Diagnosis

Long-time exercise test in the diagnosis of periodic paralysis

Ding Z, Liu M, Cui L. Reference value of long-time exercise test in the diagnosis of primary periodic paralysis. Chin Med J (Engl). 2014;127(18):3219-23.

Abstract

BACKGROUND:
The long-time exercise test (ET) is used to diagnose the primary periodic paralyses (PPs). However the reference values of ET are many and various. This study aimed to investigate the reference value of long-time ET in the diagnosis of PPs.

A Quick Overview of Hypokalemic Periodic Paralysis

 

1) Patient has "attacks" of weakness and/or floppy paralysis.

2) First attacks occur by the age of 16 in 65% of patients.

How is periodic paralysis diagnosed?

Do you suspect that you might have periodic paralysis? The periodic paralyses are a rare group of disorders and there are many conditions which cause an imbalance in serum potassium. So how does the doctor tell the difference between paralysis or weakness caused by an ion channelopathy and any of the other numerous disorders, conditions and reactions which might produce the same symptoms?

Pursuing a Diagnosis of Periodic Paralysis:


A free workbook to download for those who are in the diagnostic process, contains information on each type of PP, diagnostic testing, available therapies, how to determine episode triggers and  work out a management plan. download here

Diagnostic Flowchart for Diagnosing Hypokalemic Periodic Paralysis

The normal renal response when hypokalemia is due to non-renal causes is a TTKG <2, where a TTKG >5 is indicative of increased secretion of K+ in the cortical collecting ducts. Thus a transtubular potassium concentration gradient (TTKG) of greater than 3.0 indicates hypokalemia of renal origin, while a value below 2.0 indicates intracellular shift of K+, as found in ion channelopathy hypokalemic periodic paralysis.

Chart Comparing Periodic Paralyses

This chart allows comparison of the forms of primary periodic paralysis, symptoms, clinical and cardiac signs, effect of potassium, triggers and therapies. 

 

The Potassium Draw: Special Measures

When evaluating patients for periodic paralysis it is vital to obtain accurate results from blood draws. Because potassium levels which vary only a fraction of a point may be enough to trigger weakness in some patients, the lab may need to be made aware of specialized procedures which should ideally be used to obtain samples for K+ evaluation.

From "A Manual of Laboratory Diagnostic Tests" by Frances Fischbach, Page 247-253.

Potassium (K+) Normal Values: 3.5-5.0 mmol/liter 4.0-4.7 mmol/liter (average)

Determining the Origin of Hypokalemic Paralysis

Using the Transtubular Potassium Concentration Gradient

to Determine the Origin of Hypokalemic Paralysis

Compound Muscle Action Potential (CMAP) Test

The Compound Muscle Action Potential (CMAP)  test (aka the Exercise EMG) is now being used to diagnose the periodic paralyses. When done correctly this test is reported to positively identify 70%-80% of patients.

How to Determine What Triggers Attacks of Periodic Paralysis

What happens during an attack of Periodic Paralysis?

People with Periodic Paralysis (PP) have episodes in which their muscles become weak or paralyzed in response to variations in the amount of potassium in their blood. These variations occur during sleep, they may result from food the person has eaten, from a sudden change in a person's activity level, from rest after activity, emotional stress, from becoming chilled, or a number of other factors. Potassium levels vary all the time.

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