Localizing lesions

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.


The aim is to find the focus of the pathology. Thus all signs are assessed and judged if they can be ascribed to a single lesion or not.


Once this intellectual exercise is over, the clinician then prioritizes any tests required to diagnose the actual disease.


If the anatomical site of disease is not known, then testing cannot be performed in a logical, rational, cost-efficient and expedient manner (Table 3.1).


Table 3.1 The required thought process for diagnosis



























Question Answer
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


AETIOLOGY OF THE LESION


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).


image

Figure 3.1 Temporal patterns of neurological disease.


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.





Categories of disease



1. Malformation

2. Inflammation

3. Degeneration

4. Trauma

5. Tumour.

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.



How to prioritize diagnostic tests



1. Utility
• Will this test alter the treatment of the animal?

• Will this test give more information about possible underlying pathologies than other diagnostic options?

2. Morbidity
• What is the risk to the animal in performing the test? (How can this risk be decreased?)

• What is the risk to the animal in NOT performing the test? (Is there another diagnostic method?)

• Which is greater?

3. Interference
• Will this procedure confound the results of future tests?

4. Cost
• Can the owner afford to act upon the test results? (i.e. Will this test alter the treatment of the animal?).


Do not forget the basics … co-existing disease



1. Neurologically affected animals may have disease elsewhere, e.g. arthritis of forelimbs as well as hindlimb ataxia. Physical examination is important.

2. Animals may have new, acute deficits and long-term unrelated signs, e.g. an acute balance loss from peripheral vestibular disease and residual proprioceptive deficits from an unresolved TL cord compression secondary to IVDD. History taking is important.

3. A neurological lesion may begin as a focus and expand to cause damage to surrounding structures, giving the appearance of multifocal disease, e.g. IV disc extrusion leading to myelomalacia. Repeat neurological examinations are important.


The accuracy of neurological diagnoses


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.



Interpretation of neurological signs


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.


The trick of examination is to follow a routine, and train oneself to notice both the normal and abnormal.


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



• Altered alertness

• Altered behaviour

• Seizures

• Contralateral visual deficit

• Minimal gait change

• Compulsive walking, pacing, circling

• Contralateral proprioception deficits

• Contralateral paresis.


Box 3.2 DIENCEPHALON: thalamus, pituitary



• Altered alertness

• Altered behaviour

• Seizures

• Visual and PLR deficits

• Minimal gait change

• Compulsive walking, pacing, circling

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Sep 3, 2016 | Posted by in SMALL ANIMAL | Comments Off on Localizing lesions
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