Normally, the major salivary glands develop from oral ectoderm proliferation and outpouching during the fourth to eighth weeks of fetal development. The aplasia (agenesis) of SMG occurs due to defects during fetal development . This condition is related to the first and second branchial arch abnormalities. It is also seen in genetic syndromes such as Treacher-Collins syndrome, hemifacial microsomia, ectodermal dysplasia, and lacrimoauriculo-dento-digital syndrome (LADD) . However commonly, unilateral absence of SMG is an isolated condition without any abnormalities .
The SMG agenesis is commonly diagnosed incidentally during imaging. The bimanual clinical examination of the neck is insufficient for definitive diagnosis. The various imaging modalities can be useful such as sialography, ultrasound, CT, MRI, and nuclear medicine (technetium T99m pertechnetate scintigraphy) imaging. Ultrasound is an initial imaging investigation but it is difficult to interpret. Next, the scintigraphy gives the information only about the presence of functioning salivary tissue. For a better analysis, the cross-sectional imaging as CT or MRI is preferred and it gives the complete anatomy of salivary glands. However, among all the imaging modalities, the modality of choice for the diagnosis of salivary gland agenesis can be MRI [1, 3, 4, 7, 8, 10,11,12,13].
In our patient, neither family history nor facial or other abnormalities were reported. In general, absence of major salivary gland may lead to decrease in saliva causing symptoms such as dryness of mouth, dental caries, and difficulty in swallowing . However, our patient has not shown any such symptoms on presentation explaining the adequate flow of saliva by the other gland present.
In most of these SMG aplasia patients, a contralateral SMG hypertrophy will be present as pseudomass [3,4,5, 9, 14]. In our patient, the contralateral SMG is normal size with normal signal intensity. However, the enlargement of ipsilateral sublingual gland was noted and such hypertrophy was well identified on MRI over CT. Presence of such ipsilateral sublingual gland hypertrophy itself helped us in ascertaining the absence of SMG. In addition, the right parotid gland was also observed to be atrophic. Overall on MRI, a well-defined round homogeneously enhancing lymph node in submandibular region (level IB) was seen occupying the right submandibular region fat without an SMG.
The lymph node differentiation from normal gland was truly difficult on palpation without an MRI. Even in our patient, these preliminary MRI images might have been overlooked. But the finding of ipsilateral asymmetric sublingual gland hypertrophy has grabbed our attention for careful evaluation. Which lead to identification of right SMG aplasia, avoiding confusion with single enlarged right submandibular metastatic lymph node (right level IB). The final surgical histopathological report has also confirmed and reaffirmed the absence of right submandibular gland. In 28 dissected lymph nodes, only 1 lymph node was reported to be metastatic as seen on MRI imaging.
In general, on imaging modalities like MRI, we observe linear T2 weighted high signal intensity showing the ductal system in SMG differentiating from the lymph node. Also the metastatic lymph node will show a heterogeneous post-contrast enhancement due to necrosis, where an SMG will be showing a homogenous enhancement. Even on DWI, metastatic lymph nodes will show a restricted diffusion which would be absent in normal SMG.
From this case report, we strongly suggest that MRI can be highly beneficial in identifying and solving such conditions. However, a good understanding of submandibular space anatomy and awareness about this rare entity is necessary for this purpose. The familiarity with these imaging findings can help in optimum preoperative staging and treatment planning.