Principles of Lesion Localization

The purpose of the neurologic examination is to determine the neurologic abnormalities and based on that, the location of the lesion or lesions responsible for causing these abnormalities. 

The location is the anatomic diagnosis.  Narrowing down to which part(s) of the nervous system may be affected can undeniably present a number of advantages. 

From a diagnostic point of view, the differential diagnosis is entirely dependent on the anatomic diagnosis.  Aside for determining which part of the nervous system is affected, localising the lesion also involves determining if the problem is focal, multifocal (i.e., affecting multiple parts of the nervous system) or diffuse (i.e., affecting globally and symmetrically one or more parts of the nervous system).  Such information can then be used to narrow down even further the differential list (see section how to establish a differential diagnosis list).  Furthermore, a number of disease processes may only be diagnosed by exclusion of other causes mimicking a similar clinical history and presentation.  This process of exclusion implies evaluating the correct part of the nervous system to confidently rule-out these mimics.  Failure to localise the lesion, the interpretation of any diagnostic test results in the face of negative findings (as seen with some vascular or degenerative disease of the central nervous system) or findings that do not match the clinical history can end up a very challenging task for the clinician.  Finally, running a limited number of investigations aimed at narrowing down the differential list to a specific part of the nervous system can only result in less cost for the owners and less time spent to reach a diagnosis for the clinician.  

Before rushing into the specifics of the neurological examination, attention should be focused on what questions are aimed to be answered:

1. Is there any neurological abnormality detected?

2. Which part(s) of the nervous system may be involved to explain these abnormalities?

3. Is the lesion localisation focal, multifocal or diffuse?   

The first question does not require any detailed knowledge of neuroanatomy or neuroanatomic pathways.  By simple observation and testing a number of reflexes and responses (see section hands-on and hands-off approach), the clinician should be able to determine if the animal is neurologically sound or not.  

The neurological examination aims to test the integrity of these various components of the nervous system and, if present, detect any functional deficit.  Normal findings are as important as the abnormal ones in localising the lesion.  Neurological abnormalities detected on examination should be listed and added to the list of abnormal findings collected from the history.  Each of these abnormal findings should then be correlated to a specific region or to specific pathways within the peripheral and/or central nervous system.  Attempts should then be made to explain all the abnormal findings by a single lesion within one of the following regions of the nervous system: focal forebrain – brainstem – cerebellum - [C1 – C5 spinal cord segments] – [C6 – T2 spinal cord segments] – [T3 – L3 spinal cord segments] - [L4 – L6 spinal cord segments] – [L7 – S3 spinal cord segments], peripheral nerve - neuromuscular junction - muscle. 

Lesions within these regions of the nervous system result in predictable and specific neurological signs.  Note that in localizing a lesion, it is not necessary that all the clinical signs referable to one location or syndrome be present.  If a single lesion cannot explain all the listed abnormal findings, the lesion localisation is considered as multifocal or diffuse.  

Sites of lesion localisation for neurological disease

(Small Animal Neurological Emergencies, Platt & Garosi editors, Copyright 2012, Manson Publishing, reproduced with permission of Taylor & Francis Books UK)

 The neurological examination can be divided into two main parts:

1.  Hands-off examination

(i) State of consciousness, awareness and behaviour,

(ii) Posture and body position at rest

(iii) Gait evaluation

2.  Hands-on examination

(i) Postural reaction testing

(ii) Muscle tone and size

(iii) Spinal reflexes

(iv) Nociceptive testing

(v) Cranial nerve evaluation

It is often very tempting to rush into testing an animal’s spinal reflexes, postural reactions, or cranial nerves and neglect the hands-off part of the examination which can provide a lot of useful information.  So on the next page, that is where we will start.