Table 1.

Clinical features of familial myotonic disorders

Conditions	Inheritance	Gene	Myotonia	Episodic weakness	Fixed weakness	Major trigger	Other features
DM
DM type 1	AD	DMPK	M	Absent	Distal limbs, face	None	Frontal balding, temporal wasting, cataracts, systemic disease
Myotonic dystrophy type 2 (PROMM)	AD	CNBP (ZNF9)	M	Absent*	Proximal limbs	None	Disabling and atypical pain, cataracts, milder systemic disease
NDM
MC
AD (Thomsen)	AD	CLCN1	M	Absent	Rare	Rest	Generalized muscle hypertrophy
AR (Becker)	AR	CLCN1	M	Absent*	Proximal LE	Rest	Muscle hypertrophy in LE
PMC	AD	SCN4A	P	Present in some	Proximal LE	Cold, exercise	Most sensitive to cold
Potassium-sensitive periodic paralysis	AD	SCN4A	P, M or absent	Present	Proximal LE	K+, rest after exercise	Potassium levels may be high***
PAM
Myotonia fluctuans	AD	SCN4A	M**	Absent	Absent	K+, exercise	Have good days and bad days
Myotonia permanens	AD	SCN4A	M**	Absent	Absent	K+, exercise	Continuous muscle stiffness
Acetazolamide-responsive myotonia	AD	SCN4A	M**	Absent	Absent	K+, exercise	Respond to therapy with acetazolamide

AD: Autosomal dominant; AR: Autosomal recessive; K+: Potassium, LE: Lower extremities; M: Myotonia; P: Paramyotonia; PROMM: Proximal myotonic myopathy; DM: Dystrophic myotonias

^*PROMM patients may initially have intermittent or transient weakness; recessive MC patients may have transient weakness after severe bouts of stiffness,

^** May have eyelid paramyotonia,

^***Potassium levels may be normal during attack normokalemic periodic paralysis (normoKPP).