LMN Disorders

Question 1

Compare signs in UMN vs LMN disorders

Answer 1

UMN - no wasting, hypertonia, hyperreflexia, Babinksi response
LMN - wasting, normal or hypotonia, hyporeflexia

Question 2

Where are the different areas that a problem can occur in a LMN weakness?

Answer 2

Muscle fibres (myopathy)
Neuromuscular junction
Peripheral nerve (neuropathy)
Nerve root (radiculopathy)
Anterior horn cell

Question 3

Describe the features of a myopathy.

Answer 3

Proximal, symmetrical weakness
Can be painful
(reflexes spared until late, wasting can happen late)

Question 4

What are the features of neuromuscular junction disorders?

Answer 4

Clinical hallmark FATIGUABILITY
- extraocular muscles involved: opthalmoplegia, ptosis, diplpoia
- bulbar muscles involved: chewing, swallowing, nasal speech
- proximal + symmetrical weakness with extraocular, facial adn bulbar weakness
(most common: myasthenia gravis - young women, old men)

Question 5

What are the features of polyneuropathies?

Answer 5

Distal, symmetrical weakness (+/- sensory loss)
Reflexes absent
Distal wasting, pes cavus

Question 6

What are the features of MND?

Answer 6

Painless, progressive weakness and wasting 
Eye movements normal 
Sensation normal 
Respiratory muscle weakness
Asymmetric 
Bulbar involvement
Combination of UMN + LMN signs - increased reflexes
Fasiculations
(most common: ALS)

Question 7

What questions can you ask in a history to differentiate LMN disorders?

Answer 7

Pattern of weakness - what tasks are difficult?

• stairs/brushing hair - proximal weakness
• tripping, opening jars, buttons - distal

How quickly did weakness come on?

• years/since childhood - genetic?
• acute/over days or weeks - infectious/inflammatory cause

Pain?
- some myopathies, peripheral neuropathies, lots of pain in radiculopathy, NOT NMJ or MND

Sensory loss?

• peripheral neuropathies (not all), radiculopathies
• NOT myopathies, NMJ or MND

Symptoms of respiratory muscle weakness?

Question 8

What is important to look out for in general inspection?

Answer 8

• General inspection**
• wasting - think MND or peripheral nerve, obesity can mask
• fasiculations - use torch, look very closely, check tongue, pectoralis, possible MND (but can also be very proximal nerve lesion if with weakness + wasting, or can be due to stress or caffeine if not)
• scapular winging - myopathy
• spinal rigidity - certain myopathy
• pes cavus, hammer toes - peripheral neuropathy, esp. genetic
• injuries and callous from sensory loss

Question 9

What should you look out for in the rest of the exam?

Answer 9

Pattern of weakness

• proximal (myopathy): ask patient to stand from sitting on floor or sit without using arms
• distal symmetrical: polyneuropathy

Reflexes

• absent or reduced: peripheral neuropathy
• reduced or normal: myopathy
• increased despite other LMN signs: MND

Sensory loss

• distal symmetrical: peripheral neuropathy
• affecting one nerve: mononeuropathy
• if present, rules out myopathy, MND + myasthenia

Gait

• trendelenberg/waddling gait: myopathy
• foot drop/high steppage: peripheral neuropathy

Question 10

What investigations would be carried out in suspicion of LMN disorders?

Answer 10

Bloods: creatinine kinase (CK)
- > 320 in males or >176 in females, naturally higher in black patients and after high intensity exercise, increased in muscle disease, denervation (or sepsis, renal failure, malignancy)
Neurophysiology (nerve conduction study, EMG)
Muscle biopsy (shows if there is myopathy - atrophy of muscle)
MRI spine (exclude other causes)
Muscle MRI (specific pattern of muscle wasting)
Genetic testing