A primitive neuroectodermal tumor with extension into the cranial vault in a dog

Abstract

This paper reports the clinical findings, cytology, diagnostic imaging, and necropsy of an unusual case of a peripheral nervous system neoplasm which, subsequent to a 6-month clinical history, extended into the cranial vault. Necropsy and histology confirmed the diagnosis of a peripheral primitive neuroectodermal tumor.

A 4-year-old male castrated Labrador retriever was presented to the neurology service of the Centre Hospitalier Universitaire Vétérinaire (CHUV) due to a 6-month history of left-sided Horner’s syndrome, left facial paresis, an occasional cough, inspiratory stridor, and a change in the dog’s voice. Five months after the initial examination by the referring veterinarian, the dog had become weak, had lost weight, and was sleeping progressively more and deeper. A subcutaneous mass was palpated on the left side of the neck and was aspirated by the referring veterinarian. It was diagnosed as a hypertrophied salivary gland. Antibiotics had been prescribed by the referring veterinarian without significant improvement in the patient’s status.

Case description

Upon presentation at the CHUV, a physical examination revealed an inspiratory stridor suggestive of laryngeal paresis and a subcutaneous mass located just ventral to the left mandible. During the neurological examination, the patient had the tendency to fall asleep. The neurological deficits were mainly restricted to the left side including: Horner’s syndrome (miosis, ptosis, protusion of the 3rd eyelid, enophthalmos), facial paresis, dysphagia, decreased tear production, a slight head tilt to the left, slight atrophy of the left side of the tongue, and a slight restriction of cervical movements toward the left.

Magnetic resonance imaging (MRI) of the brain (Figures 1–3) indicated the presence of a multilobulated mass in and around the left tympanic bulla that was also present in the vertebral canal and the cranial vault, where it compressed the proximal cervical spinal cord and the brainstem, respectively. The tympanic bulla was completely filled by the mass, which was isoin-tense to the cervical spinal cord parenchyma on T1-weighted images and moderately hyperintense on T2-weighted images. The mass also extended through the stylomastoid foramen into the surrounding soft tissue and extended into the vertebral canal from C1 rostrally, within the foramen magnum to the caudal medulla (Figure 4). Moderate and slightly heterogeneous enhancement was noted within the mass following IV injection of gadolinium (gadobenate dimeglumine, 529 mg/mL) at a dose of 0.2 mL/kg BW (Figures 4 and 5). Differential diagnoses for the mass included a soft tissue sarcoma, lymphoma, and a possible carcinoma originating from the tympanic bulla.

Figure 1.

Magnetic resonance T2-weighted transverse image at the level of the rostral medulla and tympanic bullae. There is a heterogenous mostly hyperintense mass within the left cervical soft tissues invading the left tympanic bulla (arrows).

Figure 3.

Magnetic resonance T2-weighted image, caudal to the location shown in Figure 2, at the junction between the caudal medulla and the first cervical spinal cord segment. Note the slightly hyperintense mass adjacent to the left occipital condyle within the vertebral canal in continuity with a branch of the left XI cranial (accessory) nerve (arrow).

Figure 4.

Magnetic resonance T1-weighted image, post-intravenous gadolinium injection, at the level of the external auditory canals. Note the multilobulated, diffusely enhancing extension of the mass, within the left aspect of the cranial cervical vertebral canal and caudal fossa displacing the cranial spinal cord and medulla toward the right (arrow).

Figure 5.

Magnetic resonance T1-weighted image, post-intravenous gadolinium injection, at the level of the tympanic bullae. The mass is heterogeneously enhancing and involves the left cervical peripheral soft tissues, the left tympanic bulla and the left VII cranial (facial) nerve following its route within the temporal bone within the brainstem at the level of the medulla.

The results of routine hematology, serum biochemistry, and urinalysis were within reference ranges. The cytological examination of a fine-needle aspirate of the mass revealed round cells with small amounts of pale cytoplasm, round, oval, or occasionally indented nuclei with finely granular chromatin, which was interpreted as a “malignant round cell tumor” of uncertain origin (Figure 6). Similar cells were found in a smear of a sample of cerebrospinal fluid (CSF), which also had significantly elevated protein (0.6 g/L; reference range: 0.3 to 0.4 g/L). One week later, the dog’s condition had deteriorated; he had developed severe left vestibular ataxia with his body leaning drastically to the left. Due to the poor prognosis the dog was euthanized and submitted for postmortem examination.

Figure 6.

Photomicrograph of a fine-needle aspirate of the tumor mass showing round cells with small amounts of pale cytoplasm and round, oval, or occasionally indented nuclei with finely granular cytoplasm. Diff Quik stain; bar = 50 μm. Inset bar = 10 μm.

At necropsy, an approximately 4-cm long, well-delimited soft, reddish-gray mass extended from the region of the cranial cervical ganglion and the root of the left first cervical nerve just external to the lateral foramen of the atlas, into the vertebral canal and extended rostrally along the left side of the brainstem to approximately 3 mm caudal to the origin of the left VII cranial nerve (facial). The mass was loosely attached to the underlying brainstem and spinal cord which were slightly compressed (Figure 7). Another branch of the mass extended rostrally, within the extravertebral soft tissue ventrolateral to the vertebral canal, to the stylomastoid foramen and into the left tympanic bulla. There was a slight atrophy of the left side of the tongue.

Figure 7.

Postmortem photograph of the left side of the occipital cortex, cerebellum, and brainstem with the tumor mass adherent to the underlying brainstem (arrow).

Histological examination of the mass revealed a densely cellular mass that compressed the surrounding neuropil extending from the first cervical nerve (C1), enveloped the left XI cranial nerve (accessory), and extended along the left ventrolateral surface of the brainstem (Figure 8). The mass was composed of clumps and packets of round to polygonal sometimes slightly elongated or tapered cells with well-defined cell borders and small amounts of eosinophilic cytoplasm, supported by a fine fibrovascular stroma. Nuclei were oval to elongated and ranged from 7 to 14 μm in diameter with finely granlar chromatin and 1 or occasionally multiple nucleoli (Figure 9). Mitoses averaged 6 per 40× field with numerous individual necrotic cells throughout the mass. Multifocally in a zone up to 300 μm thick subjacent to the mass, the neuropil was vacuolated with numerous dilated myelin sheaths occasionally containing dilated axons (spheroids). Within the neuropil, small numbers of neurons exhibited central chromatolysis with rare neurons having condensed nuclear chromatin or pyknotic nuclei (necrosis). Rare examples of neuronal satellitosis were present. Several dilated myelin sheaths occasionally with spheroids were present within the XI cranial nerve (accessory), which was enveloped by the tumor. Within the left side of the tongue, multifocally, groups of myofibers had a loss of cross striations; they were hypereosinophilic, with vacuolated cytoplasm, and with marked variation in myofiber diameter along with separation of adjacent myofibers. Similar, although less severe, changes were present in the left cleidomastoideus muscle. There were no other significant macroscopic or microscopic pathological changes.

Figure 8.

Photomicrograph of the tumor mass surrounding branches of the left XI cranial (accessory) nerve (arrows), with the overlying brainstem in the top left of the photograph. Hematoxylin, phloxine, and saffron stain; bar = 500 μm.

Figure 9.

Photomicrograph of the packets or clumps of round, polygonal to tapered cells comprising the tumor. Note that there are numerous mitotic figures (arrows). Hematoxylin, phloxine, and saffron stain; bar = 100 μm.

Based on the macroscopic appearance of a soft reddish-gray mass and the microscopic appearance of round to polygonal or tapered cells arranged in clumps or packets with a scant fine fibrovascular stroma with abundant mitoses, a diagnosis of primitive neuroectodermal tumor (PNET) was made. Although there is no single immunohistochemical marker that is diagnostic for these tumors, a panel of antibodies were reacted with formalin-fixed, paraffin-embedded tumor tissue. Variable numbers of cells reacted positively with neuron specific enolase (NSE) and vimentin, and did not react with antibodies against glial fibrillary acidic protein (GFAP), S-100, pan-cytokeratin or CD99, which is consistent with the highly variable results reported in the literature, and is compatible with a primitive tumor of neuroectodermal origin.

The reported clinical signs along with the macroscopic and microscopic features of the tumor support the hypothesis that the tumor arose from the cranial cervical ganglion close to the level of the root of the left first cervical nerve root. It then penetrated the subdural space of the spinal cord and extended along the cranial nerve XI (accessory), the proximal cervical spinal cord, and along the brainstem to a point just caudal to the point of exit of cranial nerve VII (facial) as it left the brainstem. A tumor affecting the cranial cervical ganglion at this point could cause the Horner’s syndrome, which was one of the first clinical signs noted by the owner and the referring veterinarian (6 months prior to presentation to the CHUV). The facial paresis developed almost simultaneously with the Horner’s syndrome which suggests that the tumor had also extended within the extravertebral soft tissues to the region of the stylomastoid foramen early in the clinical course. The facial nerve exits the stylomastoid foramen in close proximity to the tympanic bulla and involvement of the tumor in this region could cause compression on numerous facial nerve branches and thus generate the facial paresis observed clinically. The facial nerve branches could also have provided a pathway for the tumor to enter the foramen and eventually the tympanic bulla, from which the facial nerve is separated by only fibrovascular stroma as it passes by the tympanic cavity.

The changes in the left cleidomastoideus muscle are consistent with neurogenic atrophy and are consistent with the tumor’s involvement of the cranial nerve XI (accessory). Degenerative changes in the adjacent neuropil of the brainstem likely affected the cranial nerve XII (hypoglossal) accounting for the atrophy of the musculature of the left side of the tongue and contributing to the dysphagia noted clinically. Degeneration in the brainstem at the level of the nucleus ambiguus could also have contributed to the dysphagia and inspiratory stridor; involvement of the proximal laryngeal nerve [a branch of cranial nerve X (vagus)] and cranial nerve XI (accessory) could also explain the same deficits. Degenerating neurons were identified in the vestibular nuclei, which along with suspected destruction of the inner ear receptors, were likely responsible for the head tilt and severe left vestibular ataxia noted with the progression of the neurological signs just prior to euthanasia. Slight compression of and degeneration within the brainstem may also account for the depression and disturbances in sleep pattern noted by the owners.

Discussion

Embryonal tumors of the nervous system are rare in animals, with the possible exception of medulloblastoma of the cerebellum, which is mostly seen in young dogs and calves (1). Embryonal tumors of the nervous system in humans are uncommon, and encompass a variety of diagnoses reflecting an origin from an embryonal cell that has the capacity for both neuronal and glial differentiation (2). Therefore, several diagnostic entities have been grouped under the term “primitive neuroectodermal tumors” (PNETs) to more accurately reflect the concept that a primitive neuroectodermal cell can differentiate in several directions, sometimes within the same tumor (1–3). The term medulloblastoma has been retained to describe those tumors that arise from the external granular layer of the cerebellum (1–4), perhaps for historical reasons since there is, in fact, no such cell as a medulloblast (4). Within the PNET group in animals, is the rare entity previously called neuroblastoma, most of which are said to arise from the adrenal medulla or the sympathetic ganglia, but have been reported in the central nervous system including the cerebrum, brainstem, and spinal cord (1). The term “neuroblast” refers to a primitive neuroectodermal cell that has differentiated to form an immature neuron and is therefore post-mitotic (5), and consequently unlikely to be a cell that will produce a neoplasm.

Primitive neuroectodermal tumor is a good descriptive compromise for tumors arising from neuroectoderm that differentiate towards neurons and/or glial cells. These tumors are considered to be malignant, grow rapidly, and may metastasize (1). They can be divided into central PNETs (arising from the brain or spinal cord) or peripheral PNETs (arising from neural tissue within bone or soft tissue, including Ewing’s sarcoma family of tumors in humans) (6). Reports of peripheral PNETs in the veterinary literature are rare (1,4,7,8), those arising from a nerve or a ganglion exceedingly so, with only one report in the veterinary literature that described a sympathetic ganglion origin (9). The case presented here represents the second reported case of a peripheral primitive neuroectodermal tumor arising from a ganglion or nerve in the dog, although it is the first case report of a histologically confirmed PNET arising from the periphery supported by a description of the clinical signs along with MRI images. Immunohistochemical studies have provided variable results in both human (2,6) and canine cases (1,4,7,9,10), with positive staining for triple neurofilament protein, neuron specific enolase (NSE), and synaptophysin providing the most consistent results. Our case is consistent with the variable results reported in the literature; however, positive reactivity with anti-NSE antibody is consistent with a primitive neuroectodermal origin for the tumor. The immunohistochemistry results reported do not seem to provide any advantage over the microscopic examination of the tumor by an experienced pathologist.

This case is unique in the veterinary literature since the dog originally presented with Horner’s syndrome and facial paresis as the only clinical signs. Speculatively, aggressive management including a neurological work-up with MRI and surgery may have prevented the tumor growth into the central nervous system, which ultimately led to the euthanasia of the dog.

This case illustrates the importance of impressing upon clients the need to find an explanation for Horner’s syndrome and facial paresis, which many clients and veterinarians may perceive as benign clinical signs. CVJ

Figure 2.

Magnetic resonance T2-weighted image, caudal to Figure 1, at the level of the caudal medulla. Note the extension of the mass within the left cranioventral aspect of the cranial vault with displacement to the right of the medulla (arrow); the bone adjacent to the tympanic bulla is slightly thickened (arrow).