A neurotropic lymphoma in the brachial plexus of a cat

Abstract

A 7-year-old, intact male domestic shorthair cat was presented with a progressive, non-weight-bearing lameness of the right forelimb. A neurological examination of the patient at the Small Animal Clinic revealed a paralysis of the radial nerve with sustained cutaneous sensation and a hyperaesthetic response. Further clinical, radiological and pathological findings lead to a diagnosis of a primary, neurotropic B-cell lymphoma in the brachial plexus.

Lymphomas are among the most important tumours of the cat. The most common forms are multicentric, thymic and intestinal lymphomas. ¹ Primary lymphomas of the nervous system of the cat are rare. They include intravascular lymphomas in the central nervous system ² and lymphomas that infiltrate the nervous tissues. ³ Lymphoma of the peripheral nerves has been described sporadically and can cause clinical and gross findings that are difficult to be reconciled with the typical appearance of lymphoma in other organs. ^4–8 Here we describe the clinical, radiological and pathological findings of a primary, neurotropic B-cell lymphoma in the brachial plexus of a cat and briefly discuss diagnostic approaches and differential diagnoses.

A 7-year-old, intact male domestic shorthair cat was presented with a progressive, non-weight-bearing lameness of the right forelimb. A neurological examination of the patient at the Small Animal Clinic led to the diagnosis of a paralysis of the radial nerve with sustained cutaneous sensation and a hyperaesthetic response. Complete blood cell count and serum biochemical analyses were within normal ranges. An enzyme-linked immunosorbent assay (ELISA) test did not detect feline leukaemia virus (FeLV) specific antigen or feline immunodeficiency virus (FIV)-specific antibodies. X-ray radiographic findings were restricted to a mild osteoarthritis in the right elbow joint. Finally a computed tomography (CT) was conducted using a Lightspeed-QXi scanner (GE, Milwaukee, USA). A spiral scan of the cervicothoracic spine and the forelimbs, especially the brachial plexus was conducted (120 kVp, 200 mA and 1.25 mm slice thickness). Native scans were followed by the use of intravenous iodine contrast medium (600 mg/kg). The dataset obtained was evaluated with an advanced work station 4.2 in axial, coronal and sagittal views. The eighth spinal ganglion and spinal nerve appeared moderately enlarged by a contrast medium accumulating mass that extended into the brachial plexus, especially into the radial and ulnar nerve (Fig 1A). The cat was euthanased due to the extent of replacement of the spinal nerve and the brachial plexus.

Fig 1.

B-cell lymphoma in the eighth spinal nerve of a cat. (A) Orthogonal CT scans of the eighth spinal nerve. The spinal nerve and its ganglion are enlarged by a contrast agent accumulating lymphoma (arrow). Orthogonal CT-scan. Bar=5 mm. (B) Transversal section of the spine and the spinal cord at the level of the eighth spinal nerve. The spinal nerve is expanded and replaced by a soft, whitish mass (arrow). Bar=5 mm.

At necropsy the right forelimb had a severe muscular atrophy. Examination of the spine revealed a complete loss and replacement of the eighth spinal nerve and its ganglion by a well circumscribed, white mass that led to a minimal dilatation of the intervertebral foramen (Fig 1B). Furthermore, the right brachial plexus was moderately enlarged in diameter (Fig 2A). Histological examination of paraffin sections of the decalcified C7 vertebra and the brachial plexus revealed a diffuse and severe replacement of neurons of the eighth spinal nerve and their axons and the spinal ganglion by an infiltration of densely packed, well differentiated neoplastic lymphocytes. Tumour cells were exclusively found within the nerve without infiltration of the surrounding epineurium or adjacent connective tissue. They had distinct cell border and small amounts of pale eosinophilic cytoplasm. The nuclei were round to oval with abundant, densely packed heterochromatin. Less than four mitotic figures were found per 400× magnification. Approximately 90% of the axons were lost and remaining axons were degenerated and presented as spheroids with severe dilation of the myelin sheaths and formation of digestion chambers with gitter cells. The infiltrating neoplastic lymphocytes were immunohistochemically positive for the B-cell marker CD79 (Fig 2B). The histological examination of representative sections of other organs and especially the peripheral nerves of the right forelimb, the spinal cord, spleen, bone marrow and lymph nodes revealed no significant microscopic findings. The radiographic, necropsy and histological findings, therefore, led to the diagnosis of primary, neurotropic B-cell lymphoma in the eighth spinal nerve and the brachial plexus.

Fig 2.

B-cell lymphoma in the brachial plexus of a cat. (A) The right brachial plexus (R) was moderately enlarged and displayed congestion of the perineuronal vessels when compared to the left brachial plexus (L). Bar=7 mm. (B) Longitudinal section of the brachial plexus. Axons are degenerated and replaced by the infiltrating lymphocytes. Haematoxylin and eosin. 400×. Inset: infiltrating lymphocytes were immunohistochemically positive for CD79a. Mayer's haematoxylin counterstain. 400×.

The case presented here shows that primary and focal, neurotropic lymphomas although very rare, have to be considered as a possible differential diagnosis for neoplasms of the spinal nerves and the brachial plexus. Other more common differential diagnoses include traumatic injury of the brachial plexus, arterial thromboembolism, ischaemic myelopathy and malignancies such as peripheral nerve sheath tumours. Obviously, without destruction of spinal bone the diagnosis of a spinal nerve tumour as in this case is difficult by X-ray radiographs alone. However, CT scans of the spine combined with myelography are capable to diagnose a tumour of spinal nerve and brachial plexus, although magnetic resonance imaging (MRI) is now the gold standard modality for both neural and vertebral tumours. ^9,10 Nevertheless, as the different tumour types are not discernable by neuroimaging alone, a biopsy with subsequent histological evaluation is the only way to come to a definitive diagnosis of neurotropic lymphoma. In contrast, fine needle aspiration often is not sufficient to delineate well-differentiated lymphomas from an abnormal collection of non-neoplastic, inflammatory lymphocytes. In summary, neurotropic lymphoma should be considered as a potential differential diagnosis even in FeLV-negative cats with a mononeuropathy caused by an extradural mass in the spinal cord or peripheral nerves.