Myotonic muscle fibers may have normal appearance, however, slight myopathic changes with increased occurrence of central nuclei and pathological variation of fiber diameter may be found. Both hypertrophy of type 2A fibers and absence of type 2B fibers can be identified (Jurkat-Rott and Lehmann-Horn 2013) (Fig. 2).

In both types of familial periodic paralysis, dilations of components of the T-tubular system and the sarcoplasmic reticulum occur. Dilation, proliferation, and regeneration result in vacuolization (Engel 1970). Contractions of nearby myofibrils and focal increases in muscle glycogen have also been noted, suggesting that the changes in several organelles accounted for the permanent myopathy of the disease. In otherwise unaffected fibers, collections of multiple closely packed tubules giving a honeycomb appearance may be viewed in cross sections, mostly at the end of fiber. These tubular aggregates are located between longitudinally running myofibrils or beneath the sarcolemma and may contain an internal circular membrane that is not normally seen in the T-tubule or the sarcoplasmic reticulum from which they originate (Jurkat-Rott and Lehmann-Horn 2013; Engel 1970).

Myotonia congenita and PAM patients do not generally present with pathological imaging findings. In potassium-aggravated myotonia, normal T1- and T2-weighted MR images of the lower legs were observed in a study of six patients (median age, 43 years) (Weber et al. 2006a). In a recent study, three severely affected RMC patients were examined using a multisequence (T1-weighted, T2-weighted, fat suppressed T2-weighted sequences) whole-body 3 Tesla MRI (Kornblum et al. 2010). In agreement with (Weber et al. 2006a), none of the patients showed skeletal muscle signal changes indicative of fatty muscle degeneration or edema. However, two patients showed muscle bulk hypertrophy of thighs and calves in line with the clinical appearance (Fig. 3) (Kornblum et al. 2010). The authors concluded that chloride channel myotonia and even serious and prolonged transient weakness alone do not result in MRI skeletal muscle signal changes, while MRI fatty skeletal muscle changes and edema that can be seen in myotonic dystrophy type 1 (Kornblum et al. 2006) are not the result of chloride channel dysfunction but clearly consequences of the dystrophic disease process. Therefore, imaging can be used to differentiate DMC and RMC from myotonic dystrophy in which there is reduced muscle mass and edema. Likewise, the prominent muscle bulks in myotonia congenita are due to genuine muscle hypertrophy and not to pseudo-hypertrophy as possible differentiation to other muscular dystrophies (Kornblum et al. 2010). In myotonic dystrophy type 1, a characteristic distribution of muscle involvement with frequent and early degeneration of the medial heads of gastrocnemius muscles, and a perifemoral semilunar pattern of quadriceps muscle affection sparing the rectus femoris has been reported using 3 Tesla MRI (Kornblum et al. 2006). The most frequently affected muscles in myotonic dystrophy type 1 were the medial heads of gastrocnemius, soleus, and vastus medialis muscles.

In PC, approximately 38 % of patients show normal muscle on T1-weighted and T2-weighted 1.5 Tesla MR imaging examinations of the lower legs (Weber et al. 2006b). In 12.5 % (two male R1448C patients of 25 and 42 years), a bilaterally symmetric homogeneous edema was observed at rest before any exercise or other provocation scheme that was confined to the medial head of the gastrocnemius muscle (Fig. 4) (Weber et al. 2006b). Edema-like muscular changes in the triceps surae muscles were evidenced in 10 % of PC patients (median age 45 years) in another cohort (Weber et al. 2006a). Up to 50 % of older patients (mean age 55 ± 13 years; mutation, R1448H in 1, T1313 M in 3, and R1448C in 4 patients) showed a bilaterally symmetric increased signal intensity of the medial head of the gastrocnemius muscle on T1- and T2-weighted images that was interpreted as a fatty infiltration. There was no distinct muscle atrophy in any of the so far studied 16 paramyotonia patients (Weber et al. 2006b). Highly specific for PC is ²³Na MRI which can visualize and monitor aspects of muscular pathophysiology, such as a disturbed muscular Na⁺ homeostasis. For example, cooling of the lower leg muscles of PC patients, opens non-inactivating Na⁺ channels and depolarizes the muscle fibers and thereby causes muscle stiffness followed by muscle weakness that lasts several hours even after immediate rewarming (Weber et al. 2006a). ²³Na MRI is able to depict a myoplasmic sodium accumulation (Fig. 5) which correlates well with the grade of paresis (Fig. 6) (Weber et al. 2006a) and is reproducible (Weber et al. 2006b). Therefore, ²³Na MRI is the method of choice for this disease. Na⁺ channel blockers such as mexiletine are able to prevent the myoplasmic sodium accumulation (Fig. 7) (Weber et al. 2006b).

In HyperPP, increased signal intensities on T2-weighted 1.5 Tesla ¹H MRI were visible in the triceps surae muscles of 43 % of the patients prior to any provocation (Fig. 8) (Weber et al. 2006a). In a more recent series using 3 Tesla MRI, eight of twelve HyperPP patients exhibited edema-like changes prior to any provocation on STIR images (Amarteifio et al. 2012). Seven of the twelve HyperPP patients showed fatty infiltration/degeneration, i.e., high signal intensities within their lower leg muscles on T1-weighted images (Amarteifio et al. 2012). HyperPP patients suffering from permanent weakness presented with a high degree of lipomatous changes compared to the group without permanent weakness and healthy volunteers (Amarteifio et al. 2012). A specific pattern of lipomatous changes was noticeable: the highest degree of fatty atrophy was observed in the triceps surae muscle (gastrocnemius followed by the soleus muscle). The peroneal muscles showed concomitantly with the lowest degree of edema-like changes also the lowest degree of fatty atrophy/degeneration (Fig. 9) (Amarteifio et al. 2012). In the hip muscles, the ischiocrural muscles are particularly affected.²³Na MRI displayed significantly higher intracellular muscular sodium signal intensities normalized to a reference phantom containing 51.3 mM Na⁺ in HyperPP patients with permanent weakness than in those without permanent weakness or healthy volunteers (0.83 vs. 0.67 vs. 0.53). Cooling induced or aggravated muscle weakness in the patients and further increased the sodium signal intensity significantly from 0.75 to 0.86 (Fig. 10). High sodium signal intensities were associated with an osmotic edema. In five patients with permanent weakness, treatment with acetazolamide or hydrochlorothiazide reduced the muscular ²³Na signal intensity from 0.85 to 0.64 and increased muscle strength (Amarteifio et al. 2012).