A 4-year-old male intact pointer hunting dog presented with an acute onset of rapidly progressive paraparesis of 4 days duration. The dog was thin but had good muscle definition. Referral bloodwork was normal except for markedly elevated creatine kinase (CK, 12,910 IU/L) activity and moderate elevations of aspartate amino transferase and alkaline phosphatase activity. There was only mild clinical improvement since the onset. No treatments were administered.
Physical and Neurological Examination
The dog was ambulatory, but on gait analysis, would walk on his “tip toes” (See figure below and Go To Video Clip). The dog tired easily and could only stand for short periods during the examination before having to sit and rest. Weakness and lameness was evident in both pelvic limbs. Hyperesthesia could not be elicited. Conscious proprioception was delayed in the left pelvic limb. Patellar reflexes showed clonus bilaterally. Cranial nerve and thoracic limb examinations were normal. Calcaneal tendons were thickened bilaterally.
An orthopedic consultation disclosed bilateral calcaneal tendon ruptures resulting in the unusual posture at the hocks and toes of both pelvic limbs. The superficial digital flexor (SDF) tendon status dictates the toe position. When the SDF tendon is intact and the calcaneal tendon ruptured, the toes are “grippy” as seen in this case. If the SDF tendon is also ruptured, a completely plantigrade stance is present with normal toe position (similar to a severe sciatic nerve injury). Typically there is an underlying cause to such bilateral ruptures such as infectious or inflammatory myopathies, endocrine associated myopathies and others. Diagnostic testing was performed to try to identify an underlying myopathy.
Resting and post-exercise plasma lactates – Normal at rest and post-exercise
Electrodiagnostics – Generalized spontaneous EMG activity with normal motor nerve conduction velocity
CSF tap – Normal
Infectious disease serology - Negative for Rocky Mountain Spotted Fever, Toxoplasmosis, Neosporosis, and Cryptococcosis. A week positive antibody titer was found for Ehrlichiosis.
Muscle biopsies – Biopsies from both the biceps femoris and cranial tibial muscles showed similar pathologic changes. Numerous necrotic fibers were observed at a similar stage of degeneration. Lymphocytic inflammation and other cytoarchitectural abnormalities were not detected. No organisms were found.
Acute necrotizing myopathy of undetermined origin resulting in bilateral calcaneal tendon rupture. Common causes of necrotizing myopathies include toxic exposures, post-infectious diseases, bites from venous insects or snakes, electrolyte abnormalities and drug exposures. Since this dog was an active hunting dog and electrolyte abnormalties were not identified on the screening laboratory work, a toxic exposure is thought to be most likely.