Brainstem Lesions & Crossed (Alternating) Syndromes

Localizing a Brainstem Lesion

The crossed-signs rule. An ipsilateral cranial nerve deficit plus contralateral long-tract signs means a brainstem lesion. The nucleus or fascicle is hit at the lesion's level on the same side, while the long tracts carry signs from the opposite body: the corticospinal tract crosses below the lesion at the pyramidal decussation, the medial lemniscus crosses in the lower medulla, and the spinothalamic tract has already crossed in the spinal cord. A hemispheric lesion instead puts face and body deficits on the same side, often with cortical signs such as aphasia, neglect, or hemianopia.

The nerve names the level. The midbrain holds III (superior colliculus level) and IV (inferior colliculus level); the pons holds V, VI, VII, and VIII, with the vestibulocochlear nuclei straddling the pontomedullary junction; the medulla holds IX, X, XI, and XII, and the spinal accessory nucleus extends into the upper cervical cord (C1–C5). A CN III palsy therefore points to the midbrain, VI or VII to the pons, and X or XII to the medulla.

Medial vs lateral. Nuclei keep their embryonic order: basal-plate motor nuclei lie medial and alar-plate sensory nuclei lateral to the sulcus limitans. The rule of 4 condenses this into four medial M's (motor tract, medial lemniscus, MLF, and motor nuclei of the nerves that divide evenly into 12: III, IV, VI, XII) and four lateral S's (spinothalamic tract, sensory nucleus of V, sympathetic pathway, spinocerebellar connections).

Blood supply. Paramedian arteries feed the medial brainstem: the anterior spinal artery and vertebral paramedian branches (medulla), basilar paramedian perforators (pons), and PCA/basilar-tip perforators (midbrain). Circumferential arteries feed the lateral brainstem: PICA (lateral medulla), AICA (lateral caudal pons, usually also giving the labyrinthine artery to the inner ear), and the superior cerebellar artery (lateral rostral pons).

Medullary Syndromes

Medial medullary (Dejerine) syndrome. Occlusion of the anterior spinal artery or vertebral paramedian branches damages the pyramid (contralateral arm and leg weakness, face spared), the medial lemniscus (contralateral loss of vibration and proprioception), and the hypoglossal nucleus or fibers (ipsilateral tongue weakness, so the protruded tongue deviates toward the lesion). Pain and temperature are spared because the spinothalamic tract is lateral.

Lateral medullary (Wallenberg) syndrome. PICA or intracranial vertebral artery occlusion infarcts the lateral medulla:

• Nucleus ambiguus: dysphagia, hoarseness, decreased gag (the defining medullary sign)
• Vestibular nuclei: vertigo, nystagmus, nausea and vomiting
• Inferior cerebellar peduncle: ipsilateral limb ataxia
• Descending sympathetics: ipsilateral Horner syndrome (ptosis, miosis, anhidrosis)
• Spinal trigeminal nucleus and spinothalamic tract: pain and temperature loss on the ipsilateral face and contralateral body

Strength is normal because the pyramids are medial. Vertebral artery dissection after neck trauma or manipulation is a classic cause in young adults, and intractable hiccups are a frequent clue.

Why the face and body split. Facial pain and temperature fibers enter with CN V and descend uncrossed in the spinal trigeminal tract to the medulla and upper cervical cord, whereas body pain and temperature fibers crossed in the spinal cord and ascend in the spinothalamic tract. A lateral medullary or lateral pontine lesion therefore abolishes pain and temperature on the ipsilateral face and the contralateral body.

Pontine Syndromes

Lateral pontine (AICA) syndrome. AICA infarcts share Wallenberg's vertigo, nystagmus, ipsilateral Horner syndrome, ipsilateral ataxia, and crossed pain and temperature loss. Instead of the nucleus ambiguus, they hit the facial nucleus and fibers (ipsilateral paralysis of the whole face with decreased lacrimation, salivation, and taste) and the cochlear nuclei or labyrinthine artery (ipsilateral sensorineural hearing loss, which is usually spared in PICA strokes). Memory hook: AICA's A is for Auditory; PICA's P is for Pharynx.

Medial pontine syndrome. Occlusion of paramedian basilar branches damages the abducens fascicles (ipsilateral lateral rectus palsy: eye turned in, horizontal diplopia), the corticospinal tract (contralateral hemiparesis), and the medial lemniscus (contralateral proprioception loss); MLF involvement adds an internuclear ophthalmoplegia. Damage to the abducens NUCLEUS instead causes an ipsilateral horizontal gaze palsy, because the nucleus contains interneurons that cross to the contralateral MLF, and the facial nerve looping over it (facial colliculus) is often hit too.

Locked-in syndrome. Bilateral ventral pontine infarction from basilar artery occlusion, or osmotic demyelination after overly rapid correction of chronic hyponatremia, destroys both corticospinal and corticobulbar tracts: quadriplegia, anarthria, and facial and bulbar paralysis. The tegmental reticular activating system is spared, so consciousness is intact, and vertical eye movements and blinking, controlled from the midbrain, are the only way to communicate.

Midbrain Syndromes

Weber syndrome (ventral midbrain). Occlusion of paramedian PCA or basilar-tip perforators damages the oculomotor fascicles, causing ipsilateral ptosis, a dilated pupil, and an eye resting down and out, and the cerebral peduncle, causing contralateral hemiparesis that includes the lower face and tongue (corticobulbar fibers). A lesion reaching the red nucleus adds contralateral tremor and ataxia (Benedikt syndrome).

Parinaud syndrome (dorsal midbrain). Compression of the pretectum, posterior commissure, and superior colliculi causes vertical upgaze palsy, light-near dissociation (pupils constrict to accommodation but poorly to light), convergence-retraction nystagmus on attempted upgaze, and eyelid retraction (Collier sign).

Causes of dorsal midbrain compression. A pineal region tumor, classically a germinoma in a boy or young man, presses on the tectum and cerebral aqueduct, causing Parinaud syndrome plus obstructive hydrocephalus (headache, papilledema). Hydrocephalus itself can compress the dorsal midbrain, giving infants a setting-sun downward gaze. Midbrain infarcts and MS plaques are other causes.

β-hCG–secreting pineal germ cell tumors can cause precocious puberty in boys by stimulating Leydig-cell LH receptors.

Putting It Together

Deviation rules for lower motor neuron lesions.

• Tongue (CN XII): deviates TOWARD the lesion, pushed across by the intact genioglossus.
• Uvula (CN X): deviates AWAY from the lesion, pulled by the intact palatal muscles.
• Jaw (CN V): deviates TOWARD the lesion on opening, pushed by the intact lateral pterygoid.