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Common and ‘forgotten’ head and neck lymphocentrums

Though veterinarians think of lymph nodes as being consistent from one animal to another, in reality, there is variation in number.

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Figure 1: A young Weinmaraner was presented with bilateral, freely movable subcutaneous swellings.

Recently, a young Weimaraner presented for eyelid surgery through our ophthalmology department, and it was noticed that he had bilaterally symmetrical subcutaneous soft tissue swellings dorsal to the maxillary third premolar teeth (Figure 1). These swellings were well circumscribed and freely movable beneath the skin. Their appearance, bilateral symmetry, and signalment suggested the most likely differential diagnosis to be buccal (a.k.a. facial) lymph nodes.

These can be a normal finding in any dog, but they are easier to see and palpate in young, large-breed dogs with short hair. Careful aspiration, with particular attention to avoiding trauma to the eye if aspirating in an awake patient, can confirm the diagnosis of lymphoid hyperplasia or normal lymph node. These nodes also may be enlarged in cases of lymphoma.

Though we think of lymph nodes as being consistent from one animal to another, in reality, there is variation in number. Therefore, rather than referring to a mandibular lymph node, for example, we refer to a mandibular lymphocentrum, which describes the region containing mandibular lymph nodes on one side of the body.

There are three commonly described lymphocentrums of the head and neck. The mandibular lymphocentrum (not submandibular, as referred to in humans) consists of one to five nodes that are nestled between the bifurcation of where the lingual and facial veins branch from the linguofacial vein, rostral and lateral to the mandibular salivary gland. The nodes normally are smaller and firm compared to the larger and more pliable mandibular salivary gland. In cats, the mandibular lymph nodes are about the size of a small pea. In dogs, depending on patient size, the nodes range between the size of a black bean and a kidney bean. They can feel larger if two nodes are very close together or if the nodes truly are enlarged due to reactive or effaced nodes. When first developing the skills to palpate the neck, it can be difficult to distinguish the mandibular lymph node from the mandibular salivary gland. The mandibular salivary gland is larger, softer, more caudal, deeper, and less freely movable than mandibular lymph nodes.

The parotid lymph node is about the size of a pea and sits rostral to the parotid salivary gland.

The medial retropharyngeal node is the largest lymph node of the head. It’s a long (approximately 5 centimeters in a large dog), relatively thin (approximately 2 centimeters in a large dog) node that sits caudal and medial to the mandibular salivary gland, just ventromedial to the brachiocephalicus and sternocephalicus muscles. It lies under the wing of the atlas in the triangle bounded by the digastricus cranially, musculus longus colli dorsally, and the larynx ventrally. Care must be taken to avoid the structures medial to the node, including the common carotid artery, internal jugular vein, hypoglossal nerve, vagus nerve, and sympathetic trunk.

Figure 2: A single-incision approach as been described for removal of parotid, mandibular and medial retropharyngeal lymph nodes.5 A cadaver specimen shows the common lymph nodes and salivary glands of the neck.

In approximately 21 percent of dogs, two medial retropharyngeal nodes are present ipsilaterally. In approximately 33 percent of dogs, a lateral retropharyngeal node is present. The lateral retropharyngeal lymph node is small (less than 1 centimeter) and circular, and lies at the dorsal border of the mandibular salivary gland and beneath the caudal portion of the parotid gland. More attention is being paid to ipsilateral and contralateral medial retropharyngeal lymph node during oncologic staging, as all lymphatic drainage of the head appears to pass through these nodes.1,2 From the mandibular lymph nodes, lymph may flow to the ipsilateral medial retropharyngeal lymph nodes, or along anastomotic connections to the contralateral node.1

The “forgotten node”

The buccal (facial) lymph node is the “forgotten node,” as it is present in only approximately 9 percent of dogs3,4 and it has not been documented in cats. Astute clients or clinicians may notice the node as a firm but movable subcutaneous swelling above the roots of the maxillary third premolar bilaterally. The node seems to be more common (or just more obvious) in large breed dogs, such as Weimaraners, vizslas, and Labrador retrievers.

What is the significance of this lymph node being enlarged? It may be visible/palpable in a young puppy whose immune system is being exposed to all new types of antigens. Unilateral enlargement may suggest a reactive node that is responding to disease on that side of the mouth. Neoplasia such as lymphoma cannot be ruled out without an aspirate. Of the times that I have aspirated a palpably large buccal lymph node, the results showed only reactive node. The main purpose for discussing this topic is to make practitioners aware that it’s normal for a node to be present in this area.

Dr. John Lewis practices veterinary dentistry and oral surgery at NorthStar Vets in Robbinsville, N.J.

References

1 Belz GT, Heath TJ. Lymph pathways of the medial retropharyngeal lymph node in dogs. J Anat. 1995 Jun;186 (Pt 3):517-526.

2 Herring ES, Smith MM, Robertson JL. Lymph node staging of oral and maxillofacial neoplasms in 31 dogs and cats. J Vet Dent. 2002 Sep;19(3):122-126.

3 Casteleyn CR, van der Steen M, Declercq J, Simoens P. The buccal lymph node (lymphonodus buccalis) in dogs: occurrence, anatomical location, histological characteristics and clinical implications. Vet J. 2008;175(3):379-383.

4 Shelton ME, Forsythe WB. Buccal lymph node in the dog. Am J Vet Res. 1979 Nov;40(11):1638-1639.

5 Smith MM. Surgical approach for lymph node staging of oral and maxillofacial neoplasms in dogs. J Am Anim Hosp Assoc. 1995;31(6):514-518.

 

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