Temporal bone CT scan in children: Evaluation of the acanthiomeatal baseline value and scanner appearances of congenital deafness

Joseph Francis NWATSOCK (jfnwatsock@yahoo.ca)
Radiology and radiation oncology, Yaoundé I
December, 2013


Introduction: Computed Tomography (CT) scan of temporal bone in flexed neck protocol (FNP) is unfortunately often associated with incidental exposure of eye lenses and excessive irradiation of this sensitive organ, with a real risk of radiation-induced opacities. Acanthiomeatal baseline (AMB) is a feasible alternative, but remains insufficiently evaluated in living patients. Our study aimed, firstly to assess the value of AMB by comparing eye lens irradiation and image quality, and secondarily to correlate the CT abnormalities found in congenital deafness with type and severity of hearing loss.

Patients and methods: We conducted a two phase's comparative study with 87 children over a total period of 44 months. In the retrospective phase of the study, we recorded the clinical and FNP temporal bone CT scan data of 72 children with congenital deafness from January 2010 to December 2012 (Group 1). The eye lens radiation doses for this phase were calculated using the Monte Carlo formula. Prospectively, from January to July 2013, we performed 15 AMB temporal bone CT scan (Group 2) on the same General Electric® 64-slice Discovery 750HD scanner, with the same parameters as low as reasonably achievable. The lens dose in this phase was measured using thermoluminescent dosimeters (TLD) placed on the closed eyelid. Three radiologists performed the analysis of images. The data were compared and correlated using Mann–Whitney, Fischer, Chi-square and Pearson’s linear regression tests with P values <0.05 considered significant.

Results: Patients of the group 1 were aged 4.6 ± 3.2 years (1 month – 14 years) versus 7.8 ± 4.4 (3 - 12 years) in group 2, with respectively 50% versus 0% of direct lenses exposure (p = 0.03). The mean values of CTDIvol and DLP were respectively 33.5 ± 7.2 mGy and 213 ± 48.9 mGy.cm without significant difference between the two groups (p=0.79 and 0.28). Concerning the primary endpoint, the lens dose was significantly lower in group 2, with a reduction of 78.7%, from 11.25 mGy in group 1 to 2.4 mGy in group 2 (p = 0.04). This eye lens dose was strongly correlated to the distance between the scanned volume and the lens (R = -0.99; p < 0.001). And this distance was significantly higher in group 2 (p=0.04). For the secondary endpoint, the images obtained from both baselines were of equal quality and were useful for imaging diagnosis. In these images, the lesions predominated in presence of a moderate perceptive deafness. Four cases of external ear aplastic and tympanic cavity abnormalities were most common in conductive hearing loss (82.6%), while 66.0% of lesions in the sensorineural deafness were of vestibular origin. Twenty-three cases of infraclinical serous otitis media with tympanic cavity fluid filling were found in patients supposed to have sensorineural hearing loss and were significantly associated with worsening of deafness (p < 0.01). Dilatation of the vestibular aqueduct was the most common abnormality in the inner ear (12 cases). Twenty-two cochlear abnormalities were found, including eight cases of Mondini dysplasia. Two patients with complete deafness had a small internal auditory canal due to an absence of the cochlear nerve.

Conclusion: The proposed AMB protocol for temporal bone CT scan is safely feasible, and reduces significantly the eye lenses dose by a 1/5 factor, without any deterioration of image quality. The lesions found in this series corroborate the literature data. CT scan revealed silent cases of fluid fillings of the tympanic cavity associated with sensorineural hearing loss. We recommend thus the use of acanthiomeatal baseline for all compliant children and a systematic research of serous otitis media in children with profound congenital sensorineural hearing loss.

Key words: CT scan temporal bone, Acanthiomeatal baseline, Pediatrics, Congenital deafness, Lens dose reduction, hearing impairment