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Melarsomine-Related Neurologic Injury: Rare but Real

Clinician's Brief (Capsule)


April 2014

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A castrated Great Dane (7 years of age)  presented with progressive right-sided pelvic limb abnormalities 8 weeks after treatment with melarsomine dihydrochloride (ie, Immiticide []; an arsenic-based compound) for heartworm disease. Significant paraspinal hyperesthesia was found at the level of the thoracolumbar junction. Mild atrophy of the infra- and supraspinatus muscles was present bilaterally, with decreased range of motion on flexion of the caudal cervical spine. Differentials included neoplasia, discospondylitis, intervertebral disk disease, meningomyelitis, and spinal cord inflammation or compression related to aberrant heartworm migration.

Cervical findings were thought to be related to breed-related common degenerative spinal diseases. MRI showed multiple desiccated cervical discs and an extradural compressive mass lesion at the thoracolumbar junction causing moderate right-sided spinal cord compression. An irregular periosteal reaction was noted on the transverse L1 process. Changes on MRI were most consistent with an inflammatory process and paraspinal and epidural abscessation associated with infection or melarsomine administration. A thin-walled cystic mass was noted at the level of T13-L1 during surgery; histopathology revealed sterile pyogranulomatous inflammation. The patient rapidly improved after surgery but was noted to have persistent paraparesis with generalized proprioceptive ataxia of the pelvic limbs at 3 months’ postoperative examination.


Melarsomine administration does not typically cause chronic progressive neurologic injury. Neurologic complications are hypothesized to be caused by migration of the compound into fascial planes causing ascending inflammation along nerve roots. Extensive inflammation and necrosis of epidural fat can cause extradural cord compression. Medical management of extradural lesions should be considered for patients that do not have refractory pain or progressive neurologic signs. Surgical decompression is indicated if the patient rapidly declines or is refractory to medical treatment, but only after appropriate diagnostics have been performed.—Heather Troyer, DVM, DABVP, CVA


Chronic compressive myelopathy and progressive neurologic signs associated with melarsomine dihydrochloride administration in a dog. Moore SA, Mariani CL, Van Wettere A, Borst LB. JAAHA 49:389-393, 2013.

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