Brainstem Disease & Diagnoses

Brainstem

Region of the CNS that connects the brain to the spinal cord and contains important structures responsible for neurologic and autonomic functions

Three regions of the brainstem:

1.) Midbrain

2.) Pons

3.) Medulla

The brainstem acts as a "funnel". How?

Information traveling from the brain to the body must pass through the brainstem

Sensory information traveling from the body to the brain (e.g., proprioception and nociception) must also pass through the brainstem

Three important structures within the brainstem:

1.) Cranial nerve nuclei

2.) Reticular activating system

3.) Medullary cardiovascular and respiratory centers

Which cranial nerve nuclei are within the brainstem?

CNs III-VII

Function of the cranial nerve nuclei within the brainstem

responsible for various sensory and motor functions of the eyes, face, head, and neck

Reticular activating system (RAS)

Network of neurons within the brainstem responsible for maintaining consciousness and wakefulness

How does the RAS maintain consciousness?

Receives sensory information from the body and cranial nerves and projects it through the thalamus to the cerebral cortex to maintain wakefulness

Medullary cardiovascular center

Regulates heart rate, heart rhythm, and blood pressure

Medullary respiratory center

Regulates respiratory rate, rhythm, and ventilation

Two major types of pathways that pass through the brainstem:

1.) Motor pathways

2.) Proprioceptive pathways

Motor pathways

Carry information from the brain to the body and limbs to regulate movement, strength, and coordination

Three important motor pathways associated with the brainstem:

1.) Rubrospinal tract

2.) Pontine reticulospinal tract

3.) Medullary reticulospinal tract

Function of the rubrospinal and reticulospinal tracts

Important for the generation and regulation of gait

Proprioceptive pathways

Carry information about body and limb position from the body toward the brain

General pathway of proprioceptive information

Spinal cord → brainstem → contralateral cerebral cortex

Six brainstem structures/functions and how they are assessed on examination:

1.) CN nuclei: cranial nerve exam

2.) RAS: mentation

3.) Motor pathways: gait

4.) Proprioceptive pathways: postural reactions

5.) Cardiovascular center: HR, rhythm, BP

6.) Respiratory center: respiratory rate, character, CO₂

Why can a brainstem lesion produce multiple different neurologic abnormalities?

The brainstem contains numerous important nuclei and pathways within a relatively small area, so a lesion may disrupt several functions simultaneously

Four major components of the neurologic exam used to identify brainstem dysfunction:

1.) Cranial nerve exam

2.) Mentation

3.) Gait

4.) Postural reactions

Two additional body systems that may be affected by brainstem lesions:

1.) Cardiovascular

2.) Respiratory

Cranial nerve deficits associated with a focal brainstem lesion are generally _________ to the lesion

ipsilateral

Three cranial nerves that innervate muscles responsible for eye movement:

1.) CN III (oculomotor)

2.) CN IV (trochlear)

3.) CN VI (abducens)

CN III (oculomotor) innervates...

most extraocular muscles, the levator palpebrae superioris, and provides parasympathetic innervation for pupillary constriction

CN IV (trochlear) innervates...

dorsal oblique muscle

CN VI (abducens) innervates...

lateral rectus and retractor bulbi muscles

Four ways to assess CN III, IV, and VI function:

1.) Pupil size/symmetry

2.) PLR

3.) Oculocephalic reflex

4.) Globe position/movement

Five signs of dysfunction of ocular movement cranial nerves:

1.) mydriasis (dilated pupil)

2.) inability to constrict pupil

3.) ptosis (dropping of upper eyelid)

4.) impaired globe movement

5.) strabismus (abnormal eye position)

Ophthalmoplegia

paralysis of the muscles responsible for ocular function

Internal ophthalmoplegia

Inability to constrict the pupil due to dysfunction of the intraocular muscles

External ophthalmoplegia

Impaired movement of the globe due to paralysis of the extraocular muscles

Both internal and external ophthalmoplegia occur with dysfunction of what cranial nerve?

CN III

Cranial nerve V

trigeminal nerve; provides sensation to face and motor function to muscles of mastication

Four ways cranial nerve V is assessed:

1) palpebral reflex

2.) nasal septum sensation

3.) masticatory muscle mass

4.) jaw tone

Five signs of cranial nerve V dysfunction:

1.) Reduced facial sensation ipsilateral to the lesion.

2.) Reduced or absent palpebral reflex when stimulating the affected side.

3.) Reduced response to nasal septum stimulation.

4.) Atrophy of the ipsilateral masticatory muscles (temporalis and masseter).

5.) Reduced jaw tone or inability to close the mouth with bilateral trigeminal dysfunction

Unilateral CN V dysfunction can result in...

Bilateral CN V dysfunction can result in...

Unilateral CN V dysfunction : unilateral masticatory muscle atrophy (ipsilateral to lesion)

Bilateral CN V dysfunction: dropped jaw

Cranial nerve VII

facial nerve; provides motor innervation to the muscles of facial expression and is responsible for blinking

Three additional functions of CN VII

1.) Blinking

2.) Sensation inside the pinna

3.) Parasympathetic innervation for tear production

Four ways to assess Cranial nerve VII

1.) Palpebral reflex (motor/efferent component)

2.) Menace response (motor/efferent component)

3.) Facial symmetry

4.) Ability to blink

Five signs of cranial nerve VII dysfunction:

1.) Inability to blink ipsilateral to the lesion

2.) Absent or reduced palpebral reflex and menace response due to impaired blinking

3.) Ipsilateral facial drooping/asymmetry

4.) Reduced sensation inside the pinna

5.) Reduced tear production

Why can CN VII dysfunction cause an absent menace response despite normal vision?

The animal can see the threatening stimulus but cannot produce the motor blink response

Why can CN VII dysfunction cause reduced tear production?

CN VII provides parasympathetic innervation to the lacrimal glands.

Cranial nerve VIII

Vestibulocochlear nerve; responsible for hearing and balance

*more involved with vestibular disease

Five signs of Cranial nerve VIII dysfunction:

1.) Head tilt

2.) Abnormal nystagmus

3.) Positional strabismus

4.) Abnormalities of the oculocephalic reflex

5.) Vestibular ataxia

Four nerves involved in pharyngeal/laryngeal/tongue function:

1.) CN IX

2.) CN X

3.) CN XI

4.) CN XII

CN IX

Sensory and motor contributions to pharyngeal function

CN X

Important for pharyngeal and laryngeal function

CN XI

Motor innervation to selected neck muscles

CN XII

Motor innervation to the tongue

Six ways to assess cranial nerves involved in pharyngeal/laryngeal/tongue function:

1.) Gag reflex (when safe to perform)

2.) Tongue symmetry and muscle mass

3.) History of swallowing/eating difficulties

4.) History of changes in vocalization

5.) Observation of eating, if indicated

6.) Assessment of breathing for signs of laryngeal dysfunction

Five signs of dysfunction of cranial nerves involved in pharyngeal/laryngeal/tongue function:

1.) Dysphagia → Difficulty swallowing

2.) Dysphonia → Change in the sound or tone of the bark/meow

3.) Tongue muscle atrophy (particularly with CN XII dysfunction)

4.) Laryngeal paralysis

5.) Abnormal gag reflex

Why is a complete neurologic exam important when evaluating CN IX-XII abnormalities?

These signs are not exclusive to brainstem disease; additional neurologic abnormalities help distinguish brainstem lesions from peripheral nerve or muscle disease

Mentation

an animal's level of consciousness, awareness, and responsiveness to its environment

Lesions in what structure of the brainstem will cause altered mentation?

reticular activating system (RAS)

Dullness/lethargy

Mild reduction in alertness or responsiveness; can occur with lesions elsewhere or systemic illness

Obtundation

Markedly subdued consciousness and decreased responsiveness, although the patient can still respond to stimulation

Stupor

Patient is otherwise unresponsive but may respond to noxious stimulation

Coma

Patient is completely unresponsive, even to noxious stimulation

Mild mentation changes are relatively ________. However, severe obtundation, stupor, and coma should raise strong suspicion for...

nonspecific; brainstem involvement, particularly when accompanied by other brainstem signs

Two major categories of gait abnormalities associated with brainstem lesions:

1.) Paresis

2.) Ataxia

Paresis

Weakness or reduced ability to generate voluntary movement

Two patterns of paresis associated with brainstem lesions:

1.) Tetraparesis

2.) Hemiparesis

Tetraparesis →

Hemiparesis →

Tetraparesis → Weakness affecting all four limbs

Hemiparesis → Weakness affecting the thoracic and pelvic limbs on the same side of the body

Ataxia

Incoordination of movement

Three types of ataxia associated with brainstem lesions:

1.) Proprioceptive

2.) Vestibular

3.) Mixed

Proprioceptive ataxia

Incoordination due to disruption of pathways responsible for body position awareness

Vestibular ataxia

Loss of balance and coordination associated with vestibular dysfunction

Mixed ataxia

Combination of proprioceptive and vestibular abnormalities

Brainstem region associated with vestibular ataxia

Rostral medulla, where the vestibular nuclei reside

Why can brainstem lesions cause paresis and ataxia?

Important descending motor pathways and ascending proprioceptive pathways originate within or pass through the brainstem

Postural reactions

Responses that assess an animal's ability to recognize limb position and appropriately correct that position

Four postural reactions used to assess proprioceptive pathway function:

1.) Paw placement

2.) Hopping

3.) Wheelbarrowing

4.) Extensor postural thrust

Postural reaction deficits associated with brainstem lesions

Delayed or absent postural reactions, generally ipsilateral to the lesion

Why are postural reaction deficits generally ipsilateral with brainstem lesions?

The relevant proprioceptive pathways travel through most of the brainstem before crossing to project to the contralateral cerebral cortex

Exception to ipsilateral postural reaction deficits with brainstem lesions

Very far rostral brainstem lesions may produce contralateral deficits depending on the pathways affected

Postural reaction laterality: forebrain vs. brainstem

Forebrain: typically contralateral

Brainstem: generally ipsilateral

Neurolocalization

Using abnormalities on the neurologic examination to determine where a lesion is located within the nervous system

Four most important examination findings for brainstem neurolocalization

1.) Mentation

2.) Gait

3.) Cranial nerve deficits

4.) Postural reaction deficits

Mentation findings supporting brainstem localization

Profound changes in consciousness, including severe obtundation, stupor, or coma

Gait findings supporting brainstem localization

Tetraparesis or hemiparesis with proprioceptive, vestibular, or mixed ataxia

Cranial nerve findings supporting brainstem localization

Ipsilateral deficits involving one or more cranial nerves associated with the brainstem

Postural reaction findings supporting brainstem localization

Generally ipsilateral delays or absent responses

General laterality rule for brainstem lesions

Cranial nerve and postural reaction deficits are generally ipsilateral to the lesion

General laterality rule for forebrain lesions

Postural reaction deficits are typically contralateral to the lesion

Left-sided cranial nerve deficits + left-sided postural reaction deficits suggest a lesion on what side?

Right-sided cranial nerve deficits + right-sided postural reaction deficits suggest a lesion on what side?

left brainstem lesion

right brainstem lesion

Why is combining cranial nerve and postural reaction findings useful for brainstem localization?

The combination suggests dysfunction of both cranial nerve structures and proprioceptive pathways within the brainstem

Most important brainstem neurolocalization rule

Cranial nerve deficits and postural reaction deficits on the same side strongly support a brainstem lesion on that side

Seven differential diagnosis categories for brainstem disease (based on DAMNIT-V)

1.) degenerative

2.) anomalous/congenital

3.) neoplastic

4.) metabolic

5.) inflammatory/infectious/idiopathic

6.) trauma/toxin

7.) vascular

degenerative differentials for brainstem disease

Progressive deterioration of nervous tissue that may affect the brainstem

Typical characteristics of degenerative diseases affecting the brainstem

Often breed-associated, may cause multifocal brain disease, and are extremely rare

anomalous/congenital differentials for brainstem disease (2)

1.) Congenital brainstem malformations

2.) Ventricular system abnormalities affecting the brainstem

metabolic differentials for brainstem disease (1)

hypothyroidism

neoplastic differentials for brainstem disease

1.) Meningioma

2.) Glioma

3.) Lymphoma

4.) Choroid plexus tumor

5.) Nerve sheath tumor

6.) Metastatic neoplasia

Most common cancer to metastasize to the brain

Hemangiosarcoma

inflammatory/infectious/idiopathic differentials for brainstem disease (2)

-Meningoencephalitis

-Meningoencephalitis of unknown origin (MUO)

Three types of MUO:

1.) GME

2.) NME

3.) NLE

What signalment is characteristic of Meningoencephalitis of unknown origin (MUO)?

small breeds that are young to middle aged!

Classic progression of Meningoencephalitis of unknown origin (MUO)

rapidly progressive and multifocal!

Why is Meningoencephalitis of unknown origin (MUO) often multifocal?

it can affect many different areas of the CNS, so can have other neurologic symptoms outside of brainstem lesions such as seizures, spinal pain, cerebellar disease, etc

Diagnostic approach for MUO

1.) Blood work

2.) MRI

3.) CSF tap

4.) infectious disease testing (to rule this out)

Three classic characteristics of CSF analysis for MUO:

1.) Elevated protein

2.) Elevated TNCC

3.) Mononuclear pleocytosis