Middle Ear Salivary Choristoma: A Rare Case Report and Update on Congenital Associations, Facial Nerve Involvement, and Treatment Strategies

Salivary gland choristoma is an extremely rare middle ear mass and is hypothesized to be caused by second branchial arch developmental anomalies. We present a 14-year-old girl with Dandy–Walker syndrome and conductive hearing loss. Middle ear exploration revealed a large middle ear mass with absent incus and stapes and displaced facial nerve. The mass was completely excised with histological confirmation of salivary gland choristoma. Her hearing was improved with bone-anchored hearing aids (BAHA). As facial nerve involvement is common, physicians should consider partial excision to avoid facial nerve palsy. Hearing restoration can be achieved with OCR or BAHA.


Introduction
A choristoma is a congenital, benign overgrowth of mature tissue found in an abnormal location of the body. On extremely rare occasions, benign salivary gland tissue can be present in the middle ear cavity. Presenting symptoms include conductive hearing loss, ear fullness, tinnitus, otalgia, and otorrhea [1,2]. ese salivary gland choristomas are also associated with malformations of the ossicular chain, particularly the incus and stapes, along with dehiscence of the facial nerve from the fallopian canal [2,3]. eir rare presentation and proximity to vital structures of the middle ear make diagnosis and treatment a challenge. We present a case report detailing this rare presentation along with an extensive review of previous cases with associated congenital findings, facial nerve involvement and its impact on surgery, and hearing augmentation outcomes.

Methods
We present a unique clinical case from our institution that is in line with the Surgical Case Report (SCARE) criteria along with a thorough literature review of all salivary gland choristomas, focusing on congenital and middle ear malformations, facial nerve involvement, treatments, and outcomes [4]. Statistical analysis was performed with Fisher's exact test using Statistical Analysis Software (SAS) version 9.4. A 14-year-old female with a history of Dandy-Walker syndrome, hydrocephalus with ventriculoperitoneal shunt placement, and recurrent otitis media presented to our academic center with left hearing loss and aural fullness. Tympanostomy tubes were placed in each ear 5 years prior. Her mother states the hearing loss was present since birth and denies any otorrhea, otalgia, aural bleeding, dizziness, vertigo, or familial hearing loss. Audiogram showed normal hearing on the right and a moderate-severe rising to moderate mixed hearing loss with 25-50 dB air-bone gap on the left. Word recognition scores were excellent bilaterally ( Figure 1). Tympanogram was type A for the right and type B for the left (Figure 2). CT temporal bone showed a left  mesotympanic and hypotympanic mass extending into the infracochlear space. e scutum was sharp. ere was no visible stapes. ere was no direct continuity between the mass and the carotid or jugular bulb (Figures 3 and 4). On examination, the left tympanic membrane had a shallow retraction pocket in the pars flaccida and a white retrotympanic mass suspicious for a possible secondaryacquired cholesteatoma. e decision was made to perform an exploratory tympanotomy and removal of the mass.

Case Report
Under continuous facial nerve monitoring, tympanomeatal flap was elevated by a senior otology surgeon who uncovered a white, smooth, and mildly pulsatile mass. Compression of the external jugular veins and increased vagal pressure did not expand the mass. Mild compression of the mass did not cause blanching. Electrical stimulation of the mass at the inferior pole of the tumor and marching superiorly showed no electromyography (EMG) response. Bone removal from the canal floor was required to gain better access to the tumor. e tumor was first mobilized from its infracochlear position followed by the superior pole using curved dissectors. Total tumor removal was possible without any facial nerve stimulation.
After complete tumor removal, middle ear inspection revealed malformed long process of the incus without a stapes superstructure or oval window niche for reconstruction. An inferiorly displaced tympanic segment of the facial nerve was identified and was able to be electrically stimulated.
Pathology showed a 1 cm polypoid mass of respiratory sinus mucosa lined with pseudostratified columnar ciliated epithelium and scattered goblet cells, consistent with a

Discussion
Salivary gland choristoma in the middle ear cavity is an extremely rare condition with fewer than 50 cases ever reported [1,2]. ere appears to be a left-sided and female predominance, with age ranging from 9 months to 52 years old [5]. Salivary gland choristoma of the middle ear is hypothesized to form from malformation of the second branchial arch prior to the fourth month of gestation [4,6]. Salivary tissue becomes trapped in the middle ear during the fusion of the tympanic ring with the temporal bone [7]. Abnormal second branchial arch development may also underpin the frequent findings of malformed incus, stapes, and facial nerve canal [7].
Although the abnormal deposition of tissue is often isolated to the middle ear, a unique developmental syndrome may be present [5,6]. Several ipsilateral auricle and Case Reports in Otolaryngology facial abnormalities have been associated with salivary choristomas, including alopecia [4,8], auricle deformities and swelling [9], preauricular fistulas [10], and facial asymmetry [6]. Dandy-Walker syndrome is a congenital hypoplasia of the cerebellar vermis, dilation of the fourth ventricle, and enlarged posterior fossa. To date, this syndrome has not been known to be related to any choristomas. However, Buckmiller et al. described a case of an encephalocele in a child with a contralateral middle ear salivary choristoma [5] (Table 1). CT imaging and otomicroscopic appearance are usually sufficient to presumptively diagnose middle ear pathology before making the first incision. However, due to the rarity of middle ear salivary choristoma, it is often misdiagnosed as cholesteatoma given its white, retrotympanic appearance [37] or otosclerosis due to the unilateral conductive hearing loss [8,38]. Only one case had concurrent cholesteatomas with a middle ear salivary choristoma [35]. Regardless of initial diagnosis, exploratory tympanotomy is warranted in all cases. Intracranial and vascular tumors can be ruled out through a preoperative CT scan, enlarging mass during increased intracranial pressure through Valsalva or occlusion of internal jugular veins (Queckenstedt maneuver), and blanching of mass during manipulation. Histopathology of the middle ear mass is required for diagnosis confirmation.
Malignant transformation of salivary choristoma is rare [16], and many authors in the literature suggest that complete tumor excision is not necessary [10,44]. However, others propose that the small chance of malignant transformation does warrant complete excision [45]. e major limiting factor for complete surgical excision is facial nerve involvement. Cases involving the facial nerve had a significantly lower complete excision rate than those in which the facial nerve was spared (44.4% vs. 84.2%, p � 0.0027, Table 2). Permanent facial nerve palsy after removal of a middle ear salivary choristoma has occurred in two reported cases [36,46], with two additional cases of transient facial nerve palsy that later recovered [38,47]. To mitigate this risk of facial nerve palsy in all cases, use of a facial nerve monitor during surgery is strongly recommended [4,44]. e abnormal and dehiscent course of the facial nerve makes it vulnerable to injury during middle ear dissection. Facial nerve electrical stimulation should be applied to the mass at areas prior to surgical manipulation to ensure the facial nerve is not in close proximity, and electrocautery of the mass should be avoided. Partial excision of mass can be considered to avoid facial nerve palsy.
Ossicular chain reconstruction has been attempted both during initial removal [45] and staged months after removal [37] for restoration of hearing, with promising results (Table 3). Hearing was significantly improved in patients who underwent OCR than those without repair (58.3% vs. 12.9%, p � 0.0047). In our present case, OCR was not attempted due to inaccessibility of the oval window niche. However, bone-anchored hearing aids (BAHA) can provide a viable alternative to hearing enhancement as our patient showed encouraging response on BAHA demo.

Conclusion
Salivary gland choristoma is an extremely rare diagnosis of a middle ear mass. Radiographic imaging and careful middle ear exploration are needed to rule out vascular and intracranial tumors. As facial nerve involvement is common, physicians may consider partial excision to avoid facial nerve palsy. Hearing restoration can be achieved with ossicular chain reconstruction or BAHA.

Consent
Written telephone informed consent was obtained from the parental guardians of the patient for publication of this case report and accompanying images.

Conflicts of Interest
e authors report no conflicts of interest.