Pathology of Muscle Disease

Question 1

What differences would you see on biopsy between a neurogenic vs. myopathic pathologic process of muscle disease?

Examples of each?

Answer 1

Neurogenic should show ANGULAR, atrophic fibers.
Example: motor neuron disease (ALS, SMA, polio), peripheral nerve disease

Myopathic (primary disease of muscle) should show ROUNDED, atrophic fibers as well as evidence of fiber degeneration/regeneration. This results in disordered internal architecture and myonecrosis. May or may not see vacuoles/ intracytoplasmic deposits such as glycogen, mitochondria, lipid. Late in disease would see fibrosis and fatty infiltration.
Example: dystrophy, congenital myopathy, inflammatory/ infectious, toxic/metabolic/drugs

Question 2

Type 1 vs. type 2 muscle fiber

Answer 2

A motor unit, made up of lower motor neuron and the skeletal muscle fibers that it innervates, determines whether muscle fiber will be type 1 or 2:

Type 1: slow twitch for weight bearing –> lipid
Type 2: fast twitch for purposeful movement –> glycogen

Question 3

Spinal Muscular Atrophy (SMA)

Answer 3

Neurogenic muscle disease.

Childhood forms linked to chromosome 5q mutations for SMN (survival motor neuron) gene.

Question 4

Biopsy for McArdle’s disease

Answer 4

Negative marker for myophophorylase

Positive PAS stain for increased glycogen due to inability to metabolize.

Question 5

Biopsy for carnitine palmityl transferase deficiency (lipid storage disease)

Answer 5

Positive staining for Oil Red O that stains muscle fibers strewn with fatty deposits