skeletal muscle channelopathies ( smscs ) are rare skeletal muscle disorders associated with altered sarcolemal excitability secondary to mutations in genes coding for voltage - gated ion channels .
smscs are caused by mutations in scn4a gene on chromosome 17q , which codes for the  subunit of human skeletal muscle sodium channel .
genetic mutations in scn4a can be easily predicted if clinico - electromyographic phenotypes are typical .
. we present a teenage boy , who had a combination of features of hyperkalemic periodic paralysis ( hyperpp ) , pc , myotonia congenita ( mc ) , and sodium channel myotonia .
the case report highlights the importance of recognizing unusual clinico - electromyographic phenotypes , which can help guide focused genetic testing .
a 14-year - old asian boy presented to pediatric neurology with 4-year history of episodes of muscle stiffness and weakness in lower limbs , occurring following sitting ( resting ) for nearly 1 h after exercise .
the muscle stiffness episode would improve following shaking of legs (  warm - up effect  ) .
there was no history of feeding or breathing difficulties in the neonatal period , and his weaning period was uneventful .
family history was remarkable for his father having episodes of muscle stiffness ; however , no specific diagnosis was assigned .
there was no evidence of calf muscle hypertrophy or limb weakness ( proximal or distal ) .
tone , power , and deep tendon reflexes were normal in all the four limbs .
creatine kinase ( ck ) was elevated at 1024 iu / l ( 25 - 200 )
there was evidence of muscle membrane irritability with myotonia , which was highly suggestive of a myotonic disorder .
as a result , it was decided to arrange for short exercise testing as part of the myotonia protocol . at room temperature
there was mild post - exercise potentiation for about half a minute , followed by recovery toward normal .
cold exposure produced reduction in muscle action potential amplitude to about 50% accompanied by severe clinical weakness .
it also exposed a post - exercise drop in amplitude with slight recovery , and a subsequent progressive further decrease in the action potential with serial exercises [ figure 2 ] , which was consistent with features of paramyotonia congenita ( pc ) ( fournier pattern i ) .
at this point the history and emg suggested a myotonic disorder , and consequently the long exercise tests were not carried out .
2111c > t ; p.thr704met in the coding regions ( c. 2020 - 2376 ) and ( c. 4571 - 4838 ) of sodium channel type iv  subunit ( scn4a ) .
emg showing irritability and myopathic features of short duration , low - amplitude polyphasia , which can be seen in hyperpp at room temperature there was mild post - exercise cmap potentiation followed by recovery toward normal .
post - cooling reduction of muscle action potential amplitude to about 50% , followed by a further post - exercise drop in amplitude with slight recovery and subsequent lower action potentials with serial exercises
hyperpp patients present with episodes of muscle weakness triggered by post - exercise rest , potassium - rich food , cold environment , emotional stress , and fasting .
weakness episodes are often interspersed with mild myotonia , which does not interfere with voluntary movements .
pc , autosomal dominant in inheritance , presents in the 1 decade of life with episodes of cold and exercise induced myotonia affecting muscles of face , tongue , and hands predominantly .
paralysis periodica paramyotonica , a myotonic form of hyperpp , represents a mix of clinical features of hyperpp and pc .
sodium channel myotonias generally present with isolated myotonia , and some of the pure sodium channel myotonias can manifest  warm - up effect ,  which is typically seen in mc . while myotonia sets in after inactivity in mc , it develops soon after exercise in pc and 10 - 30 min post - exercise in sodium channel myotonias . based on the distinct electromyographic patterns , fournier et al
repetitive short ( 10 - 20 s ) or long ( 3 - 5 min ) exercise tests , at room temperature and on cooling , generate five emg patterns : i , ii , and iii for non - dystrophic myotonias and iv and v for periodic paralyses .
hyperpp shows pattern iv , which is characterized by post - exercise increment in compound muscle action potential ( cmap ) with further improvement after serial exercise testing .
needle emg , however , does not reveal myotonic discharges in most of the cases .
pc , on the other hand , produces pattern i , which is characterized by post - exercise gradual cmap decrement , with progressive further decrements following exercise .
most of sodium channel myotonias manifest emg pattern iii , in which cmap remains unchanged following exercise with or without cooling .
pc , which shows emg pattern i both at room temperature and on cooling , is linked to scn4a : t1313 m , r1448c , or r1448h , whereas the pc , which shows emg pattern i on cooling only , is linked to a rare mutation in scn4a gene : q270k .
our case presented with episodic weakness and myotonia , mainly involving lower limbs , occurring after resting for up to 1 h , which improved after repetitive exercise reflecting  warm - up effect .
 distribution of myotonia ( lower limbs > upper limbs ) was not typical of pc .
differential diagnosis at this stage included mc and hyperpp . a negative clcn1 gene mutation , however , made the diagnosis of mc less likely .
some of the rare phenotypes of sodium channel myotonias , which are allelic to hyperpp , pc , can manifest warm - up effect and cold sensitivity , as seen in our case .
our case , therefore , clearly represents an overlapping clinic - electromyographic phenotype encompassing hyperpp , pc , and sodium channel myotonia , which can be explained on the basis of their common allelic basis . genetic mutation , scn4a : c. 2111c > t ; p.thr704met found in our patient , was originally reported in hyperpp .
kim et al . reported phenotypic variation of thr704met in a family with paralysis periodica paramyotonia , a type of hyperpp .
our case represents a further phenotypic variation with features of paralysis periodica paramyotonica with  warm - up effect  , which is typically seen in mc and some sodium channel myotonias , and exemplifies phenotypic heterogeneity of gene expression in smsc .
recognition of such atypical phenotypes encompassing features of hyperpp and pc with  warm - up effect  can enable clinicians carry out focused genetic analysis for thr704met mutation in scn4a gene and help reduce investigation cost .
skeletal muscle channelopathies are hereditary disorders of muscle cell ion channels , which result in disturbance of sarcolemal excitability , leading to muscle dysfunction .
diverse genetic mutations can produce variable phenotypes . while typical clinico - electromyographic phenotypes can help guide genetic testing
our case epitomizes phenotypic complexity stemming from genetic intricacies , which can result in diagnostic difficulties .
clinicians dealing with neuromuscular disorders in children need to be aware of the rare clinical presentations of smsc , such as paralysis periodica paramyotonica with  warm - up effect  , so that focused genetic testing can be carried out .
 skeletal muscle sodium channelopathies ( smscs ) including hyperkalemic periodic paralysis ( hyperpp ) , paramyotonia congenita ( pc ) , and sodium channel myotonia are caused by sodium channel gene ( scn4a ) mutations , with altered sarcolemal excitability , and can present as episodes of skeletal muscle weakness , paralysis , and myotonia . 
 we report a teenage boy , who presented with features of hyperpp , pc , myotonia congenita , and sodium channel myotonia . 
 his electromyography ( emg ) revealed myopathic changes , myotonia , and fournier emg pattern i , and posed a diagnostic challenge . 
 genetic analysis showed thr704met mutation in scn4a gene . while with typical clinical phenotypes 
 , the electromyographic patterns can be used to direct genetic testing , atypical phenotypes may pose diagnostic dilemmas . 
 clinicians dealing with neuromuscular disorders in children need to be aware of the unusual clinical presentations of smsc , so that focused genetic testing can be carried out . 
Introduction
Case Report
Discussion
Conclusion