Clinical patterns of UMN disorders

2 main patterns of UMN (pyramidal) lesions are recognizable:

• Hemiparesis
• Paraparesis

Hemiparesis means paralysis of the limbs of one side. It is usually (but not always) caused by a lesion within the brain.

Paraparesis means paralysis of both lower limbs and is characteristically diagnostic (but again, not always) of a spinal cord lesion.

Hemiparesis:

The level within the corticospinal tract (CoST) is recognized by various accompanying features

Motor cortex:

• Paralysis localized to one contralateral limb (monoplegia) or part of a limb (e.g. a weak hand) is characteristic of an isolated lesion of the motor cortex (e.g. a secondary neoplasm)
• There may be a defect in higher cortical function (e.g. aphasia if the speech area is affected)
• Focal epilepsy may be present

Internal capsule:

• Since all CoST fibres are tightly packed in the internal capsule (occupying ~1cm²) a small lesion causes a large deficit
• E.g. an infarct of a small branch of the middle cerebral artery causes a sudden contralateral hemiplegia that includes the face

Pons:

• A pontine lesion (e.g. a plaque of MS) is rarely confined only to the CoST
• As adjacent structures such as the 6^th and 7^th cranial nerve nuclei, MLF and PPRF are involved, there are other localizing signs:
• VI and VII nerve palsies
• Intranuclear nuclear ophthalmoplegia (INO)
• Lateral gaze palsy

Spinal cord:

• An isolated lesion of a single lateral CoST within the cord (which is unusual) causes an ipsilateral UMN lesion, the level of which is indicated by:
• A reflex level (e.g. absent biceps jerk)
• The presence of a Brown-Sēquard syndrome
• Muscle wasting at the level of the lesion

Paraparesis:

• Paraparesis (and tetraparesis when all 4 limbs are involved) indicates bilateral damage to the CoSTs
• Spinal cord compression or other cord disease is the usual cause, but cerebral lesions can occasionally produce paraparesis