From GeneReviews Overview
Long QT syndrome (LQTS) is a cardiac electrophysiologic disorder, characterized by QT prolongation and T-wave abnormalities on the EKG that are associated with tachyarrhythmias, typically the ventricular tachycardia torsade de pointes (TdP). TdP is usually self-terminating, thus causing a syncopal event, the most common symptom in individuals with LQTS. Such cardiac events typically occur during exercise and emotional stress, less frequently during sleep, and usually without warning. In some instances, TdP degenerates to ventricular fibrillation and causes aborted cardiac arrest (if the individual is defibrillated) or sudden death. Approximately 50% of untreated individuals with a pathogenic variant in one of the genes associated with LQTS have symptoms, usually one to a few syncopal events. While cardiac events may occur from infancy through middle age, they are most common from the preteen years through the 20s. Some types of LQTS are associated with a phenotype extending beyond cardiac arrhythmia. In addition to the prolonged QT interval, associations include muscle weakness and facial dysmorphism in Andersen-Tawil syndrome (LQTS type 7); hand/foot, facial, and neurodevelopmental features in Timothy syndrome (LQTS type 8); and profound sensorineural hearing loss in Jervell and Lange-Nielson syndrome.

From OMIM
Andersen cardiodysrhythmic periodic paralysis, also known as Andersen syndrome or Andersen-Tawil syndrome, is an autosomal dominant multisystem channelopathy characterized by periodic paralysis, ventricular arrhythmias, and distinctive dysmorphic facial or skeletal features. Hypoplastic kidney and valvular heart disease have also been reported. The disorder shows marked intrafamilial variability and incomplete penetrance (summary by Davies et al., 2005).  http://www.omim.org/entry/170390

From MedlinePlus Genetics
Andersen-Tawil syndrome is a disorder that causes episodes of muscle weakness (periodic paralysis), changes in heart rhythm (arrhythmia), and developmental abnormalities. Periodic paralysis begins early in life, and episodes last from hours to days. These episodes may occur after exercise or long periods of rest, but they often have no obvious trigger. Muscle strength usually returns to normal between episodes. However, mild muscle weakness may eventually become permanent.

In people with Andersen-Tawil syndrome, the most common changes affecting the heart are ventricular arrhythmia, which is a disruption in the rhythm of the heart's lower chambers (the ventricles), and long QT syndrome. Long QT syndrome is a heart condition that causes the heart (cardiac) muscle to take longer than usual to recharge between beats. The irregular heartbeats can lead to discomfort, such as the feeling that the heart is skipping beats (palpitations). Uncommonly, the irregular heartbeats can cause fainting (syncope), and even more rarely, sudden death.

Physical abnormalities associated with Andersen-Tawil syndrome typically affect the face, other parts of the head, and the limbs. These features often include a very small lower jaw (micrognathia), dental abnormalities (such as crowded teeth), low-set ears, widely spaced eyes, fusion (syndactyly) of the second and third toes, and unusual curving of the fingers or toes (clinodactyly). Some affected people also have short stature and an abnormal side-to-side curvature of the spine (scoliosis).

The signs and symptoms of Andersen-Tawil syndrome vary widely, and they can be different even among affected members of the same family. About 60 percent of affected individuals have all three major features (periodic paralysis, cardiac arrhythmia, and physical abnormalities).  https://medlineplus.gov/genetics/condition/andersen-tawil-syndrome