This talk follows on from the previous talk, so please refer to those notes prior to reading this. The NeuroMap and accompanying table are designed to help you localise neurological lesions; this is the first and most fundamental step in managing a neurological case. Once you've localised the lesion, then you can list the possible causes and make a diagnostic plan.

The NeuroMap is based on mapping the main neural functions on a diagram of the nervous system. If a lesion is in a particular area, you can read from the NeuroMap which neural functions could be disrupted and conversely, which neural functions will be normal. Or, given a set of clinical signs (normal and abnormal), you can work out from the NeuroMap where the lesion is likely to be located. The NeuroMap depicts the location of the main neural functions tested in the neurological examination, including: behaviour and arousal; ascending sensory systems (tactile, proprioception and nociception); motor function (posture and gait); cranial nerve function; and spinal reflexes.

In this session, we'll discuss further the components evaluated in a neurological examination and add them to a BrainMap (see Figure 1), from which is constructed a ready reference table (Table 2) that can be used in conjunction with the BrainMap for localizing lesions.

Functional Brain Divisions

Functionally, there are three main regions of the brain - the forebrain, brainstem (midbrain, pons and medulla oblongata), and the cerebellum. Each region is associated with characteristic neurological signs, so we talk about animals having a region-specific localisation - e.g., forebrain disease, brainstem disease, or cerebellar signs.

Functions Assessed During the Neurological Examination

1.  Behaviour is primarily associated with the forebrain (limbic system). However, it can only be expressed if other neural functions (e.g., awareness and motor function) are normal.

2.  Awareness and arousal (consciousness) are dependent on the function of the ascending reticular activating system (ARAS). This is located in the brainstem and has diffuse projections throughout the forebrain. Thus, decreased arousal can occur with focal lesions in the brainstem (e.g., tumour) or diffuse lesions of the cerebrum (e.g., encephalitis). In veterinary medicine, it is useful to talk about five different arousal levels.

Table 1.

Note: 'Depressed' is a psychological term and is inappropriate.

3.  Proprioception is awareness of body position (see previous session notes). Input from the limbs and body is via spinal nerves and the spinal cord travelling to the ipsilateral cerebellum (subconscious proprioception) or the contralateral forebrain (conscious proprioception). Head proprioception arises from the vestibular apparatus in the inner ear, travels to the brainstem and then cerebellum (SCP) or forebrain (CP).

4.  Motor function in quadrupeds is controlled by motor centres in the brainstem (for semiautomatic activities such as locomotion) and from the motor cortex in the contralateral cerebrum for voluntary motor activity (learned, skilled). Therefore, brainstem lesions can severely perturb gait (paresis), while forebrain lesions usually have minimal effect on gait but will affect learned movements, e.g., cat playing with a feather. Upper motor neurons (UMN or 'central MN') arise from both motor centres and influence brainstem and spinal cord lower motor neurons (LMN or peripheral MN). The cerebellum functions to coordinate the activity of extensor and flexor muscles to achieve the correct tone in opposing muscles for sustaining posture and smoothing movement. Dysfunction of agonist-antagonist contraction causes tremor. Loss of subconscious proprioceptive input to the cerebellum results in ataxia (incoordinated movement). The cerebellum uses SCP input to influence the output from the UMN centres, thereby ensuring that movement occurs with the correct rate, range, and force of movement. This is called metria. Overall, the output from the cerebellum is inhibitory; thus, cerebellar dysfunction can cause hypermetria and spasticity. A specific region of the cerebellum processes vestibular input; lesions in that region can cause vestibular signs (nystagmus, head tilt).

5.  Cranial nerves are primarily associated with the brain; therefore, animals with spinal cord lesions have intact cranial nerve function. As specific cranial nerves are associated with different areas of the brain, cranial nerve signs can be quite localising, e.g., strabismus may suggest a rostral brainstem lesion, as that is the site of attachment of CNN III and IV, innervating extraocular muscles.

6.  Reflexes involving both spinal and cranial nerve reflexes are tested during the neurological examination. Those reflexes involving cranial nerves may be compromised with brain lesions, but not spinal cord lesions. Conversely, spinal cord reflexes, but not cranial nerve reflexes, may be compromised with spinal cord lesions.

The NeuroMap

Figure 1 depicts the location of the main neural functions tested in the neurological examination.

If you cover a region of the brain on the NeuroMap (e.g., with a coin) representing a lesion, you will be able to see which neurological functions will be compromised and which will still function normally. Conversely, if you have a case with specific neurological deficits, identify the pathways associated with each deficit and see where they coincide - that is likely to be the location of the lesion. If you cannot account for all signs with a single lesion, then the animal may have multifocal disease, which is associated with specific aetiologies (e.g., inflammation or tumour). For example, consider an animal that is obtunded, blind in the right eye, and knuckles (CP deficit) on the right side. Identify the CP pathway on the NeuroMap: the lesion could be in the spinal cord or forebrain (contralateral). Vision is also associated with the forebrain (primarily contralateral); thus, the left forebrain is likely to be involved. Obtundation can occur with diffuse lesions of the forebrain, too, so the lesion is probably quite large. If the function of other cranial nerves (III–XII), and general locomotion, coordination, and spinal reflexes are intact, then this also supports a lesion in the forebrain and not the brainstem, cerebellum, or spinal cord. Thus, both the signs of dysfunction and signs of normal function have helped you localise the lesion.

Figure 1. The NeuroMap of the brain Roman numerals refer to cranial nerves.

Table 2. Summary of neural signs that can occur with lesions in different parts of the brain

a = Part of CN XI (accessory nerve) arises from the cervical spinal cord, but it is hard to see signs of its dysfunction with lesions in this area of spinal cord.
b = Important links between the cerebellum and vestibular system mean that some vestibular signs may occur with lesions in certain parts of the cerebellum.
Note: Seizures will only occur with forebrain disease.

Figure and tables reprinted with permission from: Thomson C, Hahn C. Veterinary Neuroanatomy: A Clinical Approach. Elsevier Health Sciences; 2012. ISBN 9780702034824.

References

1.  Thomson C, Hahn C. Veterinary Neuroanatomy: A Clinical Approach. Elsevier Health Sciences; 2012. ISBN 9780702034824.