Weakness Between Attacks
Are Weakness and Fatigue Between Paralytic Attacks Symptoms of Periodic Paralysis?
Patients frequently report that their physician denies that muscle weakness and fatigue between paralytic attacks are a part of periodic paralysis. While it has been reported in the past that patients with periodic paralysis had ‘normal’ muscle strength and no lingering weakness between episodes, it is now recognized that this is not the case for all patients. While young patients often bounce back quickly from episodes, older patients, those with more frequent episodes and those who are inadequately medicated may suffer from what have been dubbed Abortive Attacks.
Daily Fluctuations in Strength
Abortive attacks are those in which there are daily variations in strength, sometimes accompanied by stiffness. They range from mild fatigue to any degree of weakness short of paralysis, but their primary feature is the length of the attack. Abortive attacks have been reported which lasted so long as to be mistaken for permanent muscle weakness. 1
80% of Patients Report Abortive Attacks
In her large study of Dutch patients Dr. Thera Links reported that 80% of them complained of abortive attacks. Their weakness could at times be confirmed by muscle testing even though it was not accompanied by a significant change in serum potassium. Dr. Links concludes that “… long lasting episodes with fluctuating weakness… abortive attacks are probably due to a functional disturbance of the muscle membrane. We have found that the mean strength of affected persons with HypoKPP was lower than the mean average of normals. This implies that the cause of this ‘less than average strength’ could be the same as the cause of abortive attacks.” 2,3
CMAP Reduced by 5-15 mV in PP
Studies on intact intercostal muscle fibers indicate that the resting membrane potential of the muscle fibers in patients with hypokalemic periodic paralysis is depolarized by about 5-15 mV compared to the normal value of -85 mV. Reducing the external potassium concentration in the medium further depolarizes the fibers to -50 mV and renders them entirely inexcitable. Weakness is provoked by factors that lower the serum potassium.
Patients with Paramyotonia Congenita exhibit a similar though smaller depolarization factor. 4 Dr. Links’ team determined in large-scale Dutch studies that those who were genetically proven to be HypoKPP gene carriers showed a significantly lower mean MFCV muscle fiber conduction velocity (MFCV) than family members who did not carry the gene. The muscle force and the integrated EMG at maximal voluntary contraction were lower in the carrier group. A positive correlation between the surface MFCV and the neuromuscular efficiency (the quotient of force and integrated EMG) was found in the controls but not in the HypoKPP carriers. Since type II fibers have a higher neuromuscular efficiency, this suggests a preferential involvement of type II fibers in HypoKPP. 5
This test has been used to identify a sporadic case of HypoKPP in a young child, and may be a useful tool for determining familial involvement in those cases where gene tests prove unfruitful.6 Most patients with any form of PP have a greater than normal increase in the compound muscle action potential (CMAP) amplitude immediately after two to five minutes of intermittent voluntary contraction. This is followed over the next 30 to 40 minutes by a progressive decline in CMAP amplitude to well below the pre-exercise baseline. Patients with hyperkalemic PP, on average, had much greater amplitude increments and decrements than patients with hypokalemic PP, but there was overlap between individual patients.7,8
Abortive Attacks May Be More Disabling Than Paralytic Attacks
Many patients describe weakness following effort which limits their daily activity. This effect seems to increase with age and while the weakness may not progress to paralysis it tends to linger for a longer period of time. This lack of outright paralysis has led some researchers to conclude that attacks disappear with age. It is probably more accurate to say that attacks change in character. Abortive attacks which linger for days, weeks or even months on end are often more disabling than brief paralytic ones.
Abortive Attacks are Amenable to Treatment
The distinguishing factor between the Abortive Attack and permanent muscle weakness is that Abortive Attacks are amenable to treatment, where permanent muscle weakness is not. If a patient is complaining of fatigue and weakness between episodes of paralysis, or attacks of paralysis have been replaced by fatigue and fluctuating weakness consider more aggressive management strategies, i.e. more control of diet and appropriate therapies.
1. Links, T.P.; Haren, D.J.; Niemejijer – III, Familial Hypokalemic Periodic Paralysis Ed. 1992 ISBN 90-9005053-1 Chapter 2, page 13
2. ibid; Chapter 3, pp 49-52 3. ibid; Chapter 4.
3. page 105
4. Brooke M.H. Disorders of Skeletal Muscle. Neurology in Clinical Practice, Third Edition, Bradley, W.G.; Daroff, R.B.; Fenichel G.M. & Marsden, C.D. Eds. Boston, MA; Butterworth/Heinemann. 2000
5. van der Hoeven JH, Links TP, Zwarts MJ, van Weerden TW; Muscle fiber conduction velocity in the diagnosis of familial hypokalemic periodic paralysis–invasive versus surface determination. Muscle Nerve 1994 Aug; 17 (8): 898-905; PMID: 8041397
6. Brouwer OF, Zwarts MJ, Links TP, Wintzen AR; Muscle fiber conduction velocity in the diagnosis of sporadic hypokalemic periodic paralysis. Clin Neurol Neurosurg 1992; 94 (2):149-151; PMID: 1324813
7. Katz JS, Wolfe GI, Iannaccone S, Bryan WW, Barohn RJ. The exercise test in Andersen syndrome. Arch Neurol. 1999 Mar;56(3): 352-6. PMID: 10190827
8. McManis PG; Lambert EH; Daube JR; The exercise test in periodic paralysis. Muscle Nerve, 1986 Oct, 9:8, 704-10 PMID: 3785281
reviewed and updated 30 Sept, 2020