Neurologic disorders of the geriatric dog
SEATTLE, WA – Among the general causes of neurological disease in dogs, degenerative conditions, neoplasia, and vascular disease are diagnosed more frequently in older animals. Our understanding of these problems has increased significantly over the past several decades, leading to advancements in diagnostic and treatment options, explained Karen R. Muñana, DVM, MS, DACVIM (Neurology), presenting at the American College of Veterinary Internal Medicine Forum. Specific disorders of the cranium seen in geriatric dogs include cognitive dysfunction, idiopathic peripheral vestibular disease, cerebrovascular disease, and neoplasia. Spinal conditions diagnosed commonly in older dogs include degenerative myelopathy and neoplasia.
Canine cognitive dysfunction syndrome
This syndrome is characterized by age-related behavioural deficits that can not be attributed to any other medical condition present in the dog. The four categories of behavioural changes are disorientation, reduced social interactions, changes in sleep/wake cycle or activity level, and loss of housetraining. Of those dogs demonstrating characteristic behavioural changes, more than half are 15-16 years of age. Morphologically, the disorder is similar to Alzheimer’s disease in humans.
Differential diagnoses include vision loss, hearing loss, metabolic encephalopathy, brain neoplasia, cerebrovascular disease, and encephalitis. A definitive diagnosis of cognitive dysfunction syndrome can only be made after other potential causes of the clinical signs have been excluded. A thorough evaluation should be performed to assess for systemic disease, hypertension, hearing loss, vision loss, and neurological deficits. Brain imaging and cerebrospinal fluid (CSF) analysis are necessary to rule out a neoplastic, vascular, or inflammatory condition of the brain. Since other conditions in geriatric dogs can mimic signs of cognitive deficits, any identified conditions should be treated and the dog monitored for improvement in clinical signs before definitive treatment for cognitive dysfunction is instituted.
Pharmacologic treatment with L-deprenyl (selegiline, Anipryl), can improve clinical signs by inhibiting monoamine oxidase, the enzyme that breaks down the neurotransmitter dopamine, and thereby modulating the dopaminergic system in the brain. Feeding a diet enriched with antioxidants (Hill’s Prescription Diet Canine b/d) has also been evaluated. Dr. Muñana said that diet is thought to decrease the production and consequence of free radicals that have been shown to contribute to age-associated changes in cognition, though the studies for both treatments are limited. Other nutritional supplements that have been advocated include medium chain triglycerides, phosphatidylserine, and S-adenosyl-L-methionine (SAMe).
Cerebrovascular disease
Cerebrovascular disease refers to any abnormality of the brain resulting from a disruption of its blood supply. Stroke is the most common clinical presentation and is characterized by a sudden onset of focal neurological signs that persist at least 24 hours. In both dogs and humans, ischemic strokes are more common than hemorrhagic strokes. Brain ischemia results from occlusion of a blood vessel with a thrombus or embolus that deprives the brain of oxygen and energy. Cerebrovascular disease due to brain hemorrhage can be spontaneous, although this is rare in dogs, or can be associated with a concurrent medical condition.
The incidence of cerebrovascular disease is greatest in older dogs, with a median age at onset ranging from 8-13 years. Neurological signs tend to be acute and nonprogressive. Deficits are often asymmetric and reflect the location of the lesion within the brain. It has recently been suggested that episodic or transient (<24 hours in duration) vestibular dysfunction in dogs can result from systemic hypertension, analogous to transient ischemic attacks in humans.
Dr. Muñana said that dogs with signs compatible with cerebrovascular disease should undergo serial blood pressure measurements and a thorough ophthalmic examination to evaluate for hypertension induced retinal hemorrhage or detachment. Treatment with antihypertensive medication can improve neurological signs in animals with hypertensive encephalopathy. Diagnosis is based on imaging findings, and ischemia is best visualized with MRI while hemorrhage can be visualized on MRI or computerized tomography (CT). Analysis of CSF is frequently normal or findings are non-specific. Underlying causes should be investigated, including endocrine, renal and cardiac disease, metastatic neoplasia, and coagulopathy. Treatment is limited to supportive care and management of any identified underlying systemic disease. Prognosis is generally favourable, but is dependent on the size and location of the infarct. Clinical signs usually begin to improve after 24-72 hours, with recovery occurring over several weeks.
Canine geriatric vestibular disease
The disorder typically affects middle-aged or older dogs, and manifests as acute onset of vestibular signs with no evidence of limb weakness, depression of consciousness, or other cranial nerve deficits that would suggest brainstem disease. It is a common cause of peripheral vestibular signs in dogs. A presumptive diagnosis is based on the presence of compatible history and examination findings. Definite diagnosis can only be made by excluding other causes of peripheral vestibular disease, such as otitis media-interna and middle ear neoplasia.
Supportive care should be provided until the condition resolves. Antiemetics can be helpful in decreasing anorexia and vomiting associated with the disequilibrium. Prognosis is very good as signs resolve spontaneously in 2-4 weeks, with improvement typically noted within 24 hours. A residual head tilt can persist in some cases.
Neoplasia
The incidence of brain tumours has been reported to approach 3% in the dog population, with a median age of nine years. Primary brain tumours arise from cells normally present in or associated with the brain, and include meningiomas, gliomas, and choroid plexus tumours. Meningiomas are the most common primary brain tumour diagnosed in dogs. Secondary tumours originate from surrounding tissues and locally extend into the brain (such as nasal tumours), or arise from hematogenous metastasis of primary tumours in other tissues.
Seizures are the most common presenting complaint in dogs with brain tumours. Other neurological signs, when present, can be acute or insidious in onset, depending on the location, rate of growth and indirect effects of the tumour.
Diagnostic evaluation should include minimum database (hemogram, chemistry profile, urinalysis) to evaluate for any evidence of systemic disease, as well as thoracic radiographs and possibly abdominal ultrasound to screen for metastasis. MRI is the most useful diagnostic tool to confirm the presence of a brain mass.
Symptomatic treatment is aimed at decreasing peritumoral edema with corticosteroids and treatment of seizures with antiepileptic medication. Options for definitive tumour treatment include surgery, chemotherapy, and radiation therapy. Overall prognosis is fair to guarded, and dependent on location and tumour type. In most instances, the goal of therapy is remission and increased survival time rather than cure.
Spinal tumours are also more commonly diagnosed in older dogs, with a reported median age at diagnosis of 10 years. Spinal tumors tend to cause chronic progressive neurological deficits, but the clinical course can vary considerably. Pain is a common presenting sign with extradural or intradural-extramedullary lesions. Initial diagnostic evaluation should include minimum data base, thoracic radiographs to screen for metastasis, and spinal radiographs to evaluate for vertebral lysis or proliferation. MRI is the most sensitive diagnostic imaging tool. Treatment options include palliative therapy, surgery, radiation therapy, and chemotherapy. Prognosis is dependent on tumour type and location.
Degenerative myelopathy
Degenerative myelopathy is a spinal cord condition recognized in middle-aged to older dogs of various breeds, with age of onset of nine years. Affected dogs show signs of a slowly progressive ataxia and paraparesis, with no evidence of spinal pain. Dr. Muñana referred to recent work that has identified a specific mutation in superoxide dismutase that is associated with the disease trait. Furthermore, the disease has been demonstrated to have genetic and clinical similarities with ALS or Lou Gehrig’s disease in humans. She added that genetic testing is now available; dogs that are homozygous for the mutation are at risk for developing the disease. Dogs that are heterozygous for the mutation are unlikely to develop degenerative myelopathy, but can pass the mutation on to their offspring. Results from one study suggest that dogs with degenerative myelopathy that receive physiotherapy have a better prognosis than dogs that do not. No other proven definitive treatment exists and overall prognosis is poor.CVT
