Diagnosis depends on the clinical scenario. However, karyotyping is an essential test for diagnosis.

Syndactyly and other deformities are typically observed and diagnosed at birth. Long QT syndrome sometimes presents itself as a complication due to surgery to correct syndactyly. Other times, children collapse spontaneously while playing. In all cases it is confirmed with ECG measurements. Sequencing of the CACNA1C gene further confirms the diagnosis.

Genetic testing for Brugada syndrome is clinically available and may help confirm a diagnosis, as well as differentiate between relatives who are at risk for the disease and those who are not. Some symptoms when pinpointing this disease include fainting, irregular heartbeats, and chaotic heartbeats. However, just detecting the irregular heartbeat may be a sign of another disease, so the doctor must detect another symptom as well.

In some cases, the disease can be detected by observing characteristic patterns on an electrocardiogram. These patterns may be present all the time, they might be elicited by the administration of particular drugs (e.g., Class IA, such as ajmaline or procainamide, or class 1C, such as flecainide or pilsicainide, antiarrhythmic drugs that block sodium channels and cause appearance of ECG abnormalities), or they might resurface spontaneously due to as-yet unclarified triggers.

Brugada syndrome has three different ECG patterns:

- Type 1 has a coved type ST elevation with at least 2 mm (0.2 mV) J-point elevation and a gradually descending ST segment followed by a negative T-wave.

- Type 2 has a saddle-back pattern with a least 2 mm J-point elevation and at least 1 mm ST elevation with a positive or biphasic T-wave. Type 2 pattern can occasionally be seen in healthy subjects.

- Type 3 has either a coved (type 1 like) or a saddle-back (type 2 like) pattern, with less than 2 mm J-point elevation and less than 1 mm ST elevation. Type 3 pattern is not rare in healthy subjects.

The pattern seen on the ECG is persistent ST elevations in the electrocardiographic leads V-V with a right bundle branch block (RBBB) appearance, with or without the terminal S waves in the lateral leads that are associated with a typical RBBB. A prolongation of the PR interval (a conduction disturbance in the heart) is also frequently seen. The ECG can fluctuate over time, depending on the autonomic balance and the administration of antiarrhythmic drugs. Adrenergic stimulation decreases the ST segment elevation, while vagal stimulation worsens it. (There is a case report of a patient who died while shaving, presumed due to the vagal stimulation of the carotid sinus massage.)

The administration of class Ia, Ic, and III drugs increases the ST segment elevation, as does fever. Exercise decreases ST segment elevation in some people, but increases it in others (after exercise, when the body temperature has risen). The changes in heart rate induced by atrial pacing are accompanied by changes in the degree of ST segment elevation. When the heart rate decreases, the ST segment elevation increases, and when the heart rate increases, the ST segment elevation decreases. However, the contrary can also be observed.

Surgery is typically used to correct structural heart defects and syndactyly. Propanolol or beta-adrenergic blockers are often prescribed as well as insertion of a pacemaker to maintain proper heart rhythm. With the characterization of Timothy syndrome mutations indicating that they cause defects in calcium currents, it has been suggested that calcium channel blockers may be effective as a therapeutic agent.

It is associated with LAMP2. The status of this condition as a GSD has been disputed.

It is named for Mary Holt and Samuel Oram, who published a paper on it in 1960.

Danon disease was characterized by Moris Danon in 1981. Dr. Danon first described the disease in 2 boys with heart and skeletal muscle disease (muscle weakness), and intellectual disability.

The first case of Danon disease reported in the Middle East was a family diagnosed in the eastern region of United Arab Emirates with a new "LAMP2" mutation; discovered by the Egyptian cardiologist Dr. Mahmoud Ramadan the associate professor of Cardiology in Mansoura University (Egypt) after doing genetic analysis for all the family members in Bergamo, Italy where 6 males were diagnosed as Danon disease patients and 5 female were diagnosed as carriers; as published in "Al-Bayan" newspaper in 20 February 2016 making this family the largest one with patients and carriers of Danon disease.

Danon Disease has overlapping symptoms with another rare genetic condition called 'Pompe' disease. Microscopically, muscles from Danon Disease patients appear similar to muscles from Pompe disease patients. However, intellectual disability is rarely, if ever, a symptom of Pompe disease. Negative enzymatic or molecular genetic testing for Pompe disease can help rule out this disorder as a differential diagnosis.

Stokes-Adams attacks can be precipitated by this condition. These involve a temporary loss of consciousness resulting from marked slowing of the heart when the atrial impulse is no longer conducted to the ventricles. This should not be confused with the catastrophic loss of heartbeat seen with ventricular fibrillation or asystole.

It was described independently by Maurice Lev and Jean Lenègre in 1964, but the condition is generally called after Lev.

Studies have shown that patients with Pacemaker syndrome and/or with sick sinus syndrome are at higher risk of developing fatal complications that calls for the patients to be carefully monitored in the ICU. Complications include atrial fibrillation, thrombo-embolic events, and heart failure.

If undiagnosed (or untreated), Stokes–Adams attacks have a 50% mortality within a year of the first episode. The prognosis following treatment is very good.

At the time of pacemaker implantation, AV synchrony should be optimized to prevent the occurrence of pacemaker syndrome. Where patients with optimized AV synchrony have shown great results of implantation and very low incidence of pacemaker syndrome than those with suboptimal AV synchronization.

Initial treatment can be medical, involving the use of drugs like isoprenaline (Isuprel) and epinephrine (adrenaline). Definitive treatment is surgical, involving the insertion of a pacemaker – most likely one with sequential pacing such as a DDI mode as opposed to the older VVI mechanisms, and the doctor may arrange the patient to undergo electrocardiography to confirm this type of treatment.

If untreated, this abnormal heart rhythm can lead to dizziness, chest pain, a sensation of fluttering or pounding in the chest (palpitations), shortness of breath, or fainting (syncope). Atrial fibrillation also increases the risk of stroke. Complications of familial atrial fibrillation can occur at any age, although some people with this heart condition never experience any health problems associated with the disorder.

Atrial fibrillation is the most common type of sustained abnormal heart rhythm (arrhythmia), affecting more than 3 million people in the United States. The risk of developing this irregular heart rhythm increases with age. The incidence of the familial form of atrial fibrillation is unknown; however, recent studies suggest that up to 30 percent of all people with atrial fibrillation may have a history of the condition in their family.

Individuals with LGL syndrome do not carry an increased risk of sudden death. The only morbidity associated with the syndrome is the occurrence of paroxysmal episodes of tachycardia which may be of several types, including sinus tachycardia, supraventricular tachycardia, atrial fibrillation, atrial flutter, or even ventricular tachycardia.

A doctor will listen to the heart with stethoscope. A "tumor plop" (a sound related to movement of the tumor), abnormal heart sounds, or a murmur similar to the mid-diastolic rumble of mitral stenosis may be heard. These sounds may change when the patient changes position.

Right atrial myxomas rarely produce symptoms until they have grown to be at least 13 cm (about 5 inches) wide.

Tests may include:

- Echocardiogram and Doppler study

- Chest x-ray

- CT scan of chest

- Heart MRI

- Left heart angiography

- Right heart angiography

- ECG—may show atrial fibrillation

Blood tests:

A FBC may show anemia and increased WBCs (white blood cells). The erythrocyte sedimentation rate (ESR) is usually increased.

Clinically, it is often asymptomatic by itself and considered benign in nature.

Although not based on a human clinical trial, the only currently accepted disease-modifying therapeutic strategy available for familial amyloid cardiomyopathy is a combined liver and heart transplant. Treatments aimed at symptom relief are available, and include diuretics, pacemakers, and arrhythmia management. Thus, Senile systemic amyloidosis and familial amyloid polyneuropathy are often treatable diseases that are misdiagnosed.

In 2013, the European Medicines Agency approved the drug tafamidis (Vyndaqel) to slow the progression of familial amyloid polyneuropathy, a related disease caused by TTR aggregation that first presents as an autonomic and/or peripheral neuropathy (later progressing to a cardiomyopathy).

LGL syndrome is diagnosed on the basis of the surface EKG in a symptomatic individual with a PR interval less than or equal to 0.12 second (120 ms) with normal QRS complex configuration and duration. It can be distinguished from WPW syndrome because the delta waves seen in WPW syndrome are not seen in LGL syndrome. It is a clinical diagnosis that came about before the advent of electrophysiology studies. Be aware, however, that not all WPW EKG's have a delta wave; the absence of a delta wave does not conclusively rule out WPW.

Ambulatory monitoring of the electrocardiogram (ECG) may be necessary because arrhythmias are transient. The ECG may show any of the following:

- Inappropriate sinus bradycardia

- Sinus arrest

- Sinoatrial block

- Tachy-Brady Syndrome

- Atrial fibrillation with slow ventricular response

- A prolonged asystolic period after a period of tachycardias

- Atrial flutter

- Ectopic atrial tachycardia

- Sinus node reentrant tachycardia

- Wolff-Parkinson-White syndrome

Electrophysiologic tests are no longer used for diagnostic purposes because of their low specificity and sensitivity. Cardioinhibitory and vasodepressor forms of sick sinus syndrome may be revealed by tilt table testing.

The level of digoxin for treatment is typically 0.5-2 ng/mL. Since this is a narrow therapeutic index, digoxin overdose can happen. A serum digoxin concentration of 0.5-0.9 ng/mL among those with heart failure is associated with reduced heart failure deaths and hospitalizations. It is therefore recommended that digoxin concentration be maintained in approximately this range if it is used in heart failure patients.

High amounts of the electrolyte potassium (K+) in the blood (hyperkalemia) is characteristic of digoxin toxicity. Digoxin toxicity increases in individuals who have kidney impairment. This is most often seen in elderly or those with chronic renal insufficiency or end-stage kidney disease.

HFpEF is typically diagnosed with echocardiography. Techniques such as catheterization are invasive procedures and thus reserved for patients with co-morbid conditions or those who are suspected to have HFpEF but lack clear non-invasive findings. Catheterization does represent are more definitive diagnostic assessment as pressure and volume measurements are taken simultaneously and directly. In either technique the heart is evaluated for left ventricular diastolic function. Important parameters include, rate of isovolumic relaxation, rate of ventricular filling, and stiffness.

Frequently patients are subjected to stress echocardiography, which involves the above assessment of diastolic function during exercise. This is undertaken because perturbations in diastole are exaggerated during the increased demands of exercise. Exercise requires increased left ventricular filling and subsequent output. Typically the heart responds by increasing heart rate and relaxation time. However, in patients with HFpEF both responses are diminished due to increased ventricular stiffness. Testing during this demanding state may reveal abnormalities that are not as discernible at rest.

Ashman phenomenon, also known as Ashman beats, describes a particular type of wide QRS complex, often seen isolated that is typically seen in atrial fibrillation. It is more often misinterpreted as a premature ventricular complex.

It is named for Richard Ashman (of New Orleans) (1890 –1969), after first being described by Gouaux and Ashman in 1947.

Although its cause is poorly understood, situs ambiguous has been linked to family history of malformations and maternal cocaine use, suggesting both genetic and environmental factors play a role. Several genes in the TGF-beta pathway, which controls left-right patterning of viseral organs across the body axis, have been indicated in sporadic and familial cases of atrial isomerism.

There does not appear to be a screening method for prevention of heterotaxy syndrome. However, genetic testing in family members that display atrial isomerism or other cardiac malformations may help to discern risk for additional family members, especially in X-linked causes of heterotaxy syndrome.