Charcot-Marie-Tooth syndrome(CMT)
= hereditary motor and sensory neuropathy(HMSN)
Pathophysiology:
CMT is a heterogeneous group of genetically distinct disorders
with similar clinical presentations.
CMT type 1 is a disorder of peripheral myelination, resulting
from a mutation in the peripheral myelin protein-22 (PMP22)
gene. Mutations in the gene encoding the major PNS myelin protein,
myelin protein zero (MPZ), account for 5% of patients with CMT.
The mutation results in abnormal myelin that is unstable and
spontaneously breaks down. This process results in demyelination,
leading to uniform slowing of conduction velocity.
Slowing of conduction in motor and sensory nerves was believed to
cause weakness and numbness. However, a recent study by Krajewski
et al suggests that neurological dysfunction and clinical
disability in CMT 1A are caused by loss of or damage to large
diameter motor and sensory axons. Pain and temperature sensations
usually are not affected because they are carried by unmyelinated
(type C) nerve fibers.
In response to demyelination, Schwann cells proliferate and form
concentric arrays of remyelination. Repeated cycles of
demyelination and remyelination result in a thick layer of abnormal
myelin around the peripheral axons. These changes cause what is
referred to as an onion bulb appearance.
CMT type 2 primarily is a neuronal (ie, axonal) disorder, not a
demyelinating disorder. CMT type 2 results in peripheral
neuropathy through direct axonal death and Wallerian degeneration.
CMT type 3 (also known as Dejerine-Sottas disease) is
characterized by infantile onset. CMT type 3 results in severe
demyelination with delayed motor skills and is much more severe
than type 1. Marked segmental demyelination with thinning of the
myelin around the nerve is observed on histological examination.
CMT X (X-linked CMT) and CMT 4 also are demyelinating neuropathies.
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所以CMT有四種types,不過最常見的還是type 1 & 2.
他如何和muscular dystrophy區分呢? (ex. Duchenne and Becker)
muscular dystrophy是肌肉本身的問題,不過muscular atrophy是神經的問題
造成肌肉的刺激變少於是逐漸atrophy.
最常表現的是motor dysfunction of calf muscle,
尤其是peroneal muscle所以又稱做peroneal muscular atrophy
因此會看到小腿的肌肉萎縮,走路時foot drop + 腳跟先著地 + 腳趾拖著地板前進
有時也會有sensory的involvement.
對於影響到手部肌肉的人,則會抱怨拿東西容易掉,"sloppy hands"的情形
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096/03/05 成大皮膚科門診實習
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