Brainstem crossed findings: the localisation rule
A cranial nerve sign on one side with weakness on the other points to the brainstem. Learn which side each tract carries and how the cranial nerve names the level.
Chapters (11)
Explain why ipsilateral cranial nerve deficits with contralateral long-tract signs localise a lesion to the brainstem, and identify its level (midbrain, pons, medulla).
The short version
- Almost every cranial nerve nucleus sends its fibres out on its own side, so a nuclear or nerve lesion gives deficits on the same side as the lesion. The exception is the trochlear (IV) nucleus, whose fibres cross before they leave the brainstem.
- The corticospinal tract crosses at the bottom of the medulla, the spinothalamic tract in the spinal cord, and the medial lemniscus in the medulla.
- A cranial nerve sign on one side with weakness or sensory loss on the other localises the lesion to the brainstem, on the side of the cranial nerve.
- The cranial nerve names the level: III and IV midbrain; VI and VII pons; IX, X and XII medulla; V runs through all three.
- Weber syndrome is the midbrain pairing, Millard-Gubler the pontine pairing, and the XII plus pyramid combination the medullary pairing.
The puzzle: one lesion, two sides of the body
A 44-year-old man wakes with his left eyelid drooping, and the eye points down and out. His left arm and leg work perfectly, but his right arm and leg are weak. One lesion has produced signs on both sides of the body.
The solution is the brainstem crossed findings rule. It depends on knowing which side each pathway carries at each level of the brainstem, so the lesson builds that map first and then uses it to name the level.
Cranial nerve nuclei: signs on their own side
Almost every cranial nerve nucleus sends its fibres out on its own side. Damage to a nucleus or to its nerve therefore shows up on the same side as the lesion.[1][8] The exception is the trochlear (IV) nucleus: its fibres cross before they leave the back of the midbrain, so a lesion of the nucleus weakens the opposite superior oblique.
For example, the abducens nerve supplies the lateral rectus on its own side, and the facial nerve supplies the muscles of facial expression on its own side.[1]
The long tracts: which side they carry
The long tracts behave differently, because each crosses at a different level. Once a tract has crossed, it carries the opposite side of the body.
| Tract | Carries | Crosses | Side carried in the brainstem |
|---|---|---|---|
| Corticospinal tract | Voluntary movement | Pyramidal decussation, at the bottom of the medulla | Not yet crossed above the decussation, so a lesion there weakens the opposite side of the body[4][8] |
| Spinothalamic tract | Pain and temperature | In the spinal cord | Already crossed: the opposite side[5][7] |
| Medial lemniscus | Vibration and position sense | In the medulla | Crossed in the lower medulla (internal arcuate fibres): the opposite side in most of the medulla, the pons and the midbrain[3][7] |
Most corticospinal fibres cross at the pyramidal decussation, where the medulla meets the spinal cord. So above that crossing the tract has not crossed yet, and a lesion there weakens the opposite side of the body.[4][8]
The crossed findings rule
The cranial nerve nucleus is on the lesion side, and the long tract is crossed or not yet crossed, so the limbs are on the opposite side.
A lesion above the brainstem, in the internal capsule for example, has no crossed pattern. The lower face and the limbs are affected together, opposite the lesion, and the forehead is spared.[4]
The cranial nerve names the level
Once the lesion is in the brainstem, the cranial nerve tells you which level.[1][2][3]
| Level | Cranial nerves |
|---|---|
| Midbrain | III and IV |
| Pons | VI and VII |
| Medulla | IX, X and XII |
| All three levels | V (the exception: its nuclei run through the midbrain, pons and medulla) |
Three classic pairings: Weber, Millard-Gubler and the medulla
Each classic pairing is a cranial nerve plus the corticospinal tract at its level.
| Level | Cranial nerve sign (lesion side) | Corticospinal sign | Syndrome |
|---|---|---|---|
| Midbrain | Third nerve: eye points down and out, ptosis, often a large pupil[6] | Opposite arm and leg weak (cerebral peduncle) | Weber syndrome[2][12] |
| Pons | Seventh nerve: the whole face on that side is weak; the sixth nerve is often involved too, so the eye cannot look outward[1][12] | Opposite arm and leg weak | Millard-Gubler syndrome |
| Medulla | Twelfth nerve: the tongue points toward the lesion[6][11] | Opposite arm and leg weak (pyramid) | Medullary pairing |
Back to the opening patient: a left third nerve palsy with a weak right side puts the lesion in the left midbrain. The nerve sign names the side and the level, and the weak right limbs fit a corticospinal tract that has not yet crossed.
- Left eyelid droops and the eye points down and out: left third nerve
- Third nerve nucleus lies in the midbrain: level is midbrain
- Right arm and leg weak: corticospinal tract not yet crossed, so the lesion is on the left
- Left midbrain lesion: Weber pattern
Localising the lesion step by step
- Find the cranial nerve signNote its side. Almost every nucleus sends fibres out on its own side, so this is the side of the lesion (the trochlear nucleus is the exception: its fibres cross).
- Check the limbsWeakness or sensory loss on the opposite side means the lesion is in the brainstem.
- Name the levelIII and IV: midbrain. VI and VII: pons. IX, X and XII: medulla.
- Rule out a lesion above the brainstemWith no crossed pattern, think of a lesion such as the internal capsule, where the lower face and the limbs are affected together on the opposite side.
The next lesson in the series looks at medial versus lateral medullary syndrome.
Practice question
Original · USMLE styleA 58-year-old woman has sudden double vision when she looks to the right. On examination, her right eye does not move outward, the whole right side of her face is weak, including the forehead, and her left arm and leg are weak.
Common questions
What are crossed findings in a brainstem lesion?
A cranial nerve sign on one side with weakness or sensory loss on the other side. It localises the lesion to the brainstem, on the side of the cranial nerve.
Where does the corticospinal tract cross?
Most of its fibres cross at the pyramidal decussation, at the bottom of the medulla. Above that point a lesion weakens the opposite side of the body.
Which cranial nerves lie in the midbrain, pons and medulla?
The third and fourth lie in the midbrain, the sixth and seventh in the pons, and the ninth, tenth and twelfth in the medulla. The fifth is the exception: its nuclei run through all three levels.
What is Weber syndrome?
A midbrain pairing of a third nerve palsy on the lesion side (eye down and out, ptosis, often a large pupil) with weakness of the opposite arm and leg.
How does an internal capsule lesion differ from a brainstem lesion?
An internal capsule lesion has no crossed pattern. The lower face and the limbs are affected together, opposite the lesion, and the forehead is spared.
Transcript
Read the full narration, by chapter717 words
0:00The patient: eye on one side, weakness on the other
A 44-year-old man wakes with his left eyelid drooping, and the eye points down and out. His left arm and leg work perfectly, but his right arm and leg are weak. One lesion, two sides of the body. How? Today: why a cranial nerve sign on one side plus a weak limb on the other points to the brainstem, and how to name the level.
0:23Cranial nerve nuclei: signs on their own side
First, cranial nerve nuclei. Almost every one sends its fibres out on its own side, so damage to a nucleus or its nerve shows up on the same side as the lesion. Second, the long tracts.
0:36The corticospinal tract crosses low
The corticospinal tract runs down through the midbrain and pons, and most of its fibres cross at the pyramidal decussation, at the bottom of the medulla. So above that crossing the tract has not crossed yet, and a lesion there weakens the opposite side of the body.
0:56Spinothalamic tract and medial lemniscus
The spinothalamic tract carries pain and temperature, and it crosses in the spinal cord, so in the brainstem it already carries the opposite side. The medial lemniscus carries vibration and position sense. Its fibres cross in the medulla, so above that it carries the opposite side.
1:15The crossed rule
That gives the rule: a cranial nerve sign on one side, with weakness or sensory loss on the other, localises the lesion to the brainstem, on the side of the cranial nerve. A lesion above the brainstem, in the internal capsule say, has no crossed pattern: face and limbs are affected together, opposite the lesion. The cranial nerve names the level.
1:37The cranial nerve names the level
The third and fourth nerve nuclei lie in the midbrain. The sixth and seventh lie in the pons. The ninth, tenth and twelfth lie in the medulla. The fifth is the exception: its nuclei run through all three levels. Three classic pairings,
1:56Three classic pairings: Weber (midbrain)
each a cranial nerve plus the corticospinal tract at its level. In the midbrain, the third nerve plus the cerebral peduncle gives an eye that points down and out, with ptosis and often a large pupil on the lesion side, and weakness of the opposite arm and leg. That is Weber syndrome. And that is our man: a left third nerve palsy with a weak right side puts the lesion in the left midbrain.
2:24Millard-Gubler (pons) and the medulla pairing
In the pons, the seventh nerve plus the corticospinal tract: the whole face on that side is weak, and the opposite arm and leg are weak. The sixth nerve is often involved too, so the eye cannot look outward. That is Millard Gubler syndrome. In the medulla, the twelfth nerve plus the pyramid: the tongue points toward the lesion, and the opposite arm and leg are weak.
2:49Practice question
Your turn. A 58-year-old woman has sudden double vision when she looks to the right. Her right eye does not move outward, the whole right side of her face is weak, including the forehead, and her left arm and leg are weak. Where is the lesion? A, left ventral pons. B, right cerebral peduncle in the midbrain. C, right internal capsule. D, right medial medulla. E, right ventral pons. Pause here and decide.
3:24The answer, option by option
A is out: a left pontine lesion would put the nerve signs on the left and the weakness on the right. B is out: the midbrain holds the third nerve, so you would expect a down-and-out eye, not a sixth nerve palsy. C is out: an internal capsule lesion has no crossed pattern, and would not paralyse the right eye movement or the right forehead. D is out: the medulla holds the twelfth nerve, so the tongue would be affected, not the sixth and seventh nerves. So the answer is E. The sixth and seventh nerves belong to the pons and are weak on the right, and the corticospinal tract has not crossed yet, so the left side is weak: right ventral pons.
4:09High-yield recap and next lesson
Three things to keep. One: a cranial nerve sign on one side with weakness or sensory loss on the other side localises to the brainstem, on the side of the cranial nerve. Two: the corticospinal tract crosses at the bottom of the medulla, the spinothalamic tract in the cord, and the medial lemniscus in the medulla. Three: the cranial nerve names the level: three and four midbrain; 13 pons; nine, 22 medulla; five runs through all three. Next, medial versus lateral medullary syndrome.
Sources
Numbers in the text, e.g. [1], point here- Pons. TeachMeAnatomy. teachmeanatomy.info/neuroanatomy/brainstem/pons/
- Midbrain. TeachMeAnatomy. teachmeanatomy.info/neuroanatomy/brainstem/midbrain/
- Medulla Oblongata. TeachMeAnatomy. teachmeanatomy.info/neuroanatomy/brainstem/medulla-oblongata/
- Descending (motor) tracts. TeachMeAnatomy. teachmeanatomy.info/neuroanatomy/pathways/descending-tracts-motor/
- Ascending (sensory) tracts. TeachMeAnatomy. teachmeanatomy.info/neuroanatomy/pathways/ascending-tracts-sensory/
- Oculomotor and hypoglossal nerves. TeachMeAnatomy. teachmeanatomy.info/head/cranial-nerves/oculomotor/ · teachmeanatomy.info/head/cranial-nerves/hypoglossal/
- Central Processing. OpenStax Anatomy and Physiology 2e, 14.2. openstax.org/books/anatomy-and-physiology-2e/pages/14-2-central-processing
- Motor Responses. OpenStax Anatomy and Physiology 2e, 14.3. openstax.org/books/anatomy-and-physiology-2e/pages/14-3-motor-responses
- Overview of Stroke. MSD Manual Professional. msdmanuals.com/professional/neurologic-disorders/stroke/overview-of-stroke
- Brainstem stroke syndromes. StatPearls via Europe PMC (EXT_ID 32809731). europepmc.org/article/MED/32809731
- Brainstem syndromes review. Frontiers in Neurology, 2026 (PMC13053248). europepmc.org/article/PMC/PMC13053248
- Weber syndrome (StatPearls, PMID 32644584) and Millard-Gubler syndrome (StatPearls, PMID 30422502), via Europe PMC; Wikipedia articles used only as a secondary cross-check. europepmc.org/article/MED/32644584 · europepmc.org/article/MED/30422502
The practice question is original and its patient is synthetic. Every fact is checked against the sources above in independent accuracy reviews. Always follow your local guidelines and your own laboratory’s ranges.
Image credits. Brainstem and tract plates: Gray's Anatomy (1918), public domain, via Wikimedia Commons (Figs. 695, 696, 705, 711, 764). All other diagrams are original (code-drawn); no AI-generated images.
Corrections. None so far. Spotted an error? Email info@medservatory.com with the lesson code MS-S1-NEURO-002.
For exam study only, not medical advice. Read the medical disclaimer.
In this series
Playlist on YouTube- EP 01Upper vs lower motor neuron lesions: signs and localisation5:08
- EP 02Dorsal column vs spinothalamic tract: where each pathway crosses5:01
- EP 03Spinal cord syndromes compared: rebuild each from three tracts5:06
- EP 04Cranial nerve nuclei: a brainstem map by level and position5:10
- EP 05Brainstem crossed findings: the localisation rule4:50
- EP 06Medial vs lateral medullary syndrome: artery, structures and signs5:30
- EP 07Facial nerve palsy: why the forehead tells you the lesion4:59
- EP 08Stroke syndromes by artery: ACA, MCA and PCA4:41
- EP 09Lacunar stroke syndromes: pure motor, pure sensory and more4:54
- EP 10Visual pathway lesions and field defects4:42
- EP 11Basal ganglia direct and indirect pathways: D1, D2 and movement5:19
- EP 12Cerebellar lesion localisation: vermis vs hemisphere4:42
- EP 13Brain herniation syndromes: what is squeezed, and where4:56
- EP 14CSF flow and hydrocephalus: where the flow fails5:12
- EP 15Corticospinal tract: from cortex to anterior horn4:59
- EP 16Hypothalamic nuclei and their functions5:24