Lesion Localization

lesion localization

process of using the neurologic examination to determine which specific region of the nervous system is dysfunctional

The primary goal of lesion localization is to determine...

where in the nervous system the lesion is located

Why is lesion localization so important in neurology?

it determines where the problem is located, which then guides diagnostics and prevents unnecessary or inappropriate testing

Lesion localization should be done _______ diagnostic imaging. Why?

before; imaging must be targeted to the correct region

*imaging the wrong region may be useless, expensive, and potentially expose the patient to unnecessary anesthesia

The nervous system can first be divided into two major locations when localizing a neurologic lesion:

1.) central nervous system

2.) peripheral nervous system

The central nervous system is further broken down into two major strucutres:

1.) brain

2.) spinal cord

The brain is further broken down into three major regions for lesion localization:

1.) forebrain/thalamocortex

2.) brainstem

3.) cerebellum

The peripheral nervous system is further broken down into three major locations:

1.) nerves

2.) neuromuscular junction

3.) muscle

When localizing a suspected spinal cord lesion, the first major question is...

which limbs are neurologically abnormal

Spinal cord lesion localization begins with...

a gait examination

Why is gait examination important for spinal cord lesion localization?

it identifies which limbs are affected, which immediately narrows the possible lesion locations

For spinal cord lesion localization, limb involvement can initially be divided into two major patterns:

1.) all four limbs affected

2.) pelvic limbs only affected

If all four limbs are affected by a spinal cord lesion, the lesion is where?

one of the cervical regions: C1-C5 or C6-T2

If only the pelvic limbs are affected by a spinal cord lesion, the lesion is where?

in one of the more caudal regions: T3-L3 or L4-S3

The next step is then to distinguish between pelvic limb vs all four limbs is to...

use UPN vs LMN signs

Upper motor neurons (UPN)

motor pathway that originates in the brain and remains within the CNS to control movement

The upper motor neuron is considered the "_____". Why?

boss; it sends instructions and regulates movement and depends on the lower motor neuron to actually carry the signal to the muscle

Where does the upper motor neuron pathway travel?

from the brain → through the white matter of the spinal cord → toward the lower motor neurons

Functions of the upper motor neuron (3)

1.) initiate voluntary movement

2.) make postural adjustments

3.) maintain appropriate muscle tone

the combined UMN system tends to facilitate _________ muscles and inhibit ________ muscles

flexor; extensor

*UMNs exert an important inhibitory influence over extensor tone

When there is an UPM lesion, extensor tone will be ________. Why?

increased; the lesion will remove the normal inhibition of the extensor system

Lower motor neuron (LMN)

motor pathway that begins with its cell body in the gray matter of the spinal cord, leaves the CNS, and ultimately innervates skeletal muscle

Where does the lower motor neuron pathway travel?

Ventral gray column → nerve root → spinal nerve → peripheral nerve → neuromuscular junction → muscle

The lower motor neuron is considered the "________". Why?

employee; it carries out the motor command by transmitting the signal from the CNS to the muscle

Lower motor neurons are required for both:

1.) voluntary motor activity

2.) spinal reflexes

Why does LMN damage affect both voluntary movement and spinal reflexes?

the LMN provides the final motor pathway to the muscle for both voluntary movement and the local reflex arc

The LMN is aka the...

Final Common Pathway

Why are LMNs aka the Final Common Pathway?

because regardless of how well the brain and descending UMN pathways are functioning, movement cannot occur if the final connection to the muscle is disrupted

Four findings can be used to compare UMN and LMN dysfunction:

1.) motor function

2.) muscle tone

3.) spinal reflexes

4.) muscle atrophy

How can motor function be affected with either an UMN or LMN lesion?

either can cause paresis or paralysis/plegia depending on severity (so weakness alone does not distinguish the two!)

UMN dysfunction:

motor function

tone

reflexes

atrophy

motor function: paresis/ paralysis

tone: normal to increased

reflexes: normal to increased

atrophy: slower/less severe

LMN dysfunction:

motor function

tone

reflexes

atrophy

motor function: paresis/ paralysis

tone: decreased to absent

reflexes: decreased to absent

atrophy: rapid/more severe (due to denervation)

LMN signs can only occur in a limb when...

the lesion affects the LMNs that supply that limb

There are two spinal cord regions where limb LMN signs can occur:

1.) C6-T2

2.) L4-S3

C6-T2 spinal cord regions localize to _______ limbs

thoracic limbs

Why do thoracic limb LMN signs localize to C6-T2?

C6-T2 contains the LMNs that contribute to the brachial plexus and innervate the thoracic limbs

L4-S3 spinal cord regions localize to _______ limbs

pelvic limbs

Why do pelvic limb LMN signs localize to L4-S3?

L4-S3 contains the LMNs that contribute to the lumbosacral plexus and innervate the pelvic limbs

There are four major spinal cord regions used for lesion localization:

1.) C1-C5

2.) C6-T2

3.) T3-L3

4.) L4-S3

The localization process can therefore be simplified into two questions:

1. Which limbs are affected? This tells you which two regions are possible

2. Are the affected limbs showing UMN or LMN signs? This narrows those two possibilities to one region

If all four limbs are affected, the lesion can initially be localized to one of two spinal cord regions:

1.) C1-C5

2.) C6-T2

If only the pelvic limbs are affected, the lesion can initially be localized to one of two spinal cord regions:

1.) T3-L3

2.) L4-S3

Once the affected limbs have narrowed the lesion to two possible spinal cord regions, the next step is to determine...

whether the affected limbs demonstrate UMN or LMN signs

All four limbs affected: UMN vs. LMN findings distinguish between...

C1-C5 vs. C6-T2

Pelvic limbs only affected: UMN vs. LMN findings distinguish between...

T3-L3 vs. L4-S3

C1-C5 spinal cord lesions

affect the cervical spinal cord cranial to the thoracic limb LMNs

C1-C5 spinal cord lesions:

affected limbs:

thoracic limbs (UPM/LMN signs):

pelvic limbs (UPM/LMN signs):

affected limbs: all four

thoracic limbs (UPM/LMN signs): UMN signs

pelvic limbs (UPM/LMN signs): UMN signs

Why do C1-C5 spinal cord lesions affect all four limbs?

Because descending motor pathways to both the thoracic and pelvic limbs pass through this region

Clinical signs of a patient with a C1-C5 spinal cord lesion

-ataxia and tetraparesis or tetraplegia

-muscle tone normal to increased

C6-T2 spinal cord lesion

cervical spinal cord lesion that affects all four limbs and produces LMN signs in the thoracic limbs with UMN signs in the pelvic limb

C6-T2 spinal cord lesions:

affected limbs:

thoracic limbs (UPM/LMN signs):

pelvic limbs (UPM/LMN signs):

affected limbs: all four

thoracic limbs (UPM/LMN signs): LMN signs

pelvic limbs (UPM/LMN signs): UMN signs

Why does a C6-T2 lesion cause LMN signs in the thoracic limbs?

C6-T2 contains the LMNs that directly innervate the thoracic limbs

Why does a C6-T2 lesion cause UMN signs in the pelvic limbs?

the lesion interrupts descending UMN pathways to the pelvic limbs but is cranial to the pelvic limb LMNs at L4-S3, so the pelvic limb LMNs/reflex arcs remain intact

Clinical signs of a patient with a C6-T2 spinal cord lesion

-decreased to absent thoracic limb muscle tone

-normal to increased pelvic limb muscle tone

T3-L3 spinal cord lesions

thoracolumbar spinal cord lesions that leave the thoracic limbs normal and produces UMN signs in the pelvic limbs

T3-L3 spinal cord lesions:

affected limbs:

thoracic limbs (UPM/LMN signs):

pelvic limbs (UPM/LMN signs):

affected limbs: pelvic limbs only

thoracic limbs (UPM/LMN signs): remain normal

pelvic limbs (UPM/LMN signs): UMN signs

Why does a T3-L3 lesion produce UMN signs in the pelvic limbs?

it interrupts descending UMN pathways but is cranial to the pelvic limb LMNs at L4-S3, so the LMNs and local reflex arcs remain intact

Clinical signs of a patient with T3-L3 spinal cord lesions

-increased muscle tone of the pelvic limbs

-normal muscle tone of the thoracic limb

L4-S3 spinal cord lesion

caudal spinal cord lesion involving the LMNs that innervate the pelvic limbs, producing pelvic limb LMN signs

L4-S3 spinal cord lesions:

affected limbs:

thoracic limbs (UPM/LMN signs):

pelvic limbs (UPM/LMN signs):

affected limbs: pelvic limbs

thoracic limbs (UPM/LMN signs): normal

pelvic limbs (UPM/LMN signs): LMN signs

Why does an L4-S3 lesion produce LMN signs in the pelvic limbs?

the lesion directly affects the LMNs and local reflex arcs responsible for pelvic limb function

Clinical signs of a patient with a L4-S3 spinal cord lesion

-decreased to absent muscle tone of the pelvic limb

-normal muscle tone of thoracic limb

Four spinal cord regions: which limbs are affected?

C1-C5: all four limbs

C6-T2: all four limbs

T3-L3: pelvic limbs only

L4-S3: pelvic limbs only

Four spinal cord regions: UMN vs. LMN findings

C1-C5: thoracic UMN + pelvic UMN

C6-T2: thoracic LMN + pelvic UMN

T3-L3: thoracic normal + pelvic UMN

L4-S3: thoracic normal + pelvic LMN

The easiest way to understand spinal cord localization is to avoid memorizing four disconnected lists. Instead...

work through the same two questions every time (which limbs are affected? are there UMN or LMN signs?)

For a patient with a suspected spinal cord lesion, think:

1. Which limbs are abnormal?

All 4 → cervical → C1-C5 or C6-T2

Pelvic only → thoracolumbar/lumbosacral → T3-L3 or L4-S3

Then:

2. Look at muscle tone and spinal reflexes in the affected limbs.

Thoracic LMN signs? → C6-T2

No thoracic LMN signs? → C1-C5

or

Pelvic LMN signs? → L4-S3

No pelvic LMN signs / UMN signs? → T3-L3

LMN signs occur when the lesion hits the...

spinal cord region containing the LMNs that directly supply that limb

The four major spinal cord regions are useful for general localization, however...

lesions can sometimes affect only part of a spinal cord region, producing a more specific pattern of neurologic deficits

Rather than affecting the entire region, a lesion can affect particular spinal cord segments and therefore particular peripheral nerves. This can produce a mixture of...

UMN and LMN findings within the same limb!

Within the C6-T2 region, lesions can sometimes be localized more specifically to...

C6-C8

A C6-C8 lesion can produce two contradictory findings in the thoracic limbs:

1.) increased extensor tone

2.) decreased flexor reflex

Increased thoracic limb extensor tone is an ______ sign

Decreased thoracic limb flexor reflex is an ______ sign

UMN

LMN

Therefore, a C6-C8 lesion can produce both ______ and ______ signs within the same thoracic limb

UMN; LMN

Thoracic limb extension is through what nerve?

radial nerve

Thoracic limb flexion is through what nerve?

musculocutaneous nerve

Between the radial and musculoskeletal nerves, which receives more contribution from the C6-C8 spinal nerves?

musculoskeletal

Therefore, a lesion concentrated around C6-C8 can cause weakness of ________ while ________ remains increased

flexion; extension

Overall C6-C8 localization pattern

-all four limbs may be affected

-thoracic limbs have increased extensor tone but decreased flexor reflex

-pelvic limbs have UMN signs

Cervical Spondylomyelopathy Pattern

aka Wobbler syndrome; commonly produces the specific C6-C8 lesion pattern

Two anatomical locations must be distinguished when discussing spinal lesions:

1.) spinal cord segment

2.) vertebral level

spinal cord segment

functional portions of the spinal cord associated with particular spinal nerves

vertebral level

refer to the actual bones of the vertebral column surrounding the spinal cord

True or false: A spinal cord segment and its corresponding vertebra are not necessarily located at the same level

true!

Spinal cord segments and vertebral levels especially don't correspond in the ______ spinal cord

caudal

As you move caudally, spinal cord segments are located progressively more ______ than their corresponding vertebrae

cranially

Why are caudal spinal cord segments located cranial to their corresponding vertebrae?

the spinal cord ends before the vertebral column ends, so the caudal spinal cord segments become compressed into more cranial vertebral levels

Why is the knowing that spinal cord segments and vertebral levels don't always line up important?

For example, if you say there is a lesion involving the L4-S3 spinal cord segment, this does not mean it lies within the L4-S3 vertebrae

The L4-S3 spinal cord segments are actually located within which vertebral levels in most dogs?

L4-L5 vertebrae

Cauda equina

the collection of spinal nerve roots that continues caudally through the vertebral canal after the spinal cord itself has ended

At which vertebral body does the spinal cord end in the dog?

L5!!

*EXAM

At the L6-L7/L7-S1 vertebral region, there is no longer spinal cord; instead there are primarily...

nerve roots of the cauda equina

The cauda equina is part of the CNS or PNS?

PNS

Therefore, lesions affecting the caudal lumbar/lumbosacral vertebral column may affect _________ nerve roots rather than the _______ ________ itself

peripheral; spinal cord

Peripheral nerves have a greater ability to ______ following injury than the CNS

regenerate

The clinically relevant caudal spinal cord segments are primarily located around the ______ vertebral level

L5!!!

*Exam

Which spinal cord segments are particularly associated with the L5 vertebral region?

S1-S3

S1-S3 spinal cord segments are especially important for what nerve?

pudendal nerve

The pudendal nerve is important for...

urinary and fecal continence

A lesion involving the S1-S3 spinal cord segments can therefore result in...

urinary and/or fecal incontinence

The femoral nerve responsible for pelvic limb extension/weight support arises primarily from...

L4-L6

The sciatic nerve is associated primarily with...

L6-S2