STEPTRACKERNERVOUS SYSTEM | ANATOMY + PATHOLOGY | HIGH-YIELD REVIEW

Brainstem & Cortical Localization

Crossed signs point to the brainstem; the cranial nerve tells you the level. Cortical deficits follow the lobe.

FOCUS ON

Brainstem stroke = ipsilateral cranial nerve sign + contralateral body sign. Medial lesions hit motor tracts and CN III, VI, XII; lateral lesions hit spinothalamic, spinal V, sympathetics, CN V, VII, VIII, IX, X. Wallenberg (PICA): dysphagia, hoarseness, Horner, crossed pain/temp. Cortex: Broca (frontal), Wernicke (temporal), neglect (nondominant parietal).

PICTURE

SYNDROME

SITE · ARTERY

DEFICITS

CLUE

How to read: three axial slices stacked — midbrain, pons, medulla — dorsal side up, dashed line = midline. Cortical rows show the side view of one hemisphere (front is marked). Red = lesion.

Medial medullary

Pyramid, medial lemniscus, CN XII · anterior spinal artery

Contralateral hemiparesis and ↓ proprioception; tongue deviates toward lesion

Tongue points to the side of the stroke

Lateral medullary (Wallenberg)

Nucleus ambiguus, spinal V, STT, vestibular nuclei, sympathetics · PICA / vertebral

Dysphagia, hoarseness, ↓ gag; vertigo, nystagmus; ipsilateral Horner and ataxia; ipsilateral face + contralateral body pain/temp loss

Nucleus ambiguus = PICA

Lateral pontine

CN VII and VIII nuclei, spinal V, STT, cerebellar peduncles · AICA

Ipsilateral facial paralysis, ↓ taste, lacrimation; hearing loss, vertigo; crossed pain/temp; ataxia

Facial droop = AICA

Medial pontine

Corticospinal tract, CN VI · paramedian basilar branches

Contralateral hemiparesis; ipsilateral lateral gaze palsy

Eye can't abduct + opposite weakness

Locked-in syndrome

Bilateral ventral pons · basilar artery

Quadriplegia, can't speak or swallow; consciousness, blinking and vertical eye movements preserved

Communicates by blinking

Weber syndrome

Medial midbrain: CN III fascicles, cerebral peduncle · PCA branches

Ipsilateral CN III palsy (down and out, ptosis, dilated pupil) + contralateral hemiparesis

Midbrain = CN III

Parinaud syndrome

Dorsal midbrain (superior colliculi) · pinealoma, hydrocephalus

Vertical gaze palsy (upgaze), light-near dissociation, convergence-retraction nystagmus, lid retraction

Teen boy with pineal germinoma

Internuclear ophthalmoplegia

MLF (connects CN VI nucleus to opposite CN III) · MS (young, bilateral), stroke (older)

Ipsilateral eye can't adduct on lateral gaze; abducting eye has nystagmus; convergence intact

Bilateral INO in a young woman → MS

Frontal lobe

Prefrontal cortex, motor strip, Broca area, frontal eye fields

Disinhibition, apathy, poor judgment; contralateral weakness; eyes deviate toward the lesion; Broca aphasia; frontal release signs

Personality change (Phineas Gage)

Dominant parietal (Gerstmann)

Angular gyrus (usually left)

Agraphia, acalculia, finger agnosia, left-right disorientation

Can't write or calculate

Nondominant parietal

Usually right parietal cortex

Hemispatial neglect of the left side; anosognosia, dressing apraxia

Shaves only the right half of the face

Temporal lobe

Wernicke area, Meyer loop, hippocampus, amygdala

Wernicke aphasia; contralateral superior quadrantanopia ("pie in the sky"); memory loss; bilateral amygdala → Klüver-Bucy (hyperorality, hypersexuality, disinhibition)

Klüver-Bucy: HSV encephalitis

Occipital lobe

Visual cortex · PCA

Contralateral homonymous hemianopia with macular sparing (MCA collateral)

Bilateral → cortical blindness

Mechanism — localizing a brainstem lesion

Crossed signs?Ipsi face/CN + contra body

→

Level by CNIII–IV midbrain · V–VIII pons · IX–XII medulla

→

MedialMotor, lemniscus, III, VI, XII

LateralSTT, spinal V, Horner, ataxia, V, VII, VIII, IX, X

Medial structures get paramedian branches (ASA, basilar); lateral structures get circumferential branches (PICA, AICA).

Aphasias

TYPE

FLUENCY

COMPREHENSION

REPETITION

Broca (inferior frontal gyrus)

Nonfluent

Intact

Impaired

Wernicke (superior temporal gyrus)

Fluent, nonsense

Impaired

Conduction (arcuate fasciculus)

Fluent

Intact

Impaired

Global

Nonfluent

Impaired

Transcortical motor

Nonfluent

Intact

Intact

Transcortical sensory

Fluent

Impaired

Intact

How it's tested

COMMON CONFUSIONWallenberg (PICA) and lateral pontine (AICA) both give crossed pain/temperature loss and ataxia. Dysphagia and hoarseness = medulla (PICA); facial paralysis and hearing loss = pons (AICA).

THE TRAPFrontal eye field lesions push the eyes toward the lesion (the opposite field is unopposed); seizures in that area push them away.

YOUR APPROACHCrossed signs → brainstem → CN for level → medial vs lateral → artery. Uncrossed cortical deficit → which lobe and which hemisphere (dominant vs nondominant).

If you see… think…

Hoarseness, dysphagia, left Horner, left face and right body pain loss → left lateral medullary (PICA)

Right hemiparesis, tongue deviates left → left medial medullary (ASA)

Left facial paralysis, left hearing loss, vertigo → left lateral pontine (AICA)

Awake, quadriplegic, communicates by blinking → locked-in (basilar)

Right CN III palsy + left hemiparesis → right Weber (midbrain)

Teen can't look up, pupils react to near not light → Parinaud (pinealoma)

Young woman, eye fails to adduct, nystagmus in the other → INO (MS)

Ignores the left side of the world → right parietal lesion

Fluent speech, intact comprehension, can't repeat → conduction aphasia

Contralateral superior quadrantanopia → temporal lobe (Meyer loop)