Topical diagnosis algorithm: from the CNS to the brainstem
During the neurological examination, the first step is to differentiate between lesions of the central nervous system and the peripheral nervous system. Central involvement is indicated by centralparesis (increased muscle tone, hyperactive deep tendon reflexes, expansion of reflexogenic zones, pathological signs such as Babinski's sign), altered consciousness, and pathway-type sensory disturbances. Peripheral lesions are characterized by decreased tone and reflexes, rapid muscle atrophy, fasciculations, and a segmental or neuritic ("glove-and-stocking") distribution of sensory deficits.
After establishing central involvement, the next question to answer is: brain or spinal cord. Brain lesions occur significantly more frequently. Within the brain, a further distinction is made between the brainstem (medulla oblongata, pons, midbrain) and the cerebral hemispheres.
Features of brainstem lesions
The brainstem contains cranial nerve nuclei and passing nerve pathways. Functionally, a cranial nerve nucleus corresponds to the anterior or posterior horn of the spinal cord: its lesion causes symptoms of the peripheral type on the side of the lesion. Conversely, the pathways are responsible for the opposite half of the body; therefore, pyramidal and sensory disturbances manifest contralaterally.
Two phenomena are pathognomonic for the brainstem:
- Bulbar syndrome — dizziness, nausea, vomiting, dysphagia, dysphonia, dysarthria, cranial nerve involvement (most commonly ocular motility disorders), and decreased gag and palatal reflexes. It is the depression of these reflexes that distinguishes true bulbar syndrome from pseudobulbar palsy.
- Alternating syndromes — a combination of peripheral cranial nerve palsy on the side of the lesion and pathway disorders (hemiparesis or hemihypesthesia) on the opposite side.
Medulla oblongata syndromes: Jackson and Dejerine
Jackson's syndrome occurs when the hypoglossal nerve (XII) nucleus is damaged. On the side of the lesion, there is peripheralparesis of one half of the tongue with atrophy and fasciculations, along with dysarthria; when protruded, the tongue deviates toward the side of the lesion. On the opposite side, central hemiparesis of the arm and leg develops.
Dejerine's syndrome follows the same pattern, but instead of pyramidal hemiparesis on the contralateral side, there is hemihypesthesia — reduced sensation on one half of the body. The lesion of the XII nerve nucleus remains the same.
Pons syndromes: Millard-Gubler and Foville
Millard-Gubler syndrome develops when the facial nerve (VII) nucleus in the pons is damaged. Since the nucleus is affected, peripheralparesis of the facial muscles occurs on the side of the lesion: the entire half of the face is affected, including the forehead, accompanied by ptosis, incomplete eyelid closure (lagophthalmos), and drooping of the corner of the mouth. Contralaterally, there is central hemiparesis of the limbs.
Foville's syndrome is characterized by the additional involvement of the abducens nerve (VI) nucleus. Convergent strabismus is added to the peripheral facial palsy on the side of the lesion: the affected eye cannot be abducted outward. Hemiparesis persists on the opposite side.
Midbrain syndromes: Weber and Benedict
Weber's syndrome is caused by a lesion of the oculomotor nerve (III) nucleus or root. On the side of the lesion, there is ptosis, mydriasis, divergent strabismus, diplopia, and impaired upward, downward, and inward eye movements. On the opposite side, there is central hemiparesis.
Benedikt's syndrome mirrors Weber's picture but is supplemented by cerebellar ataxia and intention tremor on the side contralateral to the lesion (due to involvement of the red nucleus and its connections with the cerebellum). The patient tends to lean or list when walking. The III nerve lesion remains ipsilateral.
Key points
- Damage to a cranial nerve nucleus produces peripheral symptoms on the side of the lesion, whereas pathway damage produces contralateral symptoms.
- Bulbar syndrome includes dysphagia, dysphonia, dysarthria, and decreased pharyngeal-palatal reflexes—the key differentiator from pseudobulbar palsy.
- Alternating syndrome is a "cross-pattern": the cranial nerve is affected on one side, while pathway disorders are on the other.
- Medulla oblongata: Jackson syndrome (XII + hemiparesis), Dejerine syndrome (XII + hemihypesthesia).
- Pons: Millard-Gubler syndrome (VII + hemiparesis), Foville syndrome (VII + VI + hemiparesis).
- Midbrain: Weber syndrome (III + hemiparesis), Benedict syndrome (III + cerebellar ataxia).
- In Jackson syndrome, the tongue deviates toward the side of the damaged XII nerve nucleus.
- Precise localization of a brainstem lesion is based on the pairing of "ipsilateral cranial nerve + contralateral pathway disorders".