stanley iyadurai, phd md

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Stanley Iyadurai, PhD MD Assistant Professor of Neurology/Neuromuscular Medicine

Nationwide Children’s Hospital Myology Course 2015

Motor unit – motor neuron, its axon, and nerve terminals, and muscle fibers the axon innervates Presynaptic nerve terminal

No myelin sheath ACh is synthesized from Choline and Acetyl CoA by action of

ChAT P/Q type calcium channels Synaptic vessels – 6-10, 000 ACh molecules (quantum)

▪ Immediate (primary) store – 1, 000 quanta of ACh ▪ Secondary store – 10, 000 quanta, can resupply the

primary store after a few seconds ▪ Tertiary store in the axon and cell body – 100, 000

Synaptic space – 50 nm

Postsynaptic muscle membrane

Clefts/folds, ACh R

AChE is attached to collagen fibers of basement membrane, breaks down ACh to choline and acetate

20% of released ACh is hydrolyzed before binding to ACh R

Choline is taken up be presynaptic nerve terminal through Na-dependent active transport mechanism

nerve axon

axon terminal

synaptic cleft

muscle fiber

synaptic

vesicles

junctional

folds

Engel et al 1976

Safety factor in NMJ transmission

safety factor

Diseased endplate electrophysiology

safety factor

transmission failures

Resting state - release of single ACh quantum

produces postsynaptic depolarization – miniature EPP

amplitude – determined by amount of ACh

Duration is determined by amount of time AChR that received the quantum is open

AP depolarizes nerve terminal Calcium channel opens – influx of Ca – release of

ACh

When EPP reaches threshold voltage, muscle fiber AP is produced (all-or-none)

Quantal content – number of synaptic vesicles (quanta) released

M=N x P

M – quantal content

N – number of quanta immediately available at nerve terminal (~1000)

P – probability of quantal release (0.2 in nl)

M=N x P

At rest

Low P, high N---- low M----small number of quanta released ---- sub-threshold EPP

AP in normal subject

High P and N ---- high M ---- EPP reaches threshold ----- muscle AP

Safety margin of NMJ

Difference between actual EPP amplitude and EPP amplitude required to produce muscle AP

Determined by quantal content, efficiency of AChE and AChRs

High in normal subjects

Slow RNS (2-3 Hz) ACh quanta are progressively depleted from primary

store

Fewer quanta are released with each successive stimulation

Corresponding EPP falls in amplitude but remains above threshold

After first few seconds the secondary store begins to replace the depleted quanta with a subsequent rise in the EPP

Normal Myasthenia gravis

Rapid RNS (10-50 Hz)

Depletion of primary store is counterbalanced by both mobilization from secondary store and accumulation of calcium

100 ms is needed for Ca to be pumped out

Accumulation of Ca predominates over ACh depletion ---- increased amount of quanta being released ---- higher EPP

Voluntary muscle contraction – 30-50 Hz Post-tetanic facilitation – brief exercise (10 sec)

Postsynaptic NMJ disorders – higher EPP --- generation of MFAP. May repair a low EPP developed after slow RNS

Presynaptic NMJ disorders – if baseline EPP is below threshold --- facilitates EPP ---- generates MFAP

Post-tetanic exhaustion – after prolonged exercise NMJ disorders - slow RNS in 2-4 min can cause greater

decline of EPP ---- no MFAP

Decrement at rest

Post-tetanic facilitation

after 10 sec of exercise

Post-tetanic exhaustion

after 1 min exercise

(1,2,3 min after)

Post-tetanic facilitation

after 10 sec of exercise

IgG-directed attack on the nicotinic ACh receptor Abs are present in the serum of many MG patients

Passively transferred Ab produce experimental myasthenia in animals

Removal of Ab allows recovery

Immunization of animals with ACh receptors produces Ab and experimental myasthenia

Autoantibodies: AChR Striational MUSK Seronegative

Acetylcholine Receptor Antibodies

Normal AChR density in controls

Fambrough et al, 1973

Decreased AChR density in MG

Normal

Myasthenia gravis

Two alpha, one beta, delta and epsilon subunits Agrin, rapsyn and muscle-specific tyrosine kinase

(MuSK) – proteins important in clustering of AChR on postsynaptic membrane

Two molecules of ACh are needed to bind to each alpha subunit to open AChR channel – Sodium influx – local depolarization (EPP)

EPP size is proportional to the amount of ACh

Mechanism of Ab damage to AChR

Ab binds to ACh receptor and directly blocks the binding of the ACh

Complement-directed attack leading to destruction of AChR and postsynaptic folds

Ab binding increases removal of AChR from postsynaptic membrane

this leads to smaller endplate potential

Muscle fatigue and weakness EOM – 50% at presentation/90% at diagnosis Proximal muscles –symmetric Bulbar muscles Pathologic fatigability Transient neonatal MG

Maternal Ab passed through placenta

Self-limiting

Only 5-15% of MG patients has small CMAP amplitudes

Ensure integrity of the nerve which will be used for RNS

1. Routine motor and sensory nerve conduction studies.

Perform routine motor and sensory nerve conduction

studies, preferably a motor and sensory nerve in one uppoer

and one lower extremity. CMAP amplitudes should be

normal. If CMAP amplitudes are low or borderline, repeat

distal stimulation immediately after 10 seconds of exercise

to exclude a presynaptic NMJ transmission disorder (e.g.,

Lambert-Eaton myasthenic syndrome)

RNS is abnormal in 50-70% of generalized MG patients

2. Repetitive nerve stimulation and exercise testing.

Perform slow RNS (2-3 Hz) on at least one proximal and one

distal motor nerve. Always try to study weak muscles. If any

significant decrement (>10%) is present, repeat to ensure

decrement is reproducible. If there is no significant

decrement at baseline, exercise the muscle for 1 minute, and

repeat RNS at 1, 2, 3, and 4 minutes looking for a decrement,

secondary to post-exercise exhaustion. If at any time a

significant decrement is present (at baseline or following

post-exercise exhaustion), exercise the muscle for 10 seconds

and immediately repeat RNS, looking for post-exercise

facilitation (repair of the decrement).

Two reasons:

exclude severe denervating disorders (i.e.

MND, polyneuropathy)

show evidence of NMJ disorder

3. Needle electromyography (EMG).

Perform routine EMG of distal and proximal muscles,

especially weak muscles. Patients with moderate to severe

myasthenia gravis may display unstable or short, small,

polyphasic motor unit action potentials. Recruitment is

normal or early. Needle EMG must exclude severe

denervating disorders or myotonic disorders, which may

display an abnormal decrement on RNS.

No clinical correlate to jitter – may be abnormal even in patients w/o overt clinical symptoms. Sensitivity – 95-99% for generalized MG, but low specificity

Often done on EDC

Normal SF-EMG in clinically weak muscle rules out MG

At least 20 single-fiber pairs. Jitter in >10% of pairs - abnormal

4. Single-fiber EMG (SF-EMG).

If the above are normal, or equivocal in a patient strongly suspected

of having myasthenia gravis, perform SF-EMG in the extensor

digitorum communis and, if necessary, one other muscle, looking

for jitter and blocking. It is always best to study a weak muscle.

Normal SF-EMG in a clinically weak muscle excludes an NMJ

disorder.

Single Fiber Electromyography

Trigger on this rise

Normal jitter

Single Fiber Electromyography

Figure courtesy of W. David Arnold, MD

Trigger on this rise

Abnormal jitter

Blocking

Acetylcholine Receptor Antibodies ~80% of MG

~100% of thymoma-MG Heterogeneity of actions:

binding, blocking, activating Major pathogenic actions:

Activate complement, membrane attack complex (MAC)

Cross-link AChR, leading to increased turnover


Striational Antibodies ~33% of MG

~90% of thymoma-MG patients

Also seen in autoimmune liver disease, lung cancer, rarely in Lambert-Eaton syndrome

Pathogenic role uncertain Bind skeletal and cardiac muscle

in a cross-striational pattern. Many targets: RYR1, titin, rapsyn,

myosin


Muscle-Specific Kinase Antibodies ~10% of cases Rarely associated with thymoma May worsen with AChE inhibitors Clinical features differ:

Rarely seen in ocular MG

Subgroup with early respiratory failure, head drop

Many indistinguishable from AChR-MG


Seronegative Myasthenia Gravis ~10% of cases Clinically similar to AChR

positive MG ~60% have AChR Ab detected

with more sensitive laboratory techniques


Acetylcholine Esterase Inhibitors Pyridostigmine (Mestinon) is the

most commonly used: Edrophonium (Tensilon)

traditionally administered in a bedside diagnostic test.

Diarrhea and abdominal cramping are common dose-limiting side effects.

Enhance NMJ Transmission

Thymectomy Immune

Therapy

Thymectomy With thymoma: always remove,

variable effects on symptoms Without thymoma: controversial

Older series (before medical immunosuppression) indicate higher remission rates in thymectomized MG

Newer series indicate no significant difference from controls

Reported remission rates after thymectomy range from 11 to 32%

Improve NMJ Transmission

Thymectomy Immune

Therapy

Immune Therapies for MG Rapid: plasmapheresis, IVIG Long-term:

Corticosteroids (prednisone)

Steroid-sparing agents:

▪ azathioprine (Imuran)

▪ mycophenolate mofetil (CellCept)

▪ cyclosporine/tacrolimus (Prograf)

Investigational biologics: rituximab, belimumab, eclizumab

Treatment Approach: Improve NMJ

Transmission Thymectomy Immune

Therapy

Neuromuscul Disord. 2015 Aug;25(8):651-2.

doi: 10.1016/j.nmd.2015.03.014. Epub 2015 Apr 22.

Reduced release of ACh from presynaptic terminal

IgG Ab against presynaptic voltage-gated Ca channels

Passive transfer IgG from LEMS pts to animals causes same symptoms

Rare, 70% male, 30% female Proximal muscle weakness (esp legs) and

fatigability DTR are reduced or absent Autonomic symptoms (dry mouth) Paresthesias Bulbar symptoms are mild

Muscle facilitation After 10 sec exercise power and DTR are increased

SCLC expresses VGCC--- starts autoimmune process Found in 60% of LEMS, esp males >40, smokers

Other pts (younger women) – primary autoimmune disease

VGCC Ab testing is available

Slow RNS before and after

exercise – decrement will

be there in both conditions

but baseline CMAP

amplitude is significantly

larger after exercise

Must be suspected in any patient with small CMAP amplitude on NCS at rest with normal sensory responses

Repeat after 10 sec exercise Few patients can have signs of both MG and

LEMS (AChR AB and CMAP amplitude facilitation after exercise)

Find and treat any underlying malignancy AChE inhibitors 3,4-diaminopyridine

Immune therapy as in MG

Exotoxin of Clostridium botulinum (A,E,F) blocks presynaptic release of ACh at both somatic and autonomic synapses

NMJ and parasympathetic blockade

Food, wound infection – 2-72 hrs after Infantile botulism 2/2 GI tract colonization

with Clostridia bacteria

Nausea, vomiting, abdominal pain Blurred vision, diplopia, dysarthria Rapidly progressive descending weakness ---

flaccid areflexic quadriparesis with ophthalmoplegia

Pupils paralyzed in 50% Ileus, decreased salivation

Similar to LEMS

Similar to LEMS

Inherited defect of NMJ transmission, rare Not immune-mediated Usually presents in early childhood EO, bulbar and proximal muscles are often

affected Heterogeneous NCS/EMG results Single impulse may cause repetitive CMAP

potential Morphologic and in vitro

electrophysiological analysis of an NMJ from biopsied muscle

Disorder Onset Ocular Sx?

Bulbar Sx?

Reflexes Autonomic Sx?

Sensory Sx?

GI Sx?

MG Subacute Yes Yes Normal No No No

LEMS Subacute +/- +/- Reduced +/- +/- No

Botulism Acute Yes Yes Normal Yes No Yes

CMS Congenital Yes +/- Normal No No No

Disorder CMAP amplitude

Decrement in 3 Hz

Increment in 50 Hz

SF-EMG

Repetitive CMAP

Fibs/PSW? MUAP

MG Normal Yes No Abn No No Nml

LEMS Decreased Yes Yes Abn No No Nml

Botulism Decreased Yes Yes* Abn No Yes Nml

CMS Normal Yes No Abn Yes* No Nml

Before treatment

After treatment

Distribution of Weakness: Ocular (~25%)

can mimic CN III, IV, or VI palsy, INO Bulbar

Dysphagia, dysarthria, dysphonia, aspiration, OSA

Limb Proximal > Distal

Upper > Lower extremity Respiratory

Can lead to respiratory failure (<10%)

stanley iyadurai, phd md

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