Hereditary Sensorimotor Neuropathy (HSMN) or CMT

Question 1

What are the different types of HSMN / CMT?
(Hereditary Sensorimotor Neuropathy / Charcot Marie Tooth)

Answer 1

Type 1 - demyelination, autosomal dominant or X-linked
- NCS: velocity reduced
- Palpable nerve

Type 2 - axonal degeneration, AD or AR
- NCS: amplitude reduced
- No palpable nerve

Type 3 - demyelination, autosomal recessive
- NCS: velocity reduced
- Palpable nerve

Question 2

Examination findings/signs of HSMN/CMT

Answer 2

Distal, symmetrical weakness and sensory loss (vibration/sensation or pansensory) resulting in inverted champagne bottle appearance and foot drop
Also has palpable nerves

A. Inspection
1. Walking aids, ankle supports and orthoses
2. Foot drop - scuffing at front of shoes
2A. Look for fibula neck scars causing common peroneal nerve palsy resultant foot drop (NOT CMT)
3. Foot deformity - pes cavus, high foot arch, hammertoes, Archilles tendon contracture
4. Distal wasting up until lower third of thigh
(inverted champagne bottle or stork legs or spindle legs)
5. Visible, palpable, large firm nerves (esp lateral popliteal nerve)

B. Lower Limbs Feel and Move - symmetrical distal sensorimotor deficit
6. Reflex - reduced or absent, plantar response absent or downgoing
7. Motor distal weakness, usually dorsiflexion (with plantarflexion usually preserved)
8. Varying reduction in sensation
- Reduced vibration and proprioception (dorsal column) with normal pain and temperature (spinothalamic)
Misnomer - real reason is that spinothalamic is unmyelinated
- Or distal pansensory loss in stocking distribution

C. Gait
9. High stepping gait - reflective of foot drop
10. Sensory ataxia and unsteadiness, Romberg positive
(loss of proprioception)

D. Shocking asked to briefly examine UL
10. Distal wasting of upper limbs - small muscles of hands, guttering
11. Hyperextension deformities at MCPJ, flexion deformities of IPJ (claw hands)
11A. Look for elbow scars causing ulnar nerve palsy (NOT CMT)
12. Palpable ulnar nerve
13. Glove distribution sensation loss (proprioception and vibration&raquo_space; pain and temperature or pansensory loss)
14. Reduced reflexes, which may be stronger on reinforcement manoeuvers
15. Postural tremors

Question 3

How does CMT cause pes cavus?

Answer 3

Pes cavus - high arched foot that does not flatten when weight bear

Imbalance between 2 different muscle groups:
- Anterior tibialis and peroneus muscle weak
- Posterior tibialis and peroneus longus pull harder

Question 4

What are the causes of pes cavus?

Answer 4

Unilateral
1. Malunion of calcaneal or talar fractures
2. Burns
3. Compartment syndrome sequelae
4. Poliomyelitis
5. Spinal trauma
6. Spinal cord tumours - L4/L5 region

Bilateral
1. Friedreich’s ataxia
2. Muscular dystrophy
3. Spinal muscular atrophy
4. Cerebral palsy
5. Syringomyelia
6. Hereditary spastic paraparesis
7. Spinal cord tumours
8. CMT

Question 5

Why are pain and temperature sensations spared in CMT?

Answer 5

Carried by unmyelinated nerve fibres

Question 6

What is onion bulb appearance of nerves?

Answer 6

1. Demyelination and repair triggers Schwann cells to proliferate and form concentric layers of remyelination
2. Repetitive demyelination and remyelination results in thick layer of abnormal myelin around peripheral axons

Question 7

How would you manage CMT?

Answer 7

1. Multidisciplinary team - Neurologist, Orthopaedics, PT, OT
2. Patient education and advice
3. Regular exercise to maintain limb function
4. Walking aids, ankle/foot orthoses
5. Analgesia for neuropathic pain
6. Surgical correction of deformities

Question 8

What is hereditary neuropathy with pressure palsy (HNPP)?

Answer 8

Tomaculous (sausage swellings of myelin) neuropathy
Autosomal dominant mutation in PMP-22 gene (chromosome 17)
- Recurrent acute mononeuropathy from minor nerve compressions due to awkward limb resting position
- Resulting in weakness and sensory loss for weeks and months, which may become persistent