Myasthenia gravis

Harwood Nuss p.1002

Differential Diagnosis

1. Toxin Induced

- Botulism

- Tick Paralysis

- Envenomation (coral snake, black

widow spider), paralytic shellfish

2. Autoimmune - Eaton Lambert

3. Drug-Induced - aminogly, dilantin

procainamide, chloroquine

4. Poisoning - Organophosphates

Carbamates

5. Miller Fisher Variant Guillen Barre

6. Causes of oculomotor palsy - DM

MS, aneurysm.

Pathophys

- thymus abnl in most - thymoma,

thymitis, B cells sensitized to Ach

receptors here.

Epi

- mostly women in 20s

- peak incid in men is 60-70s

History

- pts report worse sxs as day progr

esses.

- insidious onset, can develop over

wks to months.

- precipitated by stress, preg, infec

Symptoms

- diplopia, ptosis (later in day)

- weakness in eye closure, swallowing

muscles of facial expression,

difficulty chewing, dysarthria, dys-

phagia.

Physical Exam

- Provocative tests - ptosis with pro-

longed upward gaze, hold arms up,

clench tongue blade, dysarthria w/

loud counting

- sensation, reflexes usually normal

- always eval tidal volume, FEV &

ability to handle secretions

Testing

1. Always test FEV, consider ABG

Look for infections (resp) or

meds, electrolyte problems that

may have induced problem.

2. Edrophonium - use caution in try-

ing to test for crisis vs. cholinergic

crisis

3. Ach receptor antibodies - found 90%

4. CT of thymus, TFTs, search for

other immun dz

Treatment

- Plasmapherisis or plasma exchenge

in acute setting

- Anticholinesterase agent such as

Pyridostigmine 60 mg tid

- Corticosteroids produce good results in >80% but are reserved for

those who don't respond to anti-

cholinesterases and thymectomy

due to adverse effects. Decreases

levels of antiAch receptor Ab. Also

may initially aggravate muscle

weakness so usually begun in hosp

& at low doses

Myasthenic Crisis vs. Cholinergic

1. Cholinergic - usually present w/

signs of cholinergic overactivity

(miosis, sweats, salivation, GI dis-

tress-musc) & cramps, fascicula-

tions (nicotinic)

2. Myasthenic - more common, caused by noncompliance, drug interaction, infection, stress

- aminoglycosides, flouroquinolones

clinda, sulfas, erythro, ampicillin

Dilantin, phenobarb, B blockers

Ca channel Blk, procainamide

steroids, lithium, phenothiazines

MSO4, benzos, antihistamines

VERY DANGEROUS & UNRELIALE to

use Tensilon Test to distinguish betw

the two.

OTHER NM TRANSMISSION D/O

1. Botulism - toxin binds to presyn

prevents AcH release. Wound may be benign in appearance

GI source - neuro sxs w/in 72 hrs of ingestion usually. Aticholinergic effects include dry mouth, mydriasis

ileus, urine retention then fluctuat-

ing but rapidly progressive weakness

- send cx blood, food, stool or wound

- only 33% of food borne source have

positive blood cx

- may actually see pos Tensilon test

in botulism.

Wound Botulism - high dose PCN &

debridement.

2. Eaton Lambert - rare, defect in

release of AcH from presynapse

Usually paraneoplastic (part. small

cell Ca of lung)

- Clinically proximal weakness of limb

muscles, hyporeflexia, dry mouth,

impotence.

- Extraocular & facial muscles usually

spared.

3. Tick Paralysis - acsending flaccid paralysis caused by neurotoxin

- block acH release

- late spring, summer in Rockies & NW

- female wood tick or common dog tick

- paralysis progresses over 1-2 days

to involve bulbar, extraocular muscles.

- Resp paralysis can follow

- Ataxia may be early finding

- normal sens exam usually

- DTRs decreased markedly as in GB

- fatal in 10% if tick not removed

- CHECK for ticks in someone you

think has Guillen Barre

TAKE HOME

- don't treat Myasthenic with meds

that may exacerbate weakness

- search for source of infection or

electrolyte problem w/ weakness