Asymmetric agenesis of the mastoid antrum in a neonate.

Abstract

The authors report a case of asymmetric development of the mastoid antrum in a neonate. The lack of mastoid development most likely represents a congenital maldevelopment. Agenesis of the mastoid antrum has surgical implications, as failure to recognize its absence can result in disorientation during surgery, with potential to injure adjacent structures.

Introduction

The mastoid antrum is a relatively constant bilateral structure, even in cases of chronic otitis media with sclerotic mastoids. (1) Absence of the mastoid antrum is a rare clinical finding and is usually syndromic. (2) To our knowledge, this is the first published report of a case of agenesis of the mastoid antrum in a neonate.

Case report

A female infant was delivered by cesarean section because of failure of progression of the pregnancy at a gestational age of 41 weeks, 1 day. She was born to a healthy 27-year-old woman with a normal prenatal course and no significant medical, family, or social history. Physical examination of the infant revealed multiple anomalies, including right-sided grade III microtia with absence of the external auditory canal, multiple preauricular tags and pits, hypoplasia of the right mandible, and an absent medial portion of the clavicle. There were no ocular findings or vertebral dysplasia. The anomalies were all unilateral (right-sided). The facial nerve was intact bilaterally. Magnetic resonance imaging of the brain was normal, without evidence of dysgenesis. Computed tomography (CT) of the temporal bones without contrast, obtained on day 3 of the infant's life, demonstrated on the right side what seemed to be a tiny, narrow middle ear cleft filled with soft tissue. No ossicular chain could be identified. The mastoid antrum was absent, with the mastoid process represented by a solid block of bone (figure). The cochlea, semicircular canals, and internal auditory canal were normal.

In contrast, the left middle ear was well developed and aerated, with a normal ossicular chain, and the mastoid antrum also was well developed and aerated. The left inner ear and internal auditory canal were normal.

Discussion

The development of the mastoid air cell system begins in utero. Pneumatization of the middle ear begins at week 29 and continues to week 34, when the process expands posteriorly to form the mastoid antrum. (3)

The role of the mastoid antrum in the developing ear is still controversial. There are essentially two theories: the theory of precursor cells and the theory of active invasion, which is the most popular view. According to the active-invasion theory, postnatally the antrum is the focal area for expansion of the air cell system into the mastoid process and adjacent areas. This theory is based on the presence of a well-developed mastoid antrum in the neonate and assumes active invasion of the temporal bone by diverticulae from the antrum. An animal study of chimpanzees described epithelial buds from the antrum, visible at 34 weeks, as precursors of the mastoid air cell system. (3)

The other theory of development of the mastoid air cell system suggests that the precursors of the mastoid cells are present at birth as periantral, mesenchyme-filled loculi that are visible on histologic sections. (1) This theory postulates that these loculi expand as the mastoid bone grows. Mesenchyme does not have the capacity to grow after birth. Therefore, the expanding loculi become pneumatized spaces lined by thin layers of receding mesenchyme. (1) Regardless of which theory may be correct, the developmental process has been shown to be bilateral and symmetrical.

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In a histologic study of the temporal bones of newborns, conducted by Valtonen and Karmody, the mastoid antrum was noted to be present in all subjects, without significant differences for age, gender, or laterality (right versus left). (1) Diamant has similarly shown minimal variability in mastoid pneumatization in patients of all ages. (4) Valtonen and Karmody's study also revealed that the growth and pneumatization of the temporal bone remained symmetric during the first months of life. (1) Histopathologic examination demonstrated that the periantral cells were filled with mesenchyme and grew more rapidly than the other areas of the temporal bone. (1)

Saleh et al reported the asymmetric absence of the mastoid antrum in an adult with chronic otitis media. (5) The authors believed that this was a congenital problem. Absence of the mastoid antrum previously has been reported in congenital syndromes such as trisomy 13 and mandibulofacial dysostosis (Treacher Collins syndrome). (2,6) Patients with mandibulofacial dysostosis had bilateral agenesis of the mastoid antrum. (2)

The pneumatization of the temporal bone has been shown to have a Gaussian distribution, (7) with decreased pneumatization occurring either secondary to environmental insults (Wittmaack's theory) or arising from genetic influences resulting in decreased pneumatization (Cheatle's theory). (6)

Agenesis of the mastoid antrum has surgical implications because failure to recognize its absence can result in disorientation during mastoid surgery, with potential to injure the lateral semicircular canal and/ or the facial nerve. (5)

A thorough search of the literature reveals this case to represent the first report of asymmetric development of the mastoid antrum in a neonate. This child was not exposed to the outside environment, so the undeveloped antrum could not be attributed to postinflammatory changes. Rather, the lack of mastoid development most likely represents a congenital maldevelopment.

References

(1.) Valtonen H, Karmody C. Development of mastoid antrum. In: Lim DJ, Bluestone CD, Casselbrant M, et al, eds. Recent Advances in Otitis Media: Proceedings of the 6th International Symposium on Otitis Media. Hamilton, Ontario: BC Decker, Inc.; 1996:124-6.

(2.) Hutchinson JC Jr., Caldarelli DD, Valvassori GE, et al. The otologic manifestations of mandibulofacial dysostosis. Trans Sect Otolaryngol Am Acad Ophthalmol Otolaryngol 1977;84(3 Pt. 2):ORL520-8.

(3.) Sherwood RJ. Pneumatic processes in the temporal bone of chimpanzee (Pan troglodytes) and gorilla (Gorilla gorilla). J Morphol 1999;241(2):127-37.

(4.) Diamant M. Studies on causative factors of mastoid pneumatization. AMA Arch Otolaryngol 1958;68(5):587-97.

(5.) Saleh HA, Murty GE, O'Donoghue GM. Isolated agenesis of the mastoid antrum. J Laryngol Otol 1994;108(6):497-9.

(6.) Virapongse C, Sarwar M, Bhimani S, et al. Computed tomography of temporal bone pneumatization: 1. Normal pattern and morphology. AJR Am J Roentgenol 1985;145(3):473-81.

(7.) Diamant M. Mastoid pneumatization and normal curve distribution. Acta Otolaryngol 1965;60:167-74.

Rahul K. Shah, MD; Adarsh Vasanth, MD; Collin S. Karmody, MD, FRCSE

From the Division of Otolaryngology, Children's National Medical Center, The George Washington University Medical Center, Washington, D.C. (Dr. Shah), and the Department of Otolaryngology-Head and Neck Surgery, Tufts University-New England Medical Center, Boston (Dr. Vasanth and Dr. Karmody).

Corresponding author: Rahul K. Shah, MD, Children's National Medical Center, 111 Michigan Ave., NW, Washington, DC 20010-2970. Phone: [phone]; fax: [phone]; e-mail: [e-mail]