NEUROLOCALIZATION MADE EASY
Jared B. Galle, DVM, Diplomate ACVIM (Neurology)
Dogwood Veterinary Referral Center
4920 Ann Arbor-Saline Road
Ann Arbor, MI 48103
Localizing a neurologic problem to an anatomical region of the nervous system (neuroanatomic
localization) is key to making a neurologic diagnosis. I often find that understanding how the
anatomic regions work together is intimidating and confusing for most veterinarians. A basic
knowledge and understanding of how these regions normally function and interact with each
other will make it easier to interpret the neurologic exam findings and make a neuroanatomic
localization. This lecture will review the function regionals of the nervous system and
neurolocalization.
Neuroanatomical Regions
The nervous system is divided into anatomical regions based on their functions. These regions
function much like our government. The forebrain is similar to the president. It is responsible
for making decisions, initiating movements, leading the rest of the body, and controlling
behavior. The brainstem is similar to the military. It carries out whatever functions the forebrain
tells it to do through cranial nerves and large groups of neurons (upper motor neurons - UMN)
that travel down the spinal cord and synapse on lower motor neurons (LMN). It also conveys
sensory information to the forebrain and cerebellum through sensory tracts from the limbs. The
cerebellum is similar to the CIA. It is responsible for coordinating movements and making sure
that the other regions of the nervous system know what’s going on in all the others. The spinal
cord and peripheral nerves are an extension of the military (think of them as branches of the
military). They carry out the functions generated by the brainstem via motor UMNs and convey
information to the forebrain and cerebellum though sensory nerves and tracts.
There are eight anatomical regions of the nervous system where a neurologic lesion can be
localized: Forebrain, Cerebellum, Brainstem, C1-5 spinal cord segments, C6-T2 spinal cord
segments, T3-L3 spinal cord segments, L4-S3 spinal cord segments, and the peripheral nervous
system.
Neuroanatomic Localization
When localizing a neurologic problem, all of the deficits and abnormalities on the neurologic
exam should be explained by a single lesion in one anatomical region of the nervous system (“X”
marks the spot”). Neurologists are like pirates - we want one “X” on one neuroanatomical
region to account for all the deficits detected on our exam. When a single lesion does not
account for all deficits (more than one “X”), a multifocal or diffuse disease should be considered.
1. Forebrain: Includes the right and left cerebral cortex (gray and white matter, basal nuclei) and
the diencephalon (thalamus, hypothalamus). The forebrain controls behavior, initiates
movements, interprets all sensory information, and controls endocrine/autonomic functions.
Contralateral (opposite side) sensory and motor deficits are seen with a forebrain lesion.
Forebrain Functions
Forebrain Deficits/Abnormalities
Behavior, personality
• Behavior changes
• Pacing, wander aimlessly circling, getting stuck in
corners, head pressing, “reverse does not work”, not as
social, sleeping in different places, urinating/defecating
in house
All sensory inputs terminates here
• Postural reaction deficits
• Sensory deficits on face
• Vision loss
• Decreased superficial sensation
Initiates movement
• Mild weakness
• Seizures
Endocrine and autonomic
functions
• Endocrine disorders (PU/PD, eating abnormalities)
• Changes in consciousness and sleep
Contralateral proprioceptive deficits with a normal or almost normal gait is a hallmark sign of a
cerebral lesion. Another common sign of a forebrain lesion is walking in wide circles toward the
lesion side.
2. Brainstem: Includes the midbrain, pons, and medulla oblongata. The brainstem contains
large motors centers that project UMNs down the spinal cord that are needed for walking and
supporting weight. It also contains regulatory centers for consciousness (ascending reticular
activating system - ARAS) and has ten pairs of cranial nerves (3-12) which transmit sensory and/
or motor functions. Ipsilateral (same side) sensory and motor deficits are seen with a brain stem
lesion.
Brainstem Functions
Brainstem Deficits/Abnormalities
Motor centers
Paresis/Paralysis - moderate to severe
Cranial nerves
Cranial nerve deficits 3-12
ARAS
Alteration of consciousness (depression, stupor, coma)
Links postural reactions between
forebrain and spinal cord
Postural reaction deficits
A brainstem lesion rarely causes a single cranial nerve deficit (ie. facial paralysis) without
postural reaction deficits and some degree of weakness/paralysis. Most isolated cranial nerve
deficits are due to an idiopathic cranial nerve disease.
3. Cerebellum: Regulates and coordinates movement by controlling the rate, range, and force on
the ipsilateral side (same side) of the body. It also plays a role in maintaining balance with the
vestibular system. The cerebellum does not initiate movement. A cerebellar lesion typically
causes exaggerated movements (hypermetria), ataxia without weakness, and intention tremors.
A menace deficit can also be seen with a cerebellar lesion.
Cerebellar deficits are easy to remember if you use the acronym HAT - the cerebellum is a HAT
for the brainstem.
Cerebellum Functions
Cerebellum Deficits/Abnormalities
Maintains and coordinates movement
• Hypermetria
• Ataxia
• Tremors (intention)
•
4. Spinal Cord: The spinal cord starts at the foramen magnum and extends to the 6th lumbar
vertebra in dogs and the 7th lumbar vertebra in cats. It tapers to the conus medullaris. The
spinal cord is divided into four functional segments: C1-5, C6-T2, T3-L3, and L4-S3. It contains
sensory tracts and upper motor neurons (UMN) which are located in the white matter along the
periphery of the spinal cord. The spinal cord also contains lower motor neuron cell bodies
(LMN) in the grey matter which are the beginning of peripheral nerves.
The spinal cord is responsible for relaying sensory information (proprioception and nociception)
from the limbs to the forebrain and cerebellum, and for relaying UMN information from the
brainstem to the LMNs. The UMN stimulates the LMN cell bodies to produce a gait. The UMN
are primarily facilitatory to flexor muscles and inhibitory to extensor muscles. The UMN also
inhibits the LMN component of the reflex arc. The LMNs are located along the entire spinal
cord but for localization purposes, the ones routinely evaluated are located in the C6-T2 (thoracic
limb reflexes) and L4-S3 (pelvic limb reflexes) spinal cord segments.
The deficits seen with a spinal cord lesion are dependent upon which functional segment is
affected (ie. C1-5, C6-T2, T3-L3, L4-S3). The location of a lesion determines which limbs are
affected and whether there are UMN or LMN signs in the limbs. If the UMN going to those
limbs is affected, UMN signs will be seen. If the LMN going to those limbs is affected, LMN
signs will be seen.
UMN Signs
LMN Signs
Paresis/Paralysis
Paresis/Paralysis
Normal or exaggerated reflexes
Decreased or absent reflexes
Normal or increased tone
Decreased or absent tone
Prolonged muscle atrophy
Neurogenic atrophy (rapid)
Large, turgid, hard to express bladder
Atonic, easily expressed bladder
C1-5 Myelopathy: A lesion in these segments can result in deficits in all limbs (postural reaction
deficits, tetraparesis/tetraplegia) because it affects sensory tracts coming from and UMN going to
the forelimbs and pelvic limbs. Reflexes are normal or increased with a lesion in these segment.
An UMN bladder can also occur.
C6-T2 Myelopathy: A lesion in these segments can result in deficits in all limbs (postural
reaction deficits, tetraparesis/tetraplegia). It affects the LMN going to the forelimbs, the UMN
going to the pelvic limbs, and the sensory tracts coming from all limbs. Forelimb reflexes are
decreased to absent, while pelvic limb reflexes are normal to increased. An UMN bladder can
also occur.
T3-L3 Myelopathy: A lesion in these segments can result in deficits in the rear limbs (postural
reaction deficits, paraparesis/paraplegia). The forelimbs are normal. It affects sensory tracts
coming from and UMN going to the pelvic limbs. Pelvic limb reflexes are normal to increased.
An UMN bladder can also occur.
L4-S3 Myelopathy: A lesion in these segments can result in deficits in the rear limbs (postural
reaction deficits, paraparesis/paraplegia). The forelimbs are normal. It affects sensory tracts
coming from and LMN going to the pelvic limbs. Pelvic limb reflexes are decreased to absent.
Anal tone may also be poor to absent, and the perineal reflex may be decreased to absent. A
LMN bladder can also occur.
Spinal Cord Lesion
Forelimb Signs
Pelvic Limb Signs
C1-5 myelopathy
UMN
UMN
C6-T2 myelopathy
LMN
UMN
T3-L3 myelopathy
Normal
UMN
L4-S3 myelopathy
Normal
LMN
Peripheral Nervous System (Neuromuscular)
The peripheral nerve, skeletal muscle, and neuromuscular junction are grouped into the same
neuroanatomic localization. A lesion affecting one of these components of the peripheral
nervous system results in varying amounts of weakness. Weakness can be seen in both cranial
nerves and peripheral nerves in the limbs. A key feature to help distinguish a peripheral nervous
system lesion from a central nervous system lesion is that postural reactions are typically intact
with a peripheral nervous system lesion, whereas they are delayed or absent with a central
nervous system lesion. Other neurologic deficits include: reduced or absent reflexes, decreased
or absent muscle tone, exercise intolerance, and neurogenic atrophy.
Peripheral Nervous System Lesion
Forelimbs
Rear Limbs
Peripheral nerve, neuromuscular
junction, skeletal muscle
LMN
LMN
References available upon request