Neuroanatomy and lesion localisation

Key point

■ The primary goal of the neurological examination is to anatomically localise the lesion(s). As diseases are often region specific, neuroanatomical localisation is critical for constructing a sensible list of diagnostic possibilities.

The primary goal of the neurological examination, and a good reason to study neuroanatomy, is to localise the lesion. The location of a lesion, or lesions, is fundamental to making a diagnosis in a case with neurological signs and cannot be replaced by other diagnostic tests such as advanced imaging. Not all abnormalities noted by imaging are clinically significant. The location of the abnormality must be able to explain the clinical signs.

Localising a neurological lesion relies on identifying both those neural systems that are dysfunctional AND those systems that are functioning normally (Fig. 1.14).

Fig. 1.14 A calf was presented with a left-sided head tilt. This sign combined with the loss of extensor tone on the left side strongly suggested a deficit in the vestibular system on the left side. But the head tilt did not indicate if the lesion was in the brain (vestibular nuclei in the brainstem) or inner ear (semicircular canals). However, on neurological examination, no other cranial nerve deficits, general proprioceptive deficits or upper motor neuron paresis were identified: this indicated that the lesion did not affect the motor tracts arising in the brainstem, the sensory tracts going through the brainstem, or the cranial nerve nuclei in that region of the brainstem. Thus the brainstem was unaffected and the lesion was localised to the inner ear. A presumptive inner ear infection was successfully treated with antibiotics.

It matters little where a lesion is located on a pathway (origin, midway along the pathway, or termination), it will still produce similar signs of dysfunction.

If a lesion is in a particular region, then it could cause signs of dysfunction due to damage to pathways in that region. However if a pathway does not pass through that region, then it will not be affected. Thus knowledge of those neural systems that are functioning normally indicates to the examiner that the lesion is not located in the region that those systems occupy (Fig. 13.1). For example, if the lesion is in the thoracolumbar spinal cord, then pathways passing through that part of the cord may be damaged; these include proprioception and UMN tracts to the pelvic limbs. But the lesion will not affect the cranial nerves or the function of the thoracic limbs, as the neural systems do not pass through that region. Therefore, noting the normal CNN and normal thoracic limb function is just as important as noting the UMN signs and proprioceptive deficits in the pelvic limbs.