3 Localizing lesions
William of Occam (died circa 1350) pioneered the KISS (‘Keep It Simple, Stupid!’) approach to problem-solving with his philosophical principle that the fewest possible assumptions are to be made in explaining a thing. This principle is known as Occam’s razor and is used in localizing neurological lesions.
|1. What is abnormal?||Take a history|
|2. What body system is responsible?||Examine the animal|
|3. Is it the nervous system?||Perform a neurological examination|
|4. Where is the lesion?||Interpret the exam findings|
|5. What could the lesion be?||Consider signalment, history, anatomic location|
|6. How best to diagnose?||Consider the anatomic location|
|7. How best to treat?||Consult textbooks and journals|
The history provides important information as to the nature of the underlying disease. It is wrong to assume that history-taking can be avoided by performing various blood tests and imaging techniques. In fact, that would be one of the most serious errors a clinician could make. Assessing the speed of onset, the rate of progression, either worsening or improving, and the duration of signs gives the first clues as to the diagnosis and hence the possible prognosis.
Pathological processes have typical patterns of behaviour and these can be simplistically demonstrated with graphs. These are guidelines only. Diseases have nuances and variations in clinical appearance. Textbooks only describe the common presentations (Fig. 3.1).
It is important to consider the nature of the underlying disease before planning any diagnostic tests. If all the possible underlying diseases (the differential diagnoses) are uniformly fatal within a short time span, it is only fair to the owner, who is funding the investigation, to inform them of this fact. Equally, if the suspected aetiology is one which will resolve without intervention within a short time frame, it is important to mention this to the owner, and consider using what could be termed ‘benign neglect’, i.e. wait and see. If the animal does not respond as expected within the timeframe expected, then further investigation is certainly justified.
Some owners wish to know as soon as possible what is or is not going on. The clinician must be confident in assessing the benefit of investigation to the animal. The clinician must be able to explain to the owner the significance of abnormal results (or the lack of abnormalities) on diagnostic tests.
Anatomic and aetiological diagnoses based solely on the history and examination are accurate in the majority of cases. In a study of human neurological patients, the errors in clinical diagnosis were attributed to incomplete or incorrect collection of historical and physical findings, insufficient knowledge of the spectrum of clinical manifestations associated with the patient’s disease and incorrect analysis of the findings (poor diagnostic reasoning).
An incorrect initial assessment reduces the efficiency of patient care and delays initiation of appropriate treatment. Technology cannot be relied upon as a panacea for diagnostic difficulties as test results may be negative.
Apart from inadequate history-taking, the chief hurdles of the diagnostic steeplechase are an inability to examine the nervous system, and more commonly, an inability to interpret the findings of such an exam.
Interpretation of findings requires a basic understanding of how the nervous system works. Happily, there are characteristics associated with each section of the nervous system (Boxes 3.1-3.9). Make a list of the abnormalities, apply ‘Occam’s razor’, and see if the deficits can be assigned to one area of the nervous system (Fig. 3.2).
Box 3.1 TELENCEPHALON: cerebrum