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NEURORADIOLOGY Table of Contents   
Year : 1999  |  Volume : 9  |  Issue : 1  |  Page : 5-8
Anatomy of the facial nerve in the temporal bone : HRCT

1 Department of Radiodiagnosis, All India Institute of Medical Sciences, New Delhi-110029, India
2 Department of Neuroradiology, All India Institute of Medical Sciences, New Delhi-110029, India

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Keywords: HRCT, Facial nerve, Temporal bone

How to cite this article:
Chandra S, Goyal M, Gandhi D, Gera S, Berry M. Anatomy of the facial nerve in the temporal bone : HRCT. Indian J Radiol Imaging 1999;9:5-8

How to cite this URL:
Chandra S, Goyal M, Gandhi D, Gera S, Berry M. Anatomy of the facial nerve in the temporal bone : HRCT. Indian J Radiol Imaging [serial online] 1999 [cited 2020 Sep 23];9:5-8. Available from:
HRCT scanning excels in the evaluation of bone and air space anatomy and disorders of the temporal bone. Newer software reconstruction techniques that reduce the pixel size to 0.25mm, the use of thin collimation, high frequency algorithms and special filters have improved its ability to evaluate the temporal bone. MR is very helpful in the imaging of the membranous labyrinth, facial and vestibulocochlear nerves. However, insensitivity to bone and calcified structures are disadvantages to its use [1],[2].

HRCT of the temporal bone is primarily performed in the axial, coronal and occasionally coronal oblique projections at 1.0mm intervals. Sagittal images obtained by computer reformation of the raw data from serial axial sections are rarely used to supplement the above. Axial projections are obtained by serial 1mm thin sections of the temporal bone when the patient is supine, with the line joining the infra-orbital rim and external auditory meatus perpendicular to the table and a caudal gantry tilt of 30 degrees [Figure - 1]. The coronal sections are obtained with the patient in the prone position and a gantry tilt of approximately 30 degrees, perpendicular to the plane joining the inferior orbital rim and the external auditory meatus [Figure - 2] [2].

The facial nerve is the nerve of the second branchial arch. It serves several functions - motor and sensory. It supplies the striated musculature of the face, neck, and stapedius muscle of the middle ear, parasympathetic fibres to the lacrimal, submandibular and sublingual glands, seromucinous glands of the nasal cavity and it conveys taste sensations from the anterior two-thirds of the tongue, the palate and the tonsillar fossae. It also has a small cutaneous sensory component. It has three branches: the greater superficial petrosal nerve, the nerve to the stapedius and the chorda tympani [3].

After its origin in the medulla, the facial nerve proceeds forwards and laterally in the posterior fossa into the internal auditory meatus, in conjunction with the nervus intermedius (sensory component of the facial nerve) and the acoustic nerve. This intracranial segment is 23 to 24 mm in length. The first or the internal auditory segment is 7 to 8 mm in length and lies superior to the cochlear nerve passing above the crista falciformis. While within the canal, the motor root is separated from the acoustic bundle by the nervus intermedius and the two roots unite together in the canal to form the combined trunk [Figure - 3]. Lesions at this level affect facial muscles, taste, lacrimation, the stapedial reflex, and sometimes hearing and balance due to the involvement of the VIII cranial nerve.

The second or the labyrinthine segment of the nerve (3-4mm) passes forward and laterally within its own bony channel, the ' Fallopian canal More Details'. This is the narrowest part of the facial canal and extends from the internal auditory canal to the geniculate ganglion. When the nerve reaches a point just lateral and superior to the cochlea, it angles sharply forward, nearly at right angles to the long axis of the petrous temporal, to reach the geniculate ganglion. At the ganglion, the direction of the nerve reverses itself, executing a hairpin bend so that it runs posteriorly. This is the 'first genu' of the facial nerve [Figure - 4]a. The greater superficial petrosal nerve arises from the geniculate ganglion and supplies parasympathetic fibres to the lacrimal gland and seromucinous glands of the nasal cavity.

The third or the tympanic segment (12.0mm) extends from the geniculate ganglion to the second genu of the facial nerve. It passes posteriorly and laterally along the medial wall of the tympanic cavity, perpendicular to the long axis of the petrous bone [Figure - 4]a. Here it lies above the oval window and below the bulge of the lateral semicircular canal [Figure - 4]b. At the level of the sinus tympani, the nerve changes direction at the 'second genu'.

The fourth or the mastoid segment (15-20mm) extends from the second genu to the stylomastoid foramen. Here the nerve assumes a vertical position, dropping downward in the posterior wall of the tympanic cavity and the anterior wall of the mastoid to exit at the base of the skull from the stylomastoid foramen [Figure - 5]. The nerve to the stapedius muscle is a small twig given off from the facial nerve as it descends in the posterior wall of the tympanic cavity behind the pyramidal eminence [Figure - 6]. The chorda tympani originates about 5 mm above the stylomastoid foramen and eventually joins the lingual nerve to supply taste sensation to the anterior two-thirds of the tongue. Suprastapedial lesions located between the nerve to the stapedius, and the geniculate ganglion, affect facial muscles, taste and the stapedial reflex but have no effect on lacrimation, while infrastapedial lesions above the chorda tympani branch spare the stapedial reflex too. Infrachordal lesions cause facial paralysis but spare taste, lacrimation and the stapedial reflex [4] .

Facial nerve anomalies can involve the size and the course of the canal. There may be partial or complete agenesis of the facial canal and variations of the course especially of the tympanic and mastoid segments are not uncommon [2] . A good knowledge of the anatomy of the facial nerve in the temporal bone helps in proper diagnosis.

   References Top

1.Chakeres DW, Oehler M, Schmabrock P, Wayne S. Temporal bone imaging. In: Som PM, Curtin HD. Head and Neck Imaging. 3rd ed. Philadelphia: Mosby, 1996: 1319-1350.   Back to cited text no. 1    
2.Valvassori GE, Buckingham RA. Imaging of Temporal Bone. In: Imaging of Head and Neck, 2nd ed. Stuttgart: WB Saunders Co, !995: 1-44.   Back to cited text no. 2    
3.Warwick R, Williams PL. Facial Nerve. In: Gray's Anatomy, 37th ed. London: Churchill Livingstone, 1989: 1107-1111.   Back to cited text no. 3    
4.Smark A. Anatomy and diseases of the temporal bone. In: Atlas SW. Magnetic Resonance Imaging of the Brain and Spine. 2nd ed. Philadelphia: Lippincott-Raven, 1996; 998-999.  Back to cited text no. 4    

Correspondence Address:
Manorama Berry
Department of Radiodiagnosis, All India Institute of Medical Sciences, New Delhi-110029
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Source of Support: None, Conflict of Interest: None

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[Figure - 1], [Figure - 2], [Figure - 3], [Figure - 4], [Figure - 5], [Figure - 6]

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