Myasthenia Gravis

Slide 1 · Myasthenia Gravis

Immune-mediated failure of neuromuscular transmission

Mechanism • Thymic associations • Clinical presentation • Diagnosis

Dr Abu Suraih Sakhri

MBBS, MD (Pathology)

www.abusuraihsakhri.com

Slide 2 · Learning outcomes

Localize myasthenia gravis to the postsynaptic neuromuscular junction.

Contrast anti-AChR and anti-MuSK mechanisms without conflating them.

Explain the relationship between thymic pathology and autoimmunity.

Connect ocular, bulbar, and generalized weakness to diagnostic testing.

Interpret textbook treatment principles and distinguish key mimics.

Slide 3 · The neuromuscular junction is a specialized synapse

Motor axon terminal

ACh-containing vesicles

Synaptic cleft / acetylcholine

Postsynaptic sarcolemma with junctional folds

ACh receptors = outlined rectangles

Schematic: Receptor symbols and spacing are illustrative, not counts or measurements.

Slide 4 · Normal neuromuscular transmission: A causal sequence

Nerve terminal depolarizes

Acetylcholine released

ACh binds postsynaptic receptors

Muscle activation

Presynaptic boutons face the muscle fiber across a synaptic cleft.

Postsynaptic ACh receptors initiate the response that leads to contraction.

Slide 5 · Postsynaptic specialization increases receptor access

Structural feature

Complex infoldings of the sarcolemma

Local clustering of acetylcholine receptors

Functional importance

Concentrates receptor sites at the end plate

Supports reliable translation of ACh release into a muscle response

Slide 6 · Localize dysfunction before naming a disorder

Presynaptic ACh release

Lambert–Eaton / botulism

Synaptic cleft

ACh traverses the gap

Postsynaptic ACh receptor function

Myasthenia gravis

Different mechanisms can converge on painless weakness.

Slide 7 · Myasthenia gravis: Acquired autoimmune disease

Autoantibody-mediated failure of postsynaptic ACh receptor function

The end plate receives nerve-derived ACh, but the resulting muscle response becomes less reliable.

Slide 8 · The clinical signal is painless, fatigable weakness

Neuromuscular-junction disorders typically produce painless weakness.

Myasthenia gravis produces fluctuating symptoms rather than fixed deficits.

Weakness often worsens with exertion and during the day.

Clinical distribution depends partly on the pathogenic autoantibody.

Slide 9 · Epidemiology: Prevalence and age distribution

Population frequency

A prevalence of 150–200 per million.

This estimate describes MG overall, not an antibody subgroup.

Age pattern

Age at presentation shows a bimodal distribution.

Young-adult and older-adult groups differ in sex distribution.

Slide 10 · Sex distribution varies with age

Clinical group

Reported pattern

Interpretation

Young adults

Female : male ≈ 2 : 1

Female predominance

Older adults

Male predominance

The overall sex ratio is age-dependent

Age-specific pattern as stated in the Robbins source; No age cutoffs are supplied.

Slide 11 · Autoantibodies are pathogenic, not merely associated

Patient autoantibodies

Transfer with patient serum

Experimental myasthenic phenotype

Animal transfer experiments support antibody-mediated causation.

Reducing autoantibody levels is associated with symptomatic improvement.

Slide 12 · Two antibody mechanisms dominate the textbook account

Target

Site / effect

Important distinction

ACh receptor (AChR)

Postsynaptic receptor depletion + membrane damage

Complement can contribute

Muscle-specific kinase (MuSK)

Disrupted receptor trafficking and clustering

Textbook describes non–complement-fixing antibodies

Slide 13 · Anti-AChR antibodies are the major serologic group

Anti-AChR antibodies in about 85% of affected patients.

The target is the acetylcholine receptor on the postsynaptic sarcolemma.

Receptor-directed autoimmunity reduces effective signaling at the end plate.

Slide 14 · Anti-AChR antibodies promote receptor aggregation

Antibody binds ACh receptors

Receptors become cross-linked / aggregated

Aggregates undergo removal

Simplified mechanism — aggregation is one of several pathogenic effects.

Slide 15 · Receptor degradation reduces the responsive surface

Motor axon terminal

ACh-containing vesicles

Synaptic cleft / acetylcholine

Postsynaptic sarcolemma with junctional folds

ACh receptors = outlined rectangles

Schematic: Receptor symbols and spacing are illustrative, not counts or measurements.

Slide 16 · Complement-mediated attack injures the end plate

Anti-AChR antibody binding

Complement fixation

Postsynaptic membrane damage

Reduced receptor function

Membrane injury adds to receptor loss from antibody-driven degradation.

Slide 17 · Myasthenic postsynaptic morphology is altered

Normal end plate

Specialized junctional folds

Concentrated ACh receptor clusters

Effective response to nerve-derived ACh

Anti-AChR myasthenia

Postsynaptic membrane is morphologically altered

ACh receptors are depleted

Muscle response to ACh is limited

Slide 18 · Two anti-AChR effects converge on reduced transmission

Receptor degradation

+

Complement-mediated injury

Fewer functional postsynaptic receptors

Diminished muscle response to ACh

The combined effect is functional, not primary destruction of the motor axon.

Slide 19 · Why normal ACh release may produce weakness

Presynaptic event

Motor nerve terminal releases ACh

ACh reaches the sarcolemma

Postsynaptic limitation

Receptors are fewer or poorly organized

Activation may fail despite ACh availability

Mechanistic inference from the source’s receptor-depletion model.

Slide 20 · Fatigability follows impaired transmission reliability

Repeated muscle use

Limited postsynaptic response reserve

Progressively apparent weakness

Symptoms fluctuate and usually worsen with exertion.

The source describes worsening later in the day as a typical pattern.

Slide 21 · MuSK-related disease disrupts receptor organization

Muscle-specific receptor tyrosine kinase (MuSK) is a sarcolemmal protein.

Anti-MuSK antibodies disrupt trafficking and clustering of ACh receptors.

This pathway impairs function without the complement-fixing mechanism described for anti-AChR MG.

Slide 22 · MuSK antibodies: A distinct mechanistic route

Anti-MuSK antibodies

Disordered receptor trafficking

Disrupted AChR clusters

Reduced AChR function

Slide 23 · Receptor distribution in MuSK-associated disease

Motor axon terminal

ACh-containing vesicles

Synaptic cleft / acetylcholine

Postsynaptic sarcolemma with junctional folds

ACh receptors = outlined rectangles

Schematic: Receptor symbols and spacing are illustrative, not counts or measurements.

Slide 24 · AChR and MuSK myasthenia have different pathways

Feature

Anti-AChR-associated

Anti-MuSK-associated

Primary target

ACh receptor

MuSK

Dominant mechanism

Receptor loss + membrane injury

Impaired receptor organization

Complement (source model)

Contributes to injury

Antibodies do not fix complement

Clinical emphasis

Often ocular involvement

Neck, shoulder, face, bulbar, respiratory

Slide 25 · Different antibody targets, shared final pathway

Anti-AChR: Receptor depletion and membrane damage

Anti-MuSK: Disordered receptor clustering

Reduced effective ACh receptor function → weakness

Convergence represents a shared functional endpoint, not identical upstream pathology.

Slide 26 · Thymic abnormalities are linked to anti-AChR MG

A strong association with thymic abnormalities in pathogenic anti-AChR myasthenia.

Two relevant conditions are thymoma and thymic follicular hyperplasia.

The association motivates a model of impaired thymic self-tolerance.

Slide 27 · The thymus normally exposes developing immunity to self

Thymic tissue expresses peripheral antigens

Skeletal muscle proteins are among these antigens

Normal self-tolerance is supported

Sequence summarizes the textbook’s thymic-tolerance rationale; Immune details are intentionally not extrapolated.

Slide 28 · Proposed tolerance failure links thymic disease to MG

Thymoma or thymic hyperplasia

Abnormal presentation of tissue antigens

Failure of self-tolerance (hypothesis)

Autoimmunity to AChR

Hypothesized mechanism, not proven as a universal sequence.

Slide 29 · Thymoma: Epithelial neoplasm associated with MG

Pathologic identity

Tumor of thymic epithelial cells

Thymoma in approximately 10% of MG cases

Clinical significance

A thymic lesion can coexist with an antibody-mediated neuromuscular disorder

Recognition affects evaluation and treatment planning

Slide 30 · Thymic hyperplasia: B-cell follicles are a clue

Source description

Additional approximately 30% of patients have thymic hyperplasia

Association is particularly noted in younger patients

Morphologic teaching concept

B-cell follicles appear in thymic tissue

Follicular hyperplasia is not equivalent to thymoma

Slide 31 · Distinguish thymic neoplasia from follicular hyperplasia

Feature

Thymoma

Thymic hyperplasia

Nature

Thymic epithelial tumor

Reactive follicular abnormality

Illustrative source finding

Neoplastic thymic epithelial cells

B-cell follicles within thymus

MG association in Robbins

About 10%

Additional about 30%

Histologic material in this lecture

No source photomicrograph provided

No source photomicrograph provided

Slide 32 · Thymic association depends partly on age

Younger-onset disease

Thymic hyperplasia is emphasized in younger patients.

Thymic autoimmunity offers a plausible etiologic model.

Older-onset disease

Thymic abnormalities are usually absent.

The basis of autoantibody formation is not known account.

Slide 33 · Association is not the same as proven causality

Thymic changes are strongly associated with anti-AChR disease.

Robbins frames disrupted self-antigen presentation as a hypothesis.

The model should not be presented as the proven explanation for every patient.

Older-onset disease illustrates an unresolved pathogenesis gap.

Slide 34 · A clinical syndrome of fluctuating weakness

Weakness worsens with activity in typical anti-AChR cases.

Symptoms often intensify over the course of a day.

Severity and muscle distribution vary between patients.

Extraocular involvement is diagnostically informative.

Slide 35 · Ocular involvement: Ptosis and diplopia

Ptosis

Weakness affecting eyelid elevation

A visible sign of ocular myasthenia

Diplopia

Weakness of extraocular muscles

Misalignment of ocular movements can produce double vision

Slide 36 · Why ocular symptoms are diagnostically valuable

Extraocular muscles affected

Ptosis and / or diplopia

Neuromuscular junction becomes a leading site of dysfunction

Ocular MG with most myopathies, where extraocular weakness is unusual.

Slide 37 · Myasthenia may remain limited to the eyes

Ocular-predominant presentation

Symptoms confined to extraocular muscles

Ptosis and diplopia can dominate

Generalized presentation

Weakness extends beyond ocular muscles

Severe cases may involve breathing muscles

Slide 38 · Generalized weakness can compromise ventilation

Myasthenic weakness becomes generalized

Respiratory muscle performance decreases

Mechanical ventilation may be required

Potential severe consequence, not an inevitable course.

Slide 39 · MuSK-associated myasthenia: Focal pattern matters

MuSK cases may differ from the typical anti-AChR phenotype.

Neck, shoulder, and facial muscle involvement.

Bulbar and respiratory muscles are also particularly affected.

Distribution can therefore guide targeted antibody evaluation.

Slide 40 · Match clinical findings to the implicated muscles

Finding

Muscle region

Clinicopathologic meaning

Ptosis

Eyelid elevators

Ocular muscle weakness

Diplopia

Extraocular muscles

Ocular motor imbalance

Bulbar weakness

Bulbar musculature

Potential swallowing/speech difficulties (inference)

Respiratory weakness

Breathing muscles

Risk of ventilatory compromise

Slide 41 · Characteristic course: Worsening with exertion

Activity begins

Repeated use unmasks impaired transmission

Strength declines visibly or subjectively

Fluctuation is more characteristic than continuous, stable loss of power.

Slide 42 · Clinical reasoning starts with distribution and timing

Question

Finding supportive of MG

Does strength vary over the day?

Yes: symptoms often worsen later

Is worsening linked to exertion?

Yes: use-dependent fatigability

Are ocular muscles involved?

Ptosis or diplopia is common

Is weakness painless?

Consistent with a neuromuscular-junction process

Slide 43 · Diagnosis integrates four information streams

Clinical history

Physical findings

Autoantibody identification

Electrophysiology

No single one of these elements should be treated as the entire diagnostic assessment.

Slide 44 · Serology links phenotype to an immune target

Anti-AChR result

Supports antibody-mediated postsynaptic MG

Consistent with the common textbook serologic category

Anti-MuSK result

Supports a different postsynaptic antibody mechanism

May match a distinct muscle-distribution pattern

Slide 45 · Electrophysiology can demonstrate decrement

Repeated nerve stimulation

Muscle response decreases

Supports impaired neuromuscular transmission

Qualitative description only. No unprovided cutoffs or test sensitivity are asserted.

Slide 46 · A decremental response has a mechanistic basis

Normal transmission

Repetitive stimulation activates the muscle consistently

Adequate postsynaptic receptor function

Myasthenia gravis

The evoked muscle response may diminish with repeated stimulation

Limited receptor function reduces effective transmission

Slide 47 · Avoid interpreting a test without clinical context

Compatible history suggests the neuromuscular junction.

Clinical examination determines muscle distribution and severity.

Autoantibodies can define the target mechanism.

Electrophysiologic decrement provides physiologic support.

Slide 48 · Diagnostic pathway: A source-grounded framework

Fluctuating painless weakness ± ocular signs

Clinical examination

AChR / MuSK antibody assessment

Repetitive stimulation assessment

Teaching framework; Not a validated clinical diagnostic algorithm.

Slide 49 · Myasthenia gravis versus Lambert–Eaton syndrome

Discriminating feature

Myasthenia gravis

Lambert–Eaton syndrome

Primary target

Postsynaptic AChR or MuSK

Presynaptic calcium channel

With activity

Weakness tends to worsen

Strength improves after brief activity

Repetitive stimulation

Decremental muscle response

Increment with rapid repeated stimulation

Important association

Thymoma / thymic hyperplasia

Malignancy, often small-cell lung carcinoma

Slide 50 · Why LEMS and MG respond differently to activity

Postsynaptic MG

Fewer effective ACh receptors at the end plate

Repeated activity exposes weakness

Presynaptic LEMS

Autoantibodies limit calcium-dependent ACh release

Brief activity may facilitate strength

Slide 51 · Congenital myasthenic syndromes are not autoimmune MG

Acquired autoimmune MG

Pathogenic AChR- or MuSK-directed antibodies

Immune-directed treatments are mechanistically relevant

Congenital myasthenic syndromes

Inherited defects in neuromuscular-junction proteins

Phenotype and drug response depend on causal mutation

Slide 52 · Ocular MG differs from most primary myopathies

Myasthenia gravis

Ptosis / diplopia are common

Weakness fluctuates with use

Primary lesion is at the end plate

Most myopathies

Extraocular muscle involvement is unusual

Primary myofiber injury is the defining tissue level

Clinical phenotypes vary by myopathy

Slide 53 · AChE inhibition increases acetylcholine availability

Acetylcholinesterase inhibited

Acetylcholine persists longer

Remaining receptors have greater exposure to ACh

Acetylcholinesterase inhibitors as first-line treatment.

Slide 54 · Immune-directed therapies address antibody production

Drug classes

Glucocorticoids

Cyclosporine

Rituximab

Therapeutic rationale

Immunosuppression / modulation lowers pathogenic immune activity

Symptom control can follow reduced autoantibody burden

Slide 55 · Plasmapheresis directly reduces circulating factors

Circulating pathogenic autoantibodies

Plasmapheresis removes plasma components

Antibody burden decreases

Symptoms may improve

This is a mechanistic teaching sequence, not a procedure protocol.

Slide 56 · Newer pathway-directed therapy follows disease biology

IgG catabolism

An agent that increases IgG catabolism.

Lowering IgG can lower circulating pathogenic autoantibodies.

Complement targeting

Therapies directed at complement.

The approach fits anti-AChR complement-mediated injury.

Slide 57 · Treatments can be mapped to the disease mechanism

Therapeutic strategy

Mechanistic level

Acetylcholinesterase inhibition

Increases lifetime of ACh at the synapse

Plasmapheresis

Reduces circulating antibody-associated factors

Immunosuppressive medication

Reduces pathogenic immune activity

Enhanced IgG catabolism

Accelerates removal of IgG antibodies

Complement pathway blockade

Targets complement-mediated postsynaptic injury

Slide 58 · Thymectomy: Preserve the textbook’s stated uncertainty

Thymoma-associated MG

Thymectomy as often effective in patients with thymoma.

Thymoma itself is a thymic neoplasm.

Without thymoma

Benefit with hyperplasia or no thymic abnormality as uncertain.

This is textbook wording, not an updated practice recommendation.

Treatment guidance has evolved and must be verified against current practice guidelines before clinical use.

Slide 59 · Outcomes improved as effective therapies emerged

Historical mortality

Overall mortality above 30% in the 1950s.

Severe weakness can compromise respiratory function.

Textbook contemporary mortality

Less than 5% with current therapies.

These are broad historical estimates, not subgroup predictions.

Slide 60 · Case exercise: Painless fluctuation with ocular symptoms

Hypothetical teaching vignette

An adult develops intermittent eyelid drooping and double vision that increase after prolonged activity.

Which anatomic compartment most plausibly explains the presentation?

Which additional evidence would support autoimmune MG?

Slide 61 · Case interpretation: Localize, then test

Fluctuating ptosis / diplopia

Postsynaptic NMJ dysfunction suspected

AChR / MuSK antibodies + physiology

A compatible history alone is not sufficient for etiologic classification.

Slide 62 · Case exercise: Selective bulbar and neck weakness

Hypothetical teaching vignette

An individual with fluctuating weakness has prominent neck, facial, and bulbar involvement.

Which antibody subtype is emphasized by this distribution?

How does its mechanism differ from complement-fixing anti-AChR disease?

Slide 63 · Case interpretation: MuSK points to receptor organization

Neck / facial / bulbar distribution

Consider MuSK-associated phenotype

Disrupted ACh receptor trafficking / clustering

Clinical pattern is suggestive; Confirmation rests on integrated evaluation.

Slide 64 · Case exercise: Paradoxical improvement with activity

Hypothetical teaching vignette

An adult reports extremity weakness that improves after several seconds of muscle activity.

Which alternative NMJ disorder becomes more likely?

Which presynaptic target and malignancy association should be recognized?

Slide 65 · Case interpretation: Presynaptic facilitation suggests LEMS

Strength improves after brief activity

Presynaptic ACh-release defect suggested

Consider Lambert–Eaton syndrome

LEMS is commonly associated with an underlying malignancy in the Robbins source.

Slide 66 · Five mechanism-linked points to retain

MG is an acquired autoimmune postsynaptic transmission disorder.

AChR antibodies promote receptor loss and complement-mediated membrane injury.

MuSK antibodies disrupt receptor clustering without complement fixation in the source model.

Fluctuating, often ocular weakness directs serologic and physiologic evaluation.

Thymic abnormalities and therapies illuminate—but do not fully resolve—its pathogenesis.

Slide 67 · Primary teaching source and scope

Robbins, Cotran & Kumar

Pathologic Basis of Disease, 11th edition (2026)

Myasthenia Gravis; Diseases of the Neuromuscular Junction

Comparative details: adjacent Lambert–Eaton and congenital myasthenic syndrome sections

Treatment information summarizes a pathology textbook and is not a current prescribing guideline.