JLNS, like other forms of long QT syndrome, causes the cardiac muscle to take longer than usual to recharge between beats.

Those affected have a prolonged QT interval on an electrocardiogram and are at risk of abnormal heart rhythms (arrhythmias), which can cause dizziness, blackouts, or seizures.

[5] JLNS is an autosomal recessive disorder meaning that two copies of the genetic mutation are required to produce the full syndrome.

These changes disrupt the flow of potassium ions in the inner ear and in cardiac muscle, leading to the hearing loss and irregular heart rhythm characteristic of Jervell and Lange-Nielsen syndrome.

[7] The cardiac features of JLNS can be diagnosed by measuring the QT interval corrected for heart rate (QTc) on a 12-lead electrocardiogram (ECG).

[9] Due to the higher risk of arrhythmias associated with JLNS than other forms of long QT syndrome, a defibrillator may be considered even in those without any symptoms.

[9] In those who experience recurrent arrhythmias despite medical therapy, a surgical procedure called sympathetic denervation can be used to interrupt the nerves that stimulate the heart.

[10] Although this risk is dependent on the underlying genetic defect and degree of QT prolongation, without treatment more than 50% of those affected will die before the age of 15.