A Start Codon Variant in NOG Underlies Symphalangism and Ossicular Chain Malformations Affecting Both the Incus and the Stapes

Abstract

We performed exome sequencing to evaluate the underlying molecular cause of a patient with bilateral conductive hearing loss due to multiple ossicular abnormalities as well as symphalangism of the fifth digits. This leads to the identification of a novel heterozygous start codon variant in the NOG gene (c.2T>C:p.Met1?) that hinders normal translation of the noggin protein. Variants in NOG lead to a spectrum of otologic, digit, and joint abnormalities, a combination suggested to be referred to as NOG-related-symphalangism spectrum disorder (NOG-SSD). Conductive hearing loss from such variants may stem from stapes footplate ankylosis, fixation of the malleoincudal joint, or fixation of the incus short process. In this case, the constellation of both stapes and incus fixation, an exceptionally tall stapes suprastructure, thickened long process of the incus, and enlarged incus body was encountered, leading to distinct challenges during otologic surgery to improve hearing thresholds. This case highlights multiple abnormalities to the ossicular chain in a patient with a start codon variant in NOG. We provide detailed imaging data on these malformations as well as surgical considerations and outcomes.

Figure 1

Preoperative pure tone audiometry revealed bilateral moderate rising to mild conductive hearing loss on the right (a) and left (b). Ipsilateral and contralateral stapedial reflexes were absent bilaterally.

Figure 2

Axial CT bone windows of the temporal bones revealed a markedly enlarged incus with possible fixation in both the right (a) and the left ears (a). Coronal reformats on the left (c) demonstrated an elongated stapes superstructure of approximately 4.8 mm (dashed line) with thickening of the incus long process (arrowhead). On the right (d), this measurement was 4.5 mm. Coronal views of the right ear demonstrated bony dehiscence or thinning in the tegmen mastoideum without meningoencephalic herniation. Similar findings were noted on the left.

Figure 3

Postoperative pure tone audiometry revealed improvement of the air conduction thresholds to a mild conductive hearing loss with near closure of the air-bone gap on the right. Unfortunately, the patient has not followed up for postoperative audiogram after his second operation but reports subjectively improved and symmetric hearing.