Comparative Neuromuscular Laboratory

NEUROMUSCULAR CASE OF THE MONTH - DECEMBER 2001

Stiff, rigid gait in a 15 year old male neutered Yorkshire Terrier
Contributed by Drs. Kirk Ryan, Blaine Burkhert, T.Mark Neer
Veterinary Clinical Sciences
Louisiana State University
Baton Rouge, LA

Clinical History
A 15 year old neutered male Yorkshire terrier was evaluated for a stiff, rigid gait and inability to rise (See video clip). Gait abnormalities were first detected two years prior to presentation and were attributed to osteoarthritis. In the 6 months prior to presentation, the dog's mobility progressively deteriorated. At the time of presentation, the pet was unable to rise from lateral recumbency without assistance. In addition, he would only ambulate for short distances before collapsing. The owners reported that the dog intermittently appeared disoriented or dull. Increased water consumption and urination were also reported.

Physical and Neurological Examination
Except for severe anxiety, the dog's mentation and alertness were subjectively normal. The dog's overall appearance was suggestive of endocrinopathy and age (distended abdomen, dry sparce hair coat, marked corneal opacity, weakness). Abdominal palpation was unremarkable. Thoracic auscultation identified diffusely increased bronchovesicular lung sounds. The gait was markedly abnormal and characterized by rigidity and increased muscle tone. Proprioceptive responses and tendon reflexes were within normal limits. The dog was reluctant to flex joints in all extremities (especially the elbows and stifles). Marked muscle hypertrophy was evident in the antebrachial musculature. The dog unsuccessfully struggled to right itself from lateral recumbency. A generalized myopathy was suspected.

Diagnostic Testing
CBC- within normal limits except for a low-grade inflammatory leukogram characterized by neutrophilia and monocytosis

Serum chemistry profile- mildly increased creatine kinase, cholesterol, and BUN. Alkaline phosphatase was not elevated.

Radiographs - Chest and abdominal radiographs were not abnormal

Electromyography (EMG) - EMG of fore and hind limb muscle groups revealed spontaneous myotonic discharges ("dive bomber potentials", See end of video clip) in the triceps, biceps, antebrachial muscles, quadriceps, hamstrings, and cranial tibial muscles. In addition, biphasic, high frequency spontaneous discharges were noted in these muscles. These EMG findings were suggestive of a myotonic myopathy.

Follow-up Testing
Thyroid hormone profile - Low total T4 (4.2 nmol/L; reference = 17.3-42.8 nmol/L) and free T4 (7.7 pmol/L; reference = 10.8-30.3 pmol/L) with a normal TSH concentration (0.35 ng/ml; reference = 0.1-0.43 ng/ml).

Urine cortisol:creatinine ratio (UCCR) - Markedly elevated (827 nmol/L cortisol:2.33 mmol/L creatinine = 354.9). This was a non-specific finding consistent with both stress and hyperadrenocorticism.

Thyroid stimulation test - Confirmed hypothyroidism. The baseline TT4 was 3.8 nmol/L (reference 17.3-42.8 mmol/L). Six hours post-injection (50 ug Thyrotropin IV), the TT4 was 14.7 nmol/L. Post-injection TT4 concentrations less than 15.0 nmol/L are consistent with hypothyroidism. Euthyroid dogs normally have post-injection TT4 concentrations greater than 45 nmol/L.

Diagnosis
Myotonic myopathy secondary to a chronic endocrine disorder associated at least partly with hypothyroidism. While the clinical appearance of the dog was also consistent with Cushing's syndrome, the serum alkaline phosphatase was not elevated. The owners declined further testing for hyperadrenocorticism.

Treatment and Outcome
Hypothyroidism was treated with L-thyroxine (0.04 mg/kg/day divided BID). The UCCR results were attributed to stress associated with concurrent illness and disability. A re-check two weeks later showed progression of clinical signs despite treatment. The dog was presented seven days later for profound disability and respiratory distress. At this time, the dog could not stand without support, and was unable to walk. The dog was panting uncontrollably and appeared to have labored respirations. The dog was alert but severely distressed and anxious. Humane euthanasia was performed at the owner's request. A complete necropsy was declined, but immediate postmortem muscle and peripheral nerve collection were permitted. The muscle specimens showed chronic myopathic changes consistent with an endocrine disorder including numerous lobulated fibers (Fig. 1, modified Gomori Trichrome stain) and a shift to a type 1 fiber predominance as found in chronic hypothyroidism (Fig. 2, ATPase, pH 4.3). The peripheral nerve biopsy was normal.

Figure 1.

Figure 2.