Where is the lesion?

What is the lesion?

Consists of

• Midbrain
• Pons
• Medulla oblongata

Compact and serves many vital pathways

Hence, a small lesion produces a large neurological deficit

Descending pathways: terminate in the spinal cord or brain stem

Ascending pathways: originate from the spinal cord or brain stem

Cranial nerve nuclei (except I & II Cr. Nerve nuclei)

Pathways to & from the cerebellum

Descending autonomic system pathways

Reticular activating formation

Cerebrospinal fluid pathways

Consist of branches of the vertebrobasilar system

Posterior inferior cerebellar artery

Anterior inferior cerebellar artery

Superior cerebellar artery

Posterior cerebral artery

Pontine artery

Basilar artery - median & paramedian perforators

Vertebral artery - medullary & spinal branches

Corticospinal tract

Corticonuclear tract

Corticopontine fibers

Rubrospinal tract

Tectospinal tract

Medial longitudinal fasciculus

Vestibulospinal tract

Reticulospinal tract

Central tegmental tract

Descending spinal tract of trigeminal nerve nucleus

Medial lemnicus

Spinothalamic tract

Trigeminal lemniscus

Lateral lemniscus

Reticular system fibers

Medial longitudinal fasciculus

Inferior cerebellar peduncle

Superior cerebellar peduncle

Secondary vestibular fibers

Secondary gustatory fibers

A small lesion usually affects several nuclei or pathways

Lesions can be:

• Vascular(occlusive or haemorrhagic)
• Tumour
• Trauma
• Degenerative
• Demyelinating

Depending on the level of the lesion

• Cranial nerve deficit
• Upper motor neuron signs below the level of the lesion (pyramidal pathways)
• Decreased consciousness (Reticular Activating Formation)
• Cerebellar signs (cerebellar output/input)
• Control of conjugate gaze (Medial longitudinal bundle)
• Disorder of autonomic control

A state of unarousable consciousness

Consciousness depends on the integration of 2 separate components:

Arousal (mediated via ascending reticular activating substance in brain-stem)

Content of consciousness (mediated via the integrated activity of the whole cortex of both cerebral hemispheres)

1) Diffuse damage to cerebral cortex bilaterally

2) Supratentorial mass pressing on brain stem & damage ARAS

3) Posterior fossa mass pressing on brain stem & damage ARAS

4) Intrinsic brain stem lesion directly suppressing ARAS

Table

Coma due to irreversible damage to brain stem

Coma is apnoeic & not due to drugs, metabolic disturbance, hypothermia

Absent brain stem reflexes (including oculocephalic, oculovestibular, corneal, pupillary light)

No cranial motor responses, no gag response

No spontaneous respiratory movement after disconnection from ventilator, ensure PaCO₂ is above 6.6 kPa or 50 mmHg

Repeat above after 24 hours by 2 different doctors

*EEG repeated at 12 hours apart show no signs of activity

I & II - direct evaginations of the brain

III - Oculomotor, Edinger-Westphal

IV - Trochlear

V - Main sensory, spinal (descending), mesencephalic, motor (muscles of mastication)

VI - Abducens

VII - Facial, superior salivatory, gustatory (part of solitary)

VIII - Cochlear (2 nuclei), vestibular (4 nuclei)

IX - Ambiguous, inferior salivatory, solitary

X - Dorsal motor, ambiguous, solitary

XI - Spinal accessory XI (C1-C5)

XII - Hypoglossal

Solitary nucleus is shared by nerves VII, IX, X

Ambiguous nucleus is shared by nerves IX, X,XI

Efferent Components

Somatic efferent

• Innervate striated muscles derived from somites
• External ocular movements (oculomotor of III, IV, VI) & tongue (XII)

Brachial efferent

• Muscles derived from brachial arches
• Mastication (motor of V), facial expression (motor of VII), swallowing, vocal sounds (IX, X, XI), turning of head (spinal accessory of XI)

Visceral efferent (preganglionic parasympathetic)

• Constrictor muscles of pupils (Edinger-Westphal N. of III), salivatory & lacrimal glands (VII), parotid gland (IX), heart, lung & bowels muscles (X)

Somatic afferent

• Receive & relay sensory stimuli from skin & mucosa of most of the head (mainly via V nerve to sensory, descending & mesencephalic nuclei of V)

Visceral afferent

• Relay general sensory stimuli from the viscera, specialised taste stimuli from tongue & epiglottis (IX & X to solitary nucleus from viscera; VII, IX, X to gustatory nucleus from taste)

Special sensory afferent

• VIII; vestibulocochlear (from 4 vestibular and 2 cochlear nuclei)
• I (olfactory), II (optic)

Anosmia

Head injury (shearing of the nerves)

Olfactory groove meningioma

Frontal lobe tumour

Loss of visual acuity (after correction of refraction)

Retinal lesions

Lesions of lens

Bitemporal hemianopia (pituitary lesion)

Homonymous hemianopia (postchiasmal lesion)

• Optic tract (fields often incongruous)
• Internal capsule (no macular sparing)
• Occipital cortex (macular sparing)

Quadrantic hemianopia (optic radiation)

• Parietal lobe lesion (lower quadrantic)
• Temporal lobe lesion (upper quadrantic)

Central scotoma (usually with optic atrophy)

• Optic neuritis
• Optic nerve compression
• Macula degeneration

Tunnel vision

• Papilloedema
• Glaucoma
• Retinal lesions

Pupil size, shape, symmetrical

Pupillary reflexes

Afferent pupillary defect (retinal/optic nerve lesion)

Efferent pupillary defect (III nerve lesion)

Accomodation reflex (originates from cortex & relayed via III N; optic nerve is not involved)

External eye movements (III, IV, VI): diplopia

Levator palpebrae superiores (III): ptosis

Efferent pupillary pathway (III): dilated pupils

Complete III nerve palsy (ext. compression)

• Palsy of medial & inferior recti, & inferior oblique muscles
• Complete ptosis
• Dilated pupils with absent efferent pupillary response

Partial III nerve palsy (small vessel lesions eg Diabetes Mellitus)

• Partial ptosis
• Pupillary response intact

IV nerve

• Palsy of superior oblique muscle
• Very rarely occurs on its own

VI nerve

• Palsy of lateral rectus muscle
• Diabetes mellitus
• Raised intracranial pressure

Motor part supplies muscles of mastication

Sensory (3 divisions)

• Ophthalmic (corneal reflex)
• Maxillary
• Mandibular

Cerebellopontine angle lesions (with other cranial nerves)

• Acoustic neuroma
• Nasopharyngeal carcinoma
• Meningioma
• Herpes zoster

Motor supply to muscles of facial expression, stapedius & secretomotor (lacrimation)

Sensation (taste to anterior 2/3 of tongue)

Upper motor neuron facial weakness

• Note:- lesion is not in VII nerve
• Upper part of face is spared
• Cerebrovascular accident

Lower motor neuron facial weakness

• Facial nerve is affected
• Entire side of face is affected; hyperacusis, taste
• Bell’s palsy
• Diabetes mellitus
• Intracranial lesion at cerebellopontine angle
• Lesion within temporal bone (otitis media/externa lesion; Ramsey Hunt syndrome from herpes zoster)
• Extracranial lesion (parotid tumour, trauma)

Conductive deafness (does not involve VIII nerve)

• Ear wax
• Otitis media/externa
• Otosclerosis

Sensorineural deafness

• Trauma
• Toxins, infections
• Acoustic neuroma

Tends to be affected together

Bulbar palsy (lower motor neuron lesion)

• Flaccid, fasciculating tongue, absent jaw jerk
• Motor neuron disease
• Syringobulbia
• Gllian Barre syndrome
• Jugular foramen syndrome (neuromas, meningiomas, metastases, nasopharyngeal carcinoma or lymphoma)

Pseudobulbar palsy (upper motor neuron lesion)

• Spastic tongue
• Brisk jaw jerk
• Bilateral UMN signs in limbs
• Cerebrovascular accident