Lower motor neurone signs - Patterns of neurological disorders

Patterns of neurological disorders

See Fig 7.2.

Lower motor neurone signs

Lower motor neurone (LMN) signs are due to lesions of the anterior horn cell (or cranial nerve nucleus), the motor nerve root leaving the spinal cord, or of the peripheral nerves):

·        Decreased tone (flaccidity).

·        Decreased power in the distribution of the affected nerves or nerve roots (not pyramidal).

·        Decreased or absent reflexes. Plantars remain downgoing (or are absent).

·        Wasting develops within 3 weeks of a lesion.

·        Fasciculations, which are small local contractions of muscle motor units, due to spontaneous discharge of muscle fibres innervated by a single motor nerve filament.

Patterns of LMN weakness

The pattern depends on which nerves or roots are affected, and at what level.

·        Anterior horn cell lesions occur as part of motor neurone disease, polio or other viral infections, and can affect multiple levels.

·        Spinal root damage is often due to compression for example, a disc protrusion causing compression at T1 will cause weakness and wasting of the small muscles of the hand.

·        The brachial and lumbosacral plexus can be affected by inflammation or trauma.

·        Single or multiple peripheral nerve lesions cause weakness in the distribution of that nerve or as part of a multiple neuropathy.