Hereditary motor and sensory neuropathy (HMSN) Flashcards
Definition of hereditary neuropathy
a group of inherited disorders that affect the peripheral nervous system, leading to progressive muscle weakness, sensory loss, and nerve degeneration.
What is the most common type of HMSN?
CMT1A (a type of Charcot-Marie-Tooth disease)
What types of genes does it affect?
genes involved in nerve structure and function
Cause of CMT1A
*Caused by a duplication of the PMP22 gene on chromosome 17
*PMP22 gene important in making myelin sheath
*Cause damage to Schwann cells and the myelin sheath of the axon aka demyelination of peripheral nerves
How is CMT1A inherited?
Autosomal dominant
3 common causes of HMSN:
About 90% of mutations are caused by 3 genetic defects—PMP22 duplication, GJB1 point mutations, and MPZ point mutations
HMSN classifications:
- HMSN Type I (CMT1) – Demyelinating form, slow conduction velocity.
- HMSN Type II (CMT2) – Axonal form, normal conduction velocity but low amplitude.
- HMSN Type III (Dejerine-Sottas disease) – Severe early-onset form
Clinical features of CMT1A: WALKS
*Weakness & atrophy (lower legs → hands later)
*Arch abnormalities (pes cavus = high arch) + distal weakness (hands and feet)
*Late hand involvement (writing issues)
*Keeps progressing slowly (most adults still walking)
*Sensory loss (distal, feet & hands)
Is speech or swallowing typically affected in CMT?
No
Assessment for CMT:
*Nerve conduction studies: slow conduction
*EMG: confirming the diagnosis of neuropathy and distinguishing between demyelinating and axonal types of CMT.
*Genetic testing: e.g. presence of PMP22 duplication
*Nerve biopsy
Most important ax in CMT:
Nerve conduction studies
Features of Dejerine-Sottas:
*Much rarer
*Presents with severe clinical picture of weakness, swallow, language, and learning disabilities
*Nerve conducts at really slow speeds
