Myopathies series- Part 2

METABOLIC MYOPATHIES

In previous post, I gave an introduction of metabolic myopathies.

Today we cover:-

I.Diagnostic role of creatine kinase in metabolic myopathies.

II.Metabolic myopathies and its types.

Diagnostic role of enzyme in myopathies.

The following diagram shows the enzymes related to myopathies and their associated metabolic reactions.( Note:- The metabolic pathway is not only for skeletal muscle .It is in general . My main aim is to show enzymes of liver and muscle along with the pathways.Remember urea cycle occurs in liver )

http://chemistry.elmhurst.edu/vchembook/images/641serumenzymes.gif

Creatine kinase: - This enzyme will help us to evaluate different METABOLIC myopathies.

1. ELEVATED CK: - In Glycogen storage disease associated myopathies.
    (In some GSD there will be mild elevated CK)
2. MILD ELEVATED CK:- In Fatty acid oxidation disorder.
3. NORMAL CK: - In Mitochondrial myopathies.Also in some fatty acid oxidation disorder.
   
   Metabolic myopathies types:-

I.                    DISORDER OF GLYCOGEN METABOLISM (MUSCLE GYCOGENOSES)

II.                  DISORDER OF FATTY ACID OXIDATION

III.                MITOCHONDRIAL MYOPATHIES

CLINICAL FINDINGS :-

1. Second wind phenomenon: - suggestive of GSD V / McArdle’s

2. Out-of-wind phenomenon: - suggestive of GSD VII/ Tarui

3. Myoglobinuria (Burgundy colored urine):- GSD V, GSD IX
                                                                         LDH, PGM or PGK enzyme deficiency
                                                                         CPTII Deficiency
4. Proximal weakness: - GSD II / Pompe.

5. Exercise intolerance,ataxia,multisystem involvement:- Mitochondrial disorder, Coenzyme Q10 Deficiency.

LAB TESTS:-

1. Serum CK levels.

2. Lactate 

3.Serum electrolytes.

4. ammonia

5.AST,ALT,GGT 

6. Urinalysis

7.Forearm exercise test.

8.EMG

9.Routine muscle biopsy

SPECIFIC TESTS:- 

1. Urine organic acids

2. Plasma acylcarnitine profile

CONFIRMATORY BUT COSTLY :-

1. Enzyme analysis

2. DNA Analysis on leukocytes, fibroblasts and liver.

Click on the below given link to read on how to differentiate between McArdle, CPT II deficieny and mitochondrial myopathy. (this link helped me with the notes) 

*If you are running short of time, then Read only Case 1 and Case 2

 http://www.the-rheumatologist.org/article/diagnosis-myopathy/

I hope it helped. 

-Upasana Y. :)