The risk for untreated LQTS patients having events (syncopes or cardiac arrest) can be predicted from their genotype (LQT1-8), gender, and corrected QT interval.

- High risk (> 50%) - QTc > 500 ms, LQT1, LQT2, and LQT3 (males)

- Intermediate risk (30-50%) - QTc > 500 ms, LQT3 (females) or QTc < 500 ms, LQT2 (females) and LQT3

- Low risk (< 30%) - QTc < 500 ms, LQT1 and LQT2 (males)

A 1992 study reported that mortality for symptomatic, untreated patients was 20% within the first year and 50% within the first 10 years after the initial syncope.

The diagnosis of LQTS is not easy since 2.5% of the healthy population has prolonged QT interval, and 10–15% of LQTS patients have a normal QT interval. A commonly used criterion to diagnose LQTS is the LQTS "diagnostic score", calculated by assigning different points to various criteria (listed below). With four or more points, the probability is high for LQTS; with one point or less, the probability is low. A score of two or three points indicates intermediate probability.

- QTc (Defined as QT interval / square root of RR interval)

- ≥ 480 ms - 3 points

- 460-470 ms - 2 points

- 450 ms and male gender - 1 point

- "Torsades de pointes" ventricular tachycardia - 2 points

- T wave alternans - 1 point

- Notched T wave in at least 3 leads - 1 point

- Low heart rate for age (children) - 0.5 points

- Syncope (one cannot receive points both for syncope and "torsades de pointes")

- With stress - 2 points

- Without stress - 1 point

- Congenital deafness - 0.5 points

- Family history (the same family member cannot be counted for LQTS and sudden death)

- Other family members with definite LQTS - 1 point

- Sudden death in immediate family members (before age 30) - 0.5 points

Andersen–Tawil syndrome, also called Andersen syndrome and Long QT syndrome 7, is a form of long QT syndrome. It is a rare genetic disorder, and is inherited in an autosomal dominant pattern and predisposes patients to cardiac arrhythmias. Jervell and Lange-Nielsen Syndrome is a similar disorder which is also associated with sensorineural hearing loss. It was first described by Ellen Damgaard Andersen.

This condition is incredibly rare, with only 100 cases reported worldwide, however there are thought to be many cases that have been left undiagnosed. It is either inherited from at least one parent containing the mutated gene. or it can be gained through the mutation of the KCNJ2 gene.

Some cases of myotonia congenita do not require treatment, or it is determined that the risks of the medication outweigh the benefits. If necessary, however, symptoms of the disorder may be relieved with quinine, phenytoin, carbamazepine, mexiletine and other anticonvulsant drugs. Physical therapy and other rehabilitative measures may also be used to help muscle function. Genetic counseling is available.

Diagnosis of paramyotonia congenita is made upon evaluation of patient symptoms and case history. Myotonia must increase with exercise or movement and usually must worsen in cold temperatures. Patients that present with permanent weakness are normally not characterized as having PC. Electromyography may be used to distinguish between paramyotonia congenita and myotonia congenita. Clinicians may also attempt to provoke episodes or myotonia and weakness/paralysis in patients in order to determine whether the patient has PC, hyperkalemic periodic paralysis, or one of the potassium-aggravated myotonias. Genomic sequencing of the SCN4A gene is the definitive diagnostic determinant.

In northern Scandinavia, the prevalence of myotonia congenita has been estimated at 1:10,000.

Myotonia congenita is estimated to affect 1 in 1,000,000 people worldwide.

Some horse organizations have instituted rules to attempt to eliminate this widespread disease. The American Quarter Horse Association (AQHA) mandates testing for foals descended from Impressive if both of the foal's parents were not homozygous negative (N/N) for the gene, and, since 2007, has not registered foals homozygous (H/H) for the gene. Since 2007, the Appaloosa Horse Club (ApHC) has required foals descended from Impressive to be tested, so that the results may be recorded on its certificate. The American Paint Horse Association (APHA) mandated that, after 2017, stallions must be tested for HYPP so that mare owners may make an informed decision before choosing a stallion for breeding to their mare.

Diagnosis can be achieved through a specialized form of electromyographic (EMG) testing called the long exercise test. This test measures the amplitude of a nerve response (called the Compound Muscle Action Potential or CMAP) for 40 to 50 minutes following a few minutes of exercise. In affected patients, there is a progressive fall in the amplitude of the potential. Besides the patient history or a report of serum potassium low normal or low during an attack, the long exercise test is the current standard for medical testing. Genetic diagnosis is often unreliable as only a few of the more common gene locations are tested, but even with more extensive testing 20–37% of people with a clinical diagnosis of hypokalemic periodic paralysis have no known mutation in the two known genes. Standard EMG testing cannot diagnose a patient unless they are in a full blown attack at the time of testing. Provoking an attack with exercise and diet then trying oral potassium can be diagnostic, but also dangerous as this form of PP has an alternate form known as hyperkalemic periodic paralysis. The symptoms are almost the same, but the treatment is different. The old glucose insulin challenge is dangerous and risky to the point of being life-threatening and should never be done when other options are so readily available.

People with hypokalemic periodic paralysis are often misdiagnosed as having a conversion disorder or hysterical paralysis since the weakness is muscle-based and doesn't correspond to nerve or spinal root distributions. The tendency of people with hypokalemic periodic paralysis to get paralyzed when epinephrine is released in "fight or flight" situations further adds to the temptation to misdiagnose the disorder as psychiatric.

This disease is unusually difficult to diagnose. Patients often report years of wrong diagnosis and treatments that made them worse instead of better. Part of this may be that migraines are present in up to 50% of patients and can cause a confusing array of symptoms including headaches, speech difficulties and visual, auditory or sensory auras. DNA testing is available for only a half dozen common gene mutations, while dozens of known mutations are possible but are not routinely tested. Electromyography (EMG) findings are not specific but the McManis Protocol, also called the Compound Muscle Amplitude Potential test (CMAP)can be used by a skilled neurologist capable of utilizing the EMG, which can give assistance in diagnosing several of these PP disorders. The old glucose/insulin provocative testing can cause life-threatening symptoms and should not be used.

Also of note is that potassium levels do "not" have to range outside of normal limits to cause serious, even life-threatening paralysis. These diseases are "not" the same as having a very low level of potassium (hypokalemia) or high potassium (hyperkalemia) and must not be treated as such. The total body store of potassium is usually normal; it is just in the wrong place.

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.

Some patients do not require treatment to manage the symptoms of paramyotonia congenita. Avoidance of myotonia triggering events is also an effective method of mytonia prevention.

In 1994, researchers at the University of Pittsburgh, with a grant from horse organizations, isolated the genetic mutation responsible for the problem and developed a blood test for it. Using this test, horses may be identified as:

- H/H, meaning they have the mutation and it is homozygous. These horses always pass on the disease.

- N/H, meaning they have the mutation and it is heterozygous. These horses are affected to a lesser degree and pass on the disease 50% of the time.

- N/N, meaning they do not have the mutation and cannot pass it on, even if they are descendants of Impressive.

In the case of the horse Impressive, the muscles were always contracting which was equivalent to a constant work-out. Thus the development of an "impressive" musculature.

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.

The diagnosis of TRAPS may show an increased IgD level in a possibly affected individual, other methods to ascertain a definite finding is via the following:

- Blood test

- Genetic test

- Clinical evaluation

Even in syndromes with no known etiology, the presence of the associated symptoms with a statistically improbable correlation, normally leads the researchers to hypothesize that there exists an unknown underlying cause for all the described symptoms.

In medicine a broad definition of syndrome is used, which describes a collection of symptoms and findings without necessarily tying them to a single identifiable pathogenesis. The more specific definition employed in medical genetics describes a subset of all medical syndromes.

The types in the following table are commonly accepted. Channelopathies currently under research, like Kir4.1 potassium channel in multiple sclerosis, are not included.

The prognosis for periodic paralysis varies. Overactivity, a diet that is not low in sodium and carbohydrates, or simply an unfortunate gene mutation can lead to a type of chronic, low level weakness called an "abortive attack," or to permanent muscle damage. Abortive attacks often respond to extra potassium, cutting carbohydrates, getting plenty of rest, increasing doses of medication and gentle daily exercise such as short walks. Permanent muscle weakness is just what it sounds like: Permanent, irreparable damage to the muscles and associated weakness. Vacuoles and tubular aggregates form in and destroy healthy muscle tissue. This type of damage can typically be observed via a muscle biopsy. Not even anabolic steroids can repair this type of muscular damage.

Life span is expected to be normal, but attacks can drop potassium to levels low enough to cause life-threatening breathing problems or heart arrhythmia. Patients often report muscle pain and cognitive problems during attacks. Migraines occur in up to 50% of all hypokalemic periodic paralysis patients and may include less common symptoms like phantom smells, sensitivity to light and sound or loss of words. Medical literatures states that muscle strength is normal between attacks, but patients often report that their baseline strength is in fact lower than that of healthy individuals.

Because there are dozens of possible gene mutations, some drugs and treatments that work fine for one patient will not work for another. For example, most patients do well on acetazolamide, but some don't. Some patients will do well with extra magnesium (the body's natural ion channel blocker) or fish oil, while these same nutrients will make other patients worse. Patients and caregivers should take extreme caution with all new drugs and treatment plans.

Treatment of the periodic paralyses may include carbonic anhydrase inhibitors (such as acetazolamide, methazolamide or dichlorphenamide), taking supplemental oral potassium chloride and a potassium-sparing diuretic (for hypos) or avoiding potassium (for hypers), thiazide diuretics to increase the amount of potassium excreted by the kidneys (for hypers), and significant lifestyle changes including tightly controlled levels of exercise or activity. However, treatment should be tailored to the particular type of periodic paralysis.

Treatment of periodic paralysis in Andersen-Tawil syndrome is similar to that for other types. However, pacemaker insertion or an implantable cardioverter-defibrillator may be required to control cardiac symptoms.

Channelopathies are diseases caused by disturbed function of ion channel subunits or the proteins that regulate them. These diseases may be either congenital (often resulting from a mutation or mutations in the encoding genes) or acquired (often resulting from autoimmune attack on an ion channel).

There are a large number of distinct dysfunctions known to be caused by ion channel mutations. The genes for the construction of ion channels are highly conserved amongst mammals and one condition, hyperkalemic periodic paralysis, was first identified in the descendants of Impressive, a registered Quarter Horse (see AQHA website).

The channelopathies of human skeletal muscle include hyper- and hypokalemic (high and low potassium blood concentrations) periodic paralysis, myotonia congenita and paramyotonia congenita.

Channelopathies affecting synaptic function are a type of synaptopathy.

Several disorders may appear similar to CBPS and need to be distinguished in the process of diagnosing CBPS. These include pachygyria, double cortex syndrome, and lissencephaly, all of which are classified along with CBPS as neuronal migration disorders. Diagnostic tests for CBPS include electroencephalograms, CT scanning, and magnetic resonance imaging.

Hypokalemia (low blood potassium levels) commonly occurs during attacks; levels below 3.0 mmol/l are typically encountered. Magnesium and phosphate levels are often found to be decreased. Creatine kinase levels are elevated in two thirds of cases, usually due to a degree of muscle injury; severe elevations suggestive of rhabdomyolysis (muscle tissue destruction) are rare. Electrocardiography (ECG/EKG) may show tachycardia (a fast heart rate) due to the thyroid disease, abnormalities due to cardiac arrhythmia (atrial fibrillation, ventricular tachycardia), and conduction changes associated with hypokalemia (U waves, QRS widening, QT prolongation, and T wave flattening). Electromyography shows changes similar to those encountered in myopathies (muscle diseases), with a reduced amplitude of the compound muscle action potentials (CMAPs); they resolve when treatment has commenced.

TPP is distinguished from other forms of periodic paralysis (especially hypokalemic periodic paralysis) with thyroid function tests on the blood. These are normal in the other forms, and in thyrotoxicosis the levels of thyroxine and triiodothyronine are elevated, with resultant suppression of TSH production by the pituitary gland. Various other investigations are usually performed to separate the different causes of hyperthyroidism.

In the acute phase of an attack, administration of potassium will quickly restore muscle strength and prevent complications. However, caution is advised as the total amount of potassium in the body is not decreased, and it is possible for potassium levels to overshoot ("rebound hyperkalemia"); slow infusions of potassium chloride are therefore recommended while other treatment is commenced.

The effects of excess thyroid hormone typically respond to the administration of a non-selective beta blocker, such as propranolol (as most of the symptoms are driven by increased levels of adrenaline and its effect on the β-adrenergic receptors). Subsequent attacks may be prevented by avoiding known precipitants, such as high salt or carbohydrate intake, until the thyroid disease has been adequately treated.

Treatment of the thyroid disease usually leads to resolution of the paralytic attacks. Depending on the nature of the disease, the treatment may consist of thyrostatics (drugs that reduce production of thyroid hormone), radioiodine, or occasionally thyroid surgery.

Avellis syndrome is a neurological disorder characterized by a peculiar form of alternating paralysis. There is paralysis of the soft palate and vocal cords on one side and loss of pain sensation and temperature sense on the other side, including the extremities, trunk, and neck. It usually results from occlusion of the vertebral artery in lesions of the nucleus ambiguous and pyramidal tract. Horner's syndrome may be associated. In the original description, the vagus and glossopharyngeal nerves were involved; concomitant involvement of the neighbouring cranial nerves was observed later.