral to the nose. This is an area which is anatomically difficult to reach and is surrounded by numerous important structures. We practise and advocate a direct transnasal approach[3] to the vidian nerve to effect a preganglionic section of the nerve. Having operated on 357 cases by this route, we are convinced that this is a simple and direct approach to the vidian nerve because of the special anatomical relationship between the sphenopalatine foramen on the lateral wall of the nose and the mouth of the vidian canal on the posterior wall of the pterygopalatine fossa. This paper deals with these anatomical considerations as applied to vidian neurectomy.
The anatomy of the vidian canal, the sphenopalatine foramen, and the pterygopalatine fossa was studied with special reference to the transnasal route of approach to the vidian canal. This was accomplished by means of a study of 30 dry skulls sectioned at various levels in the sagittal, coronal and horizontal planes. This was supplemented by dissections of 30 fresh cadaver specimens to determine the gross anatomy of the region. The findings were further corroborated by observations made on 357 patients who underwent a transnasal vidian neurectomy.
The following description of anatomy of the pterygopalatine fossa, although well known, is included here because of its relevance to our observation of certain anatomical features in this region which are mentioned later. The pterygopalatine fossa is bounded anteriorly by the posterior surface of the maxilla and posteriorly by the anterior surface of the pterygoid process, which has two openings, namely, the foramen rotundum and the vidian canal [Fig. 1A]. Medially, it is bounded by the vertical plate of the palatine bone. This has a notch on its upper border which is bounded anteriorly by a larger orbital process and posteriorly by a short sphenoidal process [Fig. IA]. This sphenopalatine notch is converted into the sphenopalatine foramen by the under surface of the body of the sphenoid. It transmits the sphenopalatine artery and nerves from the pterygopalatine fossa into the nose and is the main focus of interest in the transnasal vidian neurectomy. The vidian nerve enters the posterior opening of the vidian canal and traverses the canal situated on the floor of the sphenoid sinus where it often produces a ridge. The vidian canal is directed anteriorly and slightly laterally (about 5° inclination) and opens into the posterior wall of the pterygopalatine fossa (See [Fig. 1A] on page 212A). Its anterior opening is funnel shaped, and is 8-9 mm below and medial to the anterior opening of the foramen rotundum. Thus there is a distinct bony ridge between the two foramina (See [Fig. 2] on page 212B). After synapsing in the sphenopalatine ganglion, the postganglionic fibres are distributed to the mucous membrane of the nose, the palate, and the lacrimal gland. We have observed that the sphenopalatine foramen and the mouth of the vidian canal have a very consistent and special anatomical relationship to each other. The sphenopalatine foramen and the funnel shaped mouth of the vidian canal are always situated in the same horizontal plane, the former being in the medial wall and the latter being in the posterior wall of the pterygopalatine fossa [Fig. 1A]. They are separated only by a few millimetres of bone as seen in [Fig. 1A] (See page 212A). It has also been our observation that the sphenopalatine foramen is situated in level with, and posterior to the bony middle nasal turbinate. The posterior end of the bony attachment of the middle turbinate presents a sharp ridge of bone called the ethmoidal crest (See [Fig. 3] on page 212B). We find that this crest is a very constant feature and is always located at the antero-inferior margin of the sphenopalatine foramen. This constancy of location makes it an invaluable guide in locating the sphenopalatine foramen during surgery. Having identified this foramen, one has only to advance a probe through it, hugging the medial wall of the pterygopalatine fossa, to reach the funnel shaped opening of the vidian canal (See [Figs. 1B] and 2 on page 212A and 212B). It may be stressed here that a probe so advanced through the sphenopalatine foramen will invariably enter the mouth of the vidian canal. This is the essence of the transnasal vidian neurectomy. In the transantral operation,[2],[7] the maxillary antrum is entered from its anterior wall by a preliminary Caldwell-Luc operation. The posterior wall of the antrum is opened to reach the pterygopalatine fossa. To reach the vidian canal, one has to encounter and pass beyond the veins, arteries, nerves and fat in the pterygopalatine fossa, all of which make the operation very difficult. In the direct transnasal operation,[3],[8] the approach to the vidian nerve is through the sphenopalatine foramen. The middle turbinate is lifted by a Killian's speculum. The mucosa under the posterior end of the middle turbinate is incised and elevated to expose the ethmoidal crest. Postero-superior to this is the sphenopalatine foramen from which the sphenopalatine vessels may sometimes bleed. This bleeding is easily controlled by cauterization. A probe is passed through the sphenopalatine foramen and advanced postero-laterally till the anterior funnel shaped opening of the vidian canal is reached. Any further advancement of the probe is stopped by the lateral wall of the anterior opening of the vidian canal. The vidian nerve is then cauterized. The success of the preganglionic section of the vidian nerve is confirmed by decreased lacrimation on the operated side as tested by the Schirmer's test.
The most popular approach to the vidian nerve is the transantral route. This, however, is fraught with certain difficulties viz. damage to the maxillary artery or nerve, a risk of ophthalmoplegia due to deep penetration by the probe, and other complications like infra-orbital anaesthesia and neuralgia and maxillary sinusitis. The transpalatal route is cumbersome as it involves elevation of a palatal flap and bony dissection. Technical difficulties arise when extensions of the sphenoid sinus engulf the pterygoid canal. There is a risk of palatonasal fistula and injury to the internal carotid artery. In addition, there is a relatively long convalescence. The transseptal approach involves a lot of stretching and manipulation of tissues with an added risk of septal perforation. In the preganglionic section by the transnasal route, a direct approach to the vidian canal is possible as the sphenopalatine foramen and the funnel shaped anterior opening of the vidian canal are in the same horizontal plane. The identification of landmarks is easy due to the clear-cut ethmoidal crest and the presence of the sphenopalatine vessels which pinpoint the position of the sphenopalatine foramen. The contents of the pterygopalatine fossa are bypassed, thus precluding any damage to the mixillary artery, the maxillary nerve and the sphenopalatine ganglion. Even the fat in the fossa does not feature in the dissection. There is never a risk of the probe entering deep into the vidian canal due to obliquity in the directions of the probe and the vidian canal [Fig. 1B]. Because of this obliquity, a rigid probe will never be able to enter weep into the vidian canal. In the transantral operation, on the other hand, the direction of the vidian canal and that of the probe are almost in the same straight line [Fig. 1B], and hence the risk of the probe going deep into the vidian canal. Cauterization with such a deep seated probe will lead to damage to the abducent nerve intracranially. This is the cause of ophthalmoplegia and diplopia due to sixth nerve paralysis which sometimes occurs as a complication of the transantral route due to spread of the cautery burn. The whole procedure of transnasal vidian neurectomy takes less than 15 minutes for a bilateral neurectomy. It is done as an outpatient procedure under local anaesthesia, no nasal tamponade being necessary post-operatively. Further, it can be repeated easily if there is a recurrence of symptoms. In conclusion, we would like to state that the clarity of landmarks and the ease with which the vidian canal can be approached by a direct transnasal route via the sphenopalatine foramen make it the approach of choice for vidian neurectomy.
We are grateful to the Dean, Seth G.S. Medical College and K.E.M. Hospital for permission to publish this paper.
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