Myasthenia gravis pathology

Changes are found in the thymus gland and in muscle.

The gland is most active during the induction of normal immune responses in the neonatal period and attains its largest size at puberty after which it involutes.

Oesophagus u r^i fro m

In myasthenia gravis: 20%: involuted gland

Foci of lymphocytes

Epithelial cell -(Hassall's corpuscle)

Foci of lymphocytes

Epithelial cell -(Hassall's corpuscle)

Normal structure

70%: show hyperplasia

70%: show hyperplasia

with lymphoid follicles demonstrating germinal centres

10%: thymoma, an encapsulate tumour of lymphoid and epithelial cells which may be locally invasive but rarely metastasises.

Muscle biopsy may show abnormalities:

- Lymphocytic infiltration associated with small necrotic foci of muscle fibre damage.

- Muscle fibre atrophy (type I and II or type II alone).

- Diffuse muscle necrosis with inflammatory infiltration (when associated with thymoma).

Motor point biopsy may show abnormal motor endplates. Supravital methylene blue staining reveals abnormally long and irregular terminal nerve branching. Light and electron microscopy show destruction of ACh receptors with simplification of the secondary folds of the postsynaptic surface.


Up to 90% of patients present in early adult life (<40 years of age). Female: male ratio 2:1. The disorder may be selective, involving specific groups of muscles. Several clinical subdivisions are recognised: Class 1 - ocular muscles only - 20% Class 2 - Mild generalised weakness Class 3 - Moderate generalised and mild to moderate ocular-bulbar weakness 80%

Class 4 - Severe generalised and ocular-bulbar weakness Class 5 - Myasthenic crises

Approximately 40% of class I will eventually become widespread. The rest remain purely ocular throughout the illness.

Respiratory muscle involvement accompanies severe illness.

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