Weakness - Lower Motor Neurons

What parts of the body are considered to be part of the LMN pathway?

• Ventral roots

• Plexi

• Peripheral nerve

• Neuromuscular junction

• Muscle

Do nerve roots exit the spinal cord at the level it leaves the cord?

Not always - in the lower spinal cord nerve roots descend past the termination of the spinal cord as part of the cauda equina.

Symptoms of a lesion in the cauda equina

• Bilateral paralysis

• Sensory loss

• LMN signs

• Loss of bladder / bowel function

Cauda equina syndrome

Compression of the cauda equina. This is a surgical emergency - if it is not decompressed quickly there is a threat of permanent paralysis.

Where are common places for nerve roots to be compressed?

The spaces between where nerves exit the spinal canal.

Radiculopathy

Compression of a nerve root

What are some common causes of radiculopathy?

• A prolapsed intervertebral disc

• Facet joint hypertrophy

• Spondylolisthesis

• Radiculitis

Symptoms of radiculopathy

• Pain radiating down nerve

• Dermatomal sensory loss +/- weakness in the site

• LMN signs upon examination

• Provocative manouevers elicit characteristic pain

• Areflexia in an effected territory

LMN signs

• Areflexia

• Wasting

Lower Motor Neuron (LMN)

The efferent neuron of the peripheral nervous system (PNS) that connects the central nervous system (CNS) with the muscle to be innervated.

What are the symptoms of a plexus lesion?

The pattern of weakness does not conform to an individual root territory, nor peripheral nerve.

What is a serious condition that can lead to compression of the brachial plexus?

Pancoast tumour - an apical lung tumour that can invade lower segments of the brachial plexus.

Neurological symptoms of a Pancoast tumour

Weakness and wasting of the intrinsic hand muscles. This may the first sign of their cancer.

Typically any pain will be worse at night.

What can cause peripheral nerve lesions?

• Compression

• Inflammation

• Tumours

  And more

Mononeuropathy

Lesion affecting a single peripheral nerve.

Symptoms of a peripheral nerve lesion

• Loss of function in the region supplied by the specific nerve

• Usually sensory +/- motor features, though it may only have motor only features too

Saturday night palsy

Radial nerve palsy - can be more common in people who have been drunk the night before as they may fall asleep with their arm over the back of a chair and compress the nerve.

Carpal tunnel syndrome

Compression of the median nerve at the carpal tunnel

What time of day is the pain for carpal tunnel syndrome worse?

Typically night

How is carpal tunnel syndrome treated?

Surgery can be performed if it is serious, and if not serious, a splint is used.

Polyneuropathy

Lesions affecting multiple nerves in multiple limbs (+/- cranial nerves).

What is the typical presentation of polyneuropathy?

• Various patterns exist, but in general the longest nerves are damaged first creating a ‘glove and stocking’ sensory loss pattern (distal weakness)

• Reflexes diminished / absent

• Chronically, the condition can cause muscle wasting, and sometimes fasciculations

What can cause chronic polyneuropathy?

• Toxic metabolic diseases

• Diabetes

• Chemotherapy

• Alcoholism

• Autoimmune disorders

Neuromuscular Junction (NMJ)

The synapse between motor neuron and muscle.

Typical presentation of issues at the NMJ

• Sensation is normal

• Reflexes are unaffected

• Fatiguable weakness (sustained exertion on bedside testing causes decreasing power)

• Symptoms may be worse at the end of the day (diurnal pattern)

Weakness affects:

• Proximal muscles (eg. shoulders, hip girdle)

• Ocular muscles (diplopia/gaze palsy, ptosis)

• Bulbar muscles (dysarthria, dysphagia, loss of airway tone)

• Neck muscles (head drop)

What causes fatiguable weakness

The NMJ runs out of acetylcholine.

What are some more common NMJ diseases?

• Myasthenia Gravis

• Lambert-Eaton syndrome

Myasthenia Gravis (MG)

An autoimmune condition that causes destruction of acetylcholine receptors at the NMJ. This results in fatiguable weakness in muscles under voluntary.

Typical presentation of MG

• Ptosis (droopy eyelids), worsening upon sustained upgaze

• Proximal muscle weakness, worse on sustained exertion

• Intermittent double vision

• Slurred speech

• Difficult chewing and swallowing

• Symptoms worse at the end of the day (diurnal)

Lambert-Eaton syndrome

An autoimmune syndrome that causes the destruction of calcium channels that facilitate the release of acetylcholine into the synaptic cleft.

How is MG treated?

• Pyridostigmine (acetylcholinesterase)

• Immunosuppression (steroids, azathioprine, IVIg)

• Supportive measures (eg. feeding tubes, rehabilitation)

• In some cases, surgery: association with thymoma (paraneoplastic disorder)

• Avoid certain drugs which may worsen the disorder (e.g. gentamicin)

Myopathy

A general term referring to any disease that affects the muscles that control voluntary movement in the body.

Typical presentation of myopathy

• Many different patterns exist

• General pattern is proximal weakness (might cause difficulty getting up from a chair for example)

• Fatiguability is not seen

• Reflexes are generally preserved

• Sensation is normal

• Characteristic regions of atrophy (or hypertrophy)

• Contractures

• Pain

• Dysmorphic features

Does hypertrophy mean muscles are stronger?

No, despite growth, they are often weak.

Polymyositis

Autoimmune disorder causing muscle inflammation. It can sometimes be associated with underlying cancer (paraneoplastic phenomenon).

What investigations might be performed on someone with polymyositis

• Creatine Kinase test (will be elevated)

• Muscle biopsy (will show invading lymphoid cells and inflammation)

How is polymyositis treated?

• Immunosuppression: steroids, azathioprine/methotrexate, IVIg

• Supportive measures: physiotherapy, OT, SLT

• Screen for underlying cancer - and treat if present

Is MND a LMN disease?

No, it is a mixed upper and lower MN disease.

Typical presentation of MND

• Mix of UMN and LMN and bulbar features affecting multiple regions

• Gradual, insidious progression over months

Motor Neuron Disease (MND)

Motor neurone disease (MND) is a rare condition that progressively damages parts of the nervous system. This leads to muscle weakness, often with visible wasting. There is no cure, and it is unfortunately rapidly fatal.

What is the most common form of MND?

Amyotrophic lateral sclerosis

MND causes degeneration where?

In the descending corticospinal pathways, anterior horns, and peripheral motor nerves.