Brainstem lesions

• weakness of the left upper and lower limbs, with sparing of the face:
  motor (corticospinal pathway) localises the lesion to the contralateral medial brainstem
  (sparing of the face (CN7) means the lesion must be below the upper pons)
• tongue deviation to the right, with no ophthalmoplegia:
  tongue deviation indicates CN12 involvement, localising the lesion to the ipsilateral medulla
  (sparing of CN3 and CN6 means the midbrain and pons are not involved)
• loss of vibration and proprioception in the left upper and lower limbs:
  confirms localisation of the lesion to the contralateral medial brainstem

Site of the lesion: right medial medulla. Sometimes, due to the peculiar pattern of blood supply to the medulla, bilateral infarction may occur.

• weakness of the right face, upper and lower limbs:
  motor (corticospinal pathway) localises the lesion to the contralateral medial brainstem
  (involvement of the face means the lesion must be at or above the upper pons)
• the left eye is turned “down and out” and the pupil is dilated:
  CN3 involvement, localising the lesion to the ipsilateral midbrain
  (sparing of CN6 and CN12 means the pons and medulla are not involved)

Site of the lesion: left medial midbrain. A CN3 palsy (from damage to the CN3 nerve fascicle) and contralateral hemiplegia is known as Weber’s syndrome (“basal” infarction) – which can be difficult to distinguish from ‘coning’ if you don’t have a CT scanner available.

• left-sided limb ataxia:
  spinocerebellar pathway localises the lesion to the ipsilateral lateral brainstem.
• left-sided alteration of pain and temperature on the face:
  Sensory nucleus of the 5th cranial nerve localises the lesion to the ipsilateral lateral brainstem.
• left-sided ipsilateral Homer’s syndrome:
  Sympathetic pathway localises the lesion to the ipsilateral lateral brainstem.
• right-sided alteration of pain and temperature affecting the arm and leg:
  Spinothalamic pathway localises the lesion to the contralateral lateral brainstem.
• dysarthria and decreased gag reflex on the left, with the palate pulling up on the right-side:
  localises the lesion to the medulla affecting the ipsilateral CN9 and 10.

Site of the lesion: left lateral medulla. Also known as Wallenberg’s syndrome, caused by a left vertebral or left posterior inferior cerebellar artery occlusion (blood supply is variable to this region).

• right-sided limb ataxia (predominantly affecting the right upper limb):
  spinocerebellar pathway localises the lesion to the ipsilateral lateral brainstem.
• right-sided facial numbness with loss of the corneal reflex:
  Sensory nucleus of the 5th cranial nerve localises the lesion to the ipsilateral lateral brainstem.
• right-sided hemi-facial spasms:
  the lesion involves the pons affecting the ipsilateral CN7.

Site of the lesion: The findings indicate a lesion affecting the right lateral pons with evidence of spinocerebellar involvement. In this case the lesion was not vascular in origin but in fact an example of a cerebropontine angle lesion – an acoustic neuroma (or schwannoma). This demonstrates the broader utility of Gates’ Brainstem Rules of 4.