Epidural Spinal Myelolipoma in a Dog

Introduction

Myelolipoma is a benign, typically asymptomatic tumor consisting of fat, myeloid, and erythroid marrow elements.¹ The tissues affected in cats and dogs by myelolipoma have included the spleen, adrenal glands, and liver.² The etiology of myelolipoma is controversial.³ In humans, extra-adrenal myelolipomas are usually single, well-circumscribed, encapsulated masses that are most commonly seen in middle-aged to elderly persons with a 2:1 female predominance.³ Approximately half of the tumors are located in the presacral region with mediastinal, perirenal, hepatic, and gastric sites occurring in decreasing frequency.³ Extraadrenal myelolipomas are postulated to arise from metaplasia of previously uncommitted mesenchymal cells or from hematopoietic stem cells carried to ectopic sites during fetal life.³ As such, it is unclear whether these masses represent hyperplasia, choristomas, or neoplasia.⁴

An epidural myelolipoma has been previously reported in a dog that was euthanized without therapy.⁵ A thoracic spinal myelolipoma has also been reported in a man.⁴ A dorsal laminectomy and mass resection were performed in the man, and significant improvement was seen in his gait disturbance.⁴ The present report describes a case of epidural spinal myelolipoma in a dog. The diagnosis was supported by magnetic resonance imaging (MRI) and computed tomography (CT) and was confirmed by histopathology. The epidural myelolipoma was removed surgically, and the quality of life subsequently improved.

Case Report

A 13-year-old, male Siberian husky was presented with a 2-year history of progressive pelvic limb paresis and a 6-month history of urinary incontinence. Six months prior to evaluation, the dog had been examined by another veterinarian, and at this time a presumptive diagnosis of intervertebral disk disease was made and therapy was begun with prednisolone. The therapy was unsuccessful, however, and the medication was stopped. The dog was referred for further examination.

Upon presentation, general physical examination was normal except for moderate muscle atrophy in the pelvic limbs. A complete blood count and serum biochemical profile were normal. On neurological examination, mentation and cranial nerve function were normal. General proprioception was reduced in both pelvic limbs. Depression of the left patellar reflex and absence of the right patellar reflex were noted. Flexor reflexes were intact, and pain responses were normal. Spinal hyperpathia was absent. The suspected anatomical location of the lesion was within the fourth lumbar to third sacral spinal cord segments.⁶ Magnetic resonance imaging and myelography were advised, and the owner agreed to an MRI.

The MRI of the lumbar portion of the vertebral column was done using a 1.5 tesla MRI scanner.^a Sagittal plane T1-weighted and T2-weighted images showed a lesion of increased signal intensity that compressed the spinal cord from the area of the 13th thoracic to the third lumbar vertebrae. Additionally, both T1-weighted and T2-weighted images showed that the elongated mass had multiple focal and serpiginous areas of low signal intensity [Figures 1A, 1B]. Differential diagnosis was spinal arteriovenous malformation or epidural lipoma based on the MRI findings. In humans, selective spinal arteriography is performed to establish the diagnosis of arteriovenous malformation; however, selective spinal arteriography has not been performed in the dog because of technical difficulties.^7,8 Further examinations and clinical treatments were not performed at the owner’s request.

Forty-four days after the first referral, the dog was readmitted for progression of the pelvic limb paresis. Computed tomography^b with contrast medium^c was performed, and vascular anomalies were not identified. A heterogeneous mass with high attenuation was seen [Figure 2], and these CT findings were supportive of a space-occupying mass such as a lipoma.

Following CT, the dog underwent exploratory dorsal laminectomy and resection of the mass. After removal of the vertebral arches from the 13th thoracic to third lumbar vertebrae, an extradural mass (1.5 × 9.0 cm, reddish brown) intermixed with adipose tissue was found to be compressing the spinal cord [Figure 3A]. The mass was removed from the dura using a bipolar cautery. The skin was closed routinely, and recovery was uneventful. Postoperatively, intermittent catheterization was performed to decompress the bladder.

Histopathological examination of the resected tissue revealed mature adipose tissue surrounded by hematopoietic elements, which was consistent with myelolipoma. No evidence of malignancy was detected [Figure 3B].

Eight weeks after surgery, the dog was able to stand and walk without assistance, but urinary retention from neurological bladder dysfunction was still noticed. One year after surgery, the owner reported that the dog is able to walk and neurological status is improved; however, residual urinary retention persists, and intermittent catheterization of the bladder is performed every day by the owner.

Discussion

On MRI, the myelolipoma reported here had high-signal intensity with foci of low-signal intensity on both T1- and T2-weighted images. The high-signal intensity lesions were suspected to be adipose tissue.⁴ The low-signal intensity structures were thought to represent calcified masses, a chronic hematoma, and/or fast blood flow in an abnormality such as an arteriovenous malformation.⁹ The low-signal intensity tissues were most likely hematopoietic material. Magnetic resonance imaging findings of an extradural spinal myelolipoma have not been previously reported in the dog, but based on the changes in this case, extradural myelolipoma should be included as a possible diagnosis when a mixed fatty and soft-tissue lesion is identified on a spinal MRI.

On MRI, an extramedullary hematopoietic tumor may look identical to extradural spinal myelolipoma, because foci of extramedullary hematopoiesis contain both fat and soft-tissue elements.⁴ Lau et al. reported an extramedullary hematopoietic tumor that compressed the spinal cord in a human, and such tumors must be distinguished from extramedullary myelolipoma.⁹ Fowler et al. provided criteria for differentiating an extraadrenal myelolipoma from an extramedullary hematopoietic tumor in humans.¹⁰ Patients with extraadrenal myelolipomas, unlike those with extramedullary hematopoiesis tumors, typically have no history of chronic anemia, hepatosplenomegaly, or skeletal disorders.¹⁰ The clinical history and physical examination of the dog reported here favored an extraadrenal myelolipoma.

Myelography was not performed in this case at the owner’s request. Myelography may have helped distinguish between arteriovenous malformation and an epidural mass, because an arteriovenous malformation usually produces an intraspinal, space-occupying effect that is present in the intradural-extramedullary space.¹¹

Conclusion

A spinal myelolipoma was diagnosed in a 13-year-old dog. Although rare, myelolipoma should be included in the differential diagnosis when a lesion of mixed signal intensity is identified on a spinal MRI. Careful postoperative follow-up is necessary in more cases to determine the long-term outcomes of dogs with spinal myelolipoma.