When identifying differential diagnoses, you should understand the diseases' pathophysiology in relation to their presentation
and progression (Tables 2 & 3). For example, a young animal that presents with an insidious and progressive problem is more likely to have a degenerative
disease than a traumatic problem. Also, a 5-year-old cocker spaniel with a two-week history of back pain followed by an acute
onset of upper motor neuron paraplegia (T3-L3 lesion) is more likely to have a disk problem than a fibrocartilaginous embolism,
since a fibrocartilaginous embolism is unlikely to occur in a chondrodystrophic animal, is usually not painful, and should
be acute (i.e. not preceded by two weeks of pain).
Table 2. An Acronym for Categorizing Neurologic Diseases
Some clinical presentations are classic and worth mentioning. Metronidazole toxicosis causes a severe central vestibular ataxia,
possibly associated with some depression, hypermetria, and conscious proprioception deficits.8-10 A history of metronidazole administration at a high dosage (usually > 50 mg/kg/day) or long-term administration should lead
to an easy diagnosis. Treatment is mainly supportive, although administering diazepam appears to hasten recovery.10
Table 3. Categories of Neurologic Diseases by Presentation and Progression
A shaker syndrome is reported most frequently in small-breed dogs with white coats (e.g. Maltese, West Highland white terriers), although dogs with other coat colors can be affected. The disease has been called
white shaker syndrome, and affected dogs present with generalized but fine whole-body tremors.4,11,12 The tremors are usually exaggerated by excitement, handling, and movement and are usually absent during sleep. Some dogs
will also have a pathological nystagmus, although it is more common to see an oscillation of the ocular globes; seizures have
also been reported. The cause is unknown, but the lesions consist of a mild, nonsuppurative encephalitis mainly affecting
the cerebellum.4 The condition responds favorably to corticosteroid administration, and the prognosis is good.
A masticatory muscle myositis can be seen in dogs.13 In the acute form, the masticatory muscles (masseter and temporalis muscles) are swollen, firm, and quite painful. The patient
may also be febrile. The chronic form is characterized by marked muscle atrophy and fibrosis of the masticatory muscles. This
can lead to trismus with a complete inability to open the mouth, even when the patient is anesthetized. The disease is secondary
to an immune-mediated reaction against specific fibers (2M) of the masticatory muscles. The disease is diagnosed based on
positive type 2M antibody assay results or a temporalis of masseter muscle biopsy. Immunosuppressive doses of corticosteroids
is the treatment of choice and should be instituted as soon as possible to prevent extensive fibrosis.
Once you've developed a list of differential diagnoses, decide which diagnostic procedures are most pertinent, especially
since many of these procedures are expensive and can be invasive.
A minimum database should include a complete blood count, a serum chemistry profile, and urinalysis. A thoracic radiographic
examination is also helpful to identify any concomitant problem that may indicate a more diffuse or systemic involvement (e.g. megaesophagus, metastasis, lesions compatible with fungal disease). Then, based on the differential diagnoses, more specific
diagnostic procedures can be considered. In cases of multifocal central nervous system disease, an inflammatory cause is the
first consideration. In these cases, a funduscopic evaluation can reveal lesions of chorioretinitis or anterior uveitis suggestive
of an infectious disease. Measuring titers for specific diseases (fungal, rickettsial, protozoal) is noninvasive and can provide
a definitive diagnosis. Cerebrospinal fluid analysis is the next logical step and can be regarded as the "blood test" of the
central nervous system. Bacterial culture or titer testing can also be performed on a cerebrospinal fluid sample.
In cases of diffuse disease of the central nervous system, multiple blood glucose, electrolyte, bile acid, and thyroid hormone
evaluations and blood pressure monitoring are warranted. Specific tests can also be performed for suspected intoxication (e.g. blood lead concentration, ethylene glycol evaluation, cholinesterase activity).