Disorders of the Forebrain

Forebrain (prosencephalon)

The portion of the brain responsible for conscious awareness, sensory integration, cognition, behavior, and voluntary movement

The forebrain consists of two major divisions:

1.) Telencephalon

2.) Diencephalon

Telencephalon

Division of the forebrain that includes the cerebral cortex and is responsible for higher cognitive functions, conscious perception, and voluntary behavior

Diencephalon

Division of the forebrain that includes the thalamus and participates in sensory relay and integration

Five major functions of the forebrain:

1.) Consciousness and awareness

2.) Sensory integration

3.) Voluntary motor function

4.) Cognition

5.) Appropriate behavior

Sensory integration

The process of receiving and interpreting sensory information to produce an appropriate response

Conscious perception of sensory information requires processing by the ______

Forebrain/cerebral cortex

The cerebral cortex is the primary site of ______. What does this mean?

Integration; it processes sensory information and helps initiate appropriate responses

True or false: an animal may respond reflexively to a stimulus without consciously perceiving it

true!

Most sensory and voluntary motor pathways associated with the forebrain undergo _________

decussation

Decussation

Crossing of nerve pathways from one side of the nervous system to the other

Due to decussation, lesions generally cause sensory and motor deficits on the ______ side of the body

Contralateral

A left forebrain lesion generally causes deficits on the ______ side

A right forebrain lesion generally causes deficits on the ______ side

Bilateral or centrally located lesions may produce deficits on ______ sides

Right

Left

Both

Four normal functions of the forebrain:

1.) mentation

2.) sensory integration

3.) voluntary motor function

4.) cognition

Mentation

An animal's mental state, including its level of consciousness and appropriateness of behavior

Two components of mentation:

1.) Quantity (level of consciousness)

2.) Quality (appropriateness of behavior)

Quantity of mentation

The animal's level of consciousness or arousal

Quality of mentation

Whether an animal's behavior is appropriate for its surroundings and circumstances

Ex:

-a frightened cat attempting to escape an unfamiliar examination room may be behaving normally

-However, a dog repeatedly running into walls, pacing aimlessly, or becoming stuck in corners may be demonstrating inappropriate behavior

Five categories of level of consciousness:

1.) Normal

2.) Lethargic/depressed

3.) Obtunded

4.) Stuporous

5.) Comatose

Changes in the quantity of mentation are ______ specific to forebrain disease. Why?

Not; altered consciousness can also occur with brainstem disease or systemic illness

Inappropriate mentation

Abnormal behavior that does not make sense in the animal's circumstances

Five examples of inappropriate mentation:

1.) Aimless pacing

2.) Head pressing

3.) Wandering

4.) Becoming stuck in corners

5.) Inappropriate environmental interaction

The forebrain receives and processes sensory information, including...

vision, touch, pain, and conscious proprioception

Forebrain processing allows an animal to ______ a stimulus and produce an appropriate ________

recognize; response

Reflex vs. conscious response

A reflex occurs without cortical processing, while a conscious response requires sensory information to reach and be processed by the forebrain

Voluntary motor function of the forebrain

forebrain participates in planning and initiating voluntary movement

Due to voluntary motor function of the forebrain, the forebrain provides the _______ behind locomotion

purpose

How is the forebrain involved in cognition?

it is involved in learning, memory, and higher intellectual functions

Cognition allows an animal to do three things:

1.) recognize familiar individuals and environments

2.) remember previously learned behaviors

3.) adapt its responses to changing circumstances

Therefore, degenerative forebrain diseases can result in...

confusion, altered interactions, and behavioral changes

Four conscious responses evaluated during the neurological examination:

1.) Menace response

2.) Nasal sensation

3.) Conscious pain perception

4.) Postural reactions

Menace response

A learned, conscious blink response to an approaching threatening visual gesture

What does the menace response evaluate?

the animal's ability to see the approaching gesture, consciously interpret it as threatening, and initiate a blink

Three major structures involved in the menace response:

1.) CN II

2.) Cerebral cortex/cerebellum

3.) CN VII

CN II function during the menace response

Carries visual information from the retina

Cerebral cortex function during the menace response

Consciously processes the approaching visual stimulus

CN VII function during the menace response

Activates the muscles responsible for blinking

Menace response pathway

Retina → CN II → visual pathways → visual cortex → motor pathways → CN VII → blink (with cerebellar involvement)

A unilateral forebrain lesion may cause an absent menace response ______ to the lesion

Contralateral

The PLR generally remains ______ with an isolated forebrain lesion. Why?

Intact; the PLR does not require cerebral cortical processing

Therefore, absent menace response with an intact PLR may indicate dysfunction involving the...

Forebrain/cerebral cortex

Nasal sensation

evaluates conscious recognition of tactile stimulation applied to the nose

Two major structures involved in nasal sensation:

1.) CN V (sensory)

2.) Thalamocortex

CN V function during nasal sensation

Carries sensory information from the nose

Thalamocortex function during nasal sensation

Relays and processes sensory information so the animal can consciously acknowledge the stimulus

A unilateral forebrain lesion may cause decreased nasal sensation ______ to the lesion, despite intact peripheral sensory pathways

Contralateral

Conscious pain perception

involves the conscious recognition of a painful stimulus by the forebrain

Withdrawal reflex vs. conscious pain perception

Withdrawal is a spinal reflex; conscious pain perception requires cortical processing and a behavioral acknowledgment

An animal may retain its withdrawal reflex despite lacking...

Conscious pain perception

Three examples of conscious acknowledgment of a painful stimulus:

1.) Turning toward stimulus

2.) Vocalizing

3.) Attempting to escape

A unilateral forebrain lesion may cause decreased conscious pain acknowledgment ______ to the lesion

Contralateral

Ex: dog with a left forebrain lesion may withdraw its right pelvic limb when stimulated but fail to turn toward or otherwise acknowledge the stimulus

Proprioception

The nervous system's ability to detect the position and movement of the body and limbs

Two types of proprioception

1.) Conscious proprioception

2.) Unconscious proprioception

Conscious proprioception

Awareness of limb position and movement; involves the forebrain

Unconscious proprioception

Automatic detection of body position and movement for coordination; involves the cerebellum

Postural reactions

Responses that assess the integrity of sensory and motor pathways throughout the nervous system

Two postural reactions in forebrain localization:

1.) Proprioceptive placing

2.) Hopping

Postural reaction deficits have ______ localizing value when evaluated alone. Why?

Limited; the pathways involve multiple regions of the nervous system

Forebrain lesions generally produce postural reaction deficits ______ to the lesion

Contralateral

Seven clinical signs of forebrain dysfunction:

1.) abnormal mentation

2.) hemineglect

3.) head turn and pleurothotonus

4.) circling and gait abnormalities

5.) cranial nerve abnormalities

6.) spinal reflexes and muscle tone

7.) seizures

examples of abnormal mentation

pacing, head pressing, wandering, becoming stuck in corners, and inappropriate interactions with the environment

hemineglect

failure to consciously acknowledge sensory information originating from one side of the body or surrounding environment, usually contralateral to a forebrain lesion

Due to hemineglect, an animal with a right forebrain lesion may ignore stimuli on its ________ side

left

An animal with hemineglect may:

-Ignore people or objects approaching from one side.

-Fail to respond to tactile stimulation on the affected side.

-Turn its head toward the unaffected side.

-Circle toward the side of the lesion.

-Become stuck against obstacles because it does not recognize part of its environment

head turn

sustained deviation of the head toward one side without the characteristic rotation or tilt associated with vestibular dysfunction

Pleurothotonus

sustained turning or curvature of the body toward one side

In a unilateral forebrain lesion, the head and body typically turn _______ the lesion!

toward

Why will an animal turn toward the lesion with a unilateral forebrain lesion?

the animal neglects the contralateral side of its environment; since it only acknowledges the unaffected portion of its surroundings, it tends to orient its head and body toward that side

A right forebrain lesion may produce a _______ head turn because the animal neglects its _______ side

right; left

Animals with unilateral forebrain lesions frequently ________ toward the side of the lesion

circle

Why do animals circle with unilateral forebrain lesions?

the animal continuously orients itself toward the side of its environment that it can consciously acknowledge

What will the gait of an animal with forebrain disease look like?

Normal or nearly normal gait combined with contralateral postural reaction deficits

ex: a dog may walk normally but have markedly delayed proprioceptive placing in the right limbs due to a left forebrain lesion

What type of cranial nerve tests will a forebrain lesion affect?

cranial nerve tests requiring conscious cortical processing

Three cranial nerve tests that require conscious cortical processing:

1.) menace response

2.) nasal sensation

3.) facial stimulation

In contrast, ________ mediated cranial nerve reflexes will stay intact with forebrain lesions, such as...

brainstem; PLR

Spinal reflexes are generally ______ with isolated forebrain lesions. Why?

Normal; spinal reflex arcs do not require cortical processing.

Muscle tone with forebrain disease may be ______ or ______

Normal; increased

Seizure

Episode of abnormal, excessive, synchronous electrical activity within cerebral neuronal networks

Seizures are a particularly important indicator of forebrain dysfunction because...

epileptic seizures originate from abnormal electrical activity within cerebral neuronal networks

For a left forebrain lesion, what would be your expected findings for the following?

-mentation

-hemineglect

-head/body turn

-circling

-menace response

-nasal sensation

-postural reaction deficits

-spinal reflexes

-mentation --> inappropriate mentation

-hemineglect --> right-sided hemineglect

-head/body turn --> left head/body turn

-circling --> left circling

-menace response --> decreased right menace response

-nasal sensation --> decreased right nasal sensation

-postural reaction deficits --> right-sided postural reaction deficits

-spinal reflexes --> normal spinal reflexes

*opposite would be true for a right forebrain lesion

What is the foundation of the diagnostic approach for forebrain disease?

neurological examination and medical history (signalment + history)

Eight major categories of forebrain diseaes:

1.) degenerative disease

2.) congenital/developmental

3.) metabolic

4.) neoplastic

5.) infectious meningoencephalitis

6.) immune-mediated meningoencephalitis

7.) traumatic brain injury

8.) cerebrovascular disease

degenerative disease

involve progressive deterioration of nervous tissue and may affect the forebrain alone or multiple regions of the nervous system

degenerative disease:

-development speed

-rate of progression

-laterality

-symmetry

-pain

-development speed: slowly developing

-rate of progression: progressive

-laterality: bilateral

-symmetry: symmetrical

-pain: non-painful

Signalment of degenerative disease

young, adult, and older dogs! depends on disease

Three degenerative forebrain diseases:

1.) lysosomal storage diseases

2.) leukodystrophy/spongy degeneration

3.) cognitive dysfunction syndrome (CDS)

lysosomal storage diseases

inherited disorders involving abnormal accumulation of substances within cells due to defects in lysosomal function

leukodystrophy/spongy degeneration

involves abnormalities or degeneration of nervous tissue, particularly white matter

cognitive dysfunction syndrome (CDS)

age-associated neurodegenerative disorder that causes progressive deterioration of cognitive abilities; aka canine cognitive dysfunction

*comparable to dementia in humans

Four major clinical signs of cognitive dysfunction syndrome (CDS):

1.) abnormal sleep wake cycles

2.) anxiety/restlessness

3.) decreased interactions with owners/toys/environment

4.) confusion

Congenital/developmental forebrain diseases

arise from abnormalities in brain development

Congenital/developmental forebrain diseases:

-rate of progression

-distribution

-symmetry

-pain

-rate of progression: progressive or nonprogressive

-distribution: focal or multifocal

-symmetry: symmetrical or asymmetrical

-pain: often non painful

Clinical signs of congenital/developmental forebrain diseases often occur when?

early in life

Three congenital/developmental forebrain diseases:

1.) congenital hydrocephalus

2.) intracranial arachnoid diverticulum

3.) neuronal migration disorders

congenital hydrocephalus

abnormal accumulation of CSF within the ventricular system that causes ventricular enlargement and may compress surrounding brain tissue

congenital hydrocephalus is associated with what breeds?

toy and brachycephalic breeds

clinical presentation of a dog with congenital hydrocephalus

puppy with markedly enlarged skull and persistent open fontanelles (membrane where skull has not yet fused); may or may not have neurological deficits

intracranial arachnoid diverticulum

CSF-filled expansion associated with the arachnoid membrane that may compress adjacent brain tissue

neuronal migration disorders

Developmental abnormality resulting from improper migration or organization of neurons during cerebral cortex formation (affects sulci and gyri)

Metabolic encephalopathy

neurological dysfunction caused by systemic metabolic abnormalities rather than a primary structural lesion of the brain