Congenital Unilateral Agenesis of the Parotid Gland: A Case Report and Review of the Literature

Congenital unilateral agenesis of the parotid gland is a rare condition with only few cases reported in the literature. A review of 21 cases in the available literature is presented in this article. We report on a further case of a 34-year-old woman with agenesis of the left parotid gland and lipoma of the right cheek. Clinicopathological characteristics of described cases in the literature were discussed.


Introduction
The major salivary glands start to develop between the sixth and seventh week of gestation beginning with the parotid gland which arises from ectodermal lining of the stomatodeum [1]. The submandibular and sublingual glands develop later and arise from the endodermal layer of the floor of the stomatodeum. Congenital absence of major salivary glands is a rare condition of unclear etiology. It is usually bilateral and sometimes associated with other development anomalies of the head and neck area. Unilateral agenesis of the parotid gland, especially, is an extremely rare condition with only few cases reported in the literature. The first report of a salivary gland agenesis was mentioned in 1885 by Gruber [2]. Since then, few cases of the unilateral submandibular gland agenesis have been reported in the literature [3].
Agenesis of parotid glands may occur alone or in association with anomalies of the submandibular or lacrimal gland, first brachial arch developmental disturbances, or other congenital anomalies [4][5][6][7]. The true incidence of agenesis of the parotid gland is difficult to ascertain because the condition is often asymptomatic [8]. Because saliva is mostly produced by other major and minor salivary glands, xerostomia does not occur and the absence of parotid gland is not noticed by the patient in the majority of cases [4].
We present a case of unilateral agenesis of the parotid gland in combination with a lipoma of the cheek on the opposite site. The clinical and radiological findings in this patient are described. A review of the unilateral parotid gland agenesis in the literature is also presented considering a summary of the data regarding gender, age, defect site, and combined manifestations.

Case Report
A 34-year-old woman was referred to our department for evaluation of painless swelling of the right cheek over the last seven months. In addition, she often bit her right cheek. The swelling did not vary in size during eating and the patient had no other clinical symptoms and no history of recurrent parotitis. Xerostomia was not noted. There was no other relevant medical history and no family history of similar problems was reported. On clinical examination the oral mucosa was moistened by saliva. Bilateral hemifacial contour was normal, and there were no depressions in either preauricular region. Physical examination of the head and neck was without pathological findings, except for the absence of the left parotid gland papilla ( Figure 1).
Ultrasonographic examination of the head and neck area showed that the parotid gland on the left side was totally absent. The other major salivary glands were present without any pathology. A tumor in the right cheek ventral to parotid gland was observed with characteristic sonographic appearance of lipoma. For further evaluation of the tumor in the right cheek and assessment of the function of the other salivary glands magnetic resonance imaging (MRI) and

Discussion
Congenital absence of the salivary glands is a rare condition which has been described to affect the parotid or submandibular glands [26]. Agenesis of salivary glands may be unilateral or bilateral and multiple major salivary glands can be involved [27][28][29].
The true incidence of unilateral agenesis of the parotid gland is difficult to ascertain because it is often asymptomatic [10]. Congenital unilateral absence of the parotid gland is uncommon with only few cases reported. The absence of bilateral parotid glands has been observed in lacrimoauriculodentodigital (LADD) syndrome [30], in hypoplasia of the lacrimal glands or absence of lacrimal puncta [31], in hemifacial microstomia, and in ectodermal dysplasia. The resulting disturbances affect primarily the lacrimal glands, the inner and outer ear, the salivary glands, and the osseous frame work [24,30]. Aplasia of the major salivary glands may be associated with aplasia/hypoplasia of the lacrimal glands. This condition is confirmed as autosomal dominant disorder [32]. Single cases of bilateral parotid gland agenesis associated with cleft lip and palate, Down syndrome, or Klinefelter syndrome have been reported [1,[33][34][35]. Some cases of familial salivary gland agenesis have also been documented [31]. Bilateral forms of agenesis could be responsible for a severe lack of saliva causing dental caries, periodontal disease, and candidosis [8].
In the available literature, only 22 cases of unilateral agenesis of the parotid gland have been described including the present case (Table 1). Among the 22 cases, 11 (50%) of the patients were male and 11 (50%) were female. At the time of diagnosis the youngest patient was 50 days old and the oldest was 75 years old with an average age of 34.7 years. The unilateral absence of the right parotid gland was nearly twice as frequent as the left side (14/8 cases). The papilla of Stensen's duct was present in only one case. In the other cases the parotid papilla was absent ( = 12) or the presence of the parotid papilla was not documented ( = 9).
In most reported cases the unilateral agenesis of the parotid gland was associated with a painless swelling of the contralateral parotid gland or facial asymmetry without any other significant clinical symptoms [9, 10, 13-15, 17, 20, 25]. According to the authors the swelling of the contralateral parotid gland was as a compensatory functional hypertrophy of the parotid gland [14,15,17,25]. Association with other pathologies of the head and neck area could not be found in those cases. Sialosis of the contralateral parotid gland was found in one case; the diagnosis was confirmed by  an open biopsy of the parotid gland [9]. One case showed ipsilateral agenesis of the parotid gland in association with first branchial cleft cysts [12]. Another two patients suffered from lateral facial cleft associated with accessory mandible [7,24]. Pleomorphic adenoma of the contralateral parotid gland, the ipsilateral accessory parotid gland, or buccal space was reported in each case [18,19,23]. In another case agenesis of parotid gland was masqueraded in I-123 metaiodobenzylguanidine scan with SPECT/CT by a metastasis of a left craniocervical neuroblastoma [21]. In other cases agenesis of the parotid gland was associated with ipsilateral angioma of the cheek and ipsilateral accessory parotid tissue [8,11,16,22]. The present case described a patient with agenesis of the left parotid gland and a lipoma of the right cheek.
The unilateral agenesis of the parotid gland may be clinically silent. Clinical suspicion should arise in cases of asymmetrical parotid areas and a painless unilateral swelling of the parotid gland. Clinical examination, especially the absence of the papilla of Stensen's duct, could be helpful for diagnosis. Mostly the unilateral agenesis of the parotid gland seems to be a coincident finding. We were able to confirm the diagnosis of parotid gland agenesis by using a combination of MRI and salivary gland scintigraphy.