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.

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.

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

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.

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.

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.

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.

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.

The use of oral potassium and avoiding high carbohydrate meals can help treat it according to resent tests. Dr. Jacob O. Levitt, a dermatologist who has hypokalemic periodic paralysis, recently conducted research in to it.

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.

Because CAPS is extremely rare and has a broad clinical presentation, it is difficult to diagnose, and a significant delay exists between symptom onset and definitive diagnosis. There are currently no clinical or diagnostic criteria for CAPS based solely on clinical presentation. Instead, diagnosis is made by genetic testing for "NLRP3" mutations. Acute phase reactants and white blood cell count are usually persistently elevated, but this is aspecific for CAPS.

In terms of treatment for TNF receptor associated periodic syndrome, corticosteroids can be administered for the reduction of the severity of this condition, NSAIDS may be used for fever.

The diagnosis for DMSA1 is usually masked by a diagnosis for a respiratory disorder. In infants, DMSAI is usually the cause of acute respiratory insufficiency in the first 6 months of life. The respiratory distress should be confirmed as diaphragmatic palsy by fluoroscopy or by electromyography. Although the patient may have a variety of other symptoms the diaphragmatic palsy confirmed by fluoroscopy or other means is the main criteria for diagnosis. This is usually confirmed with genetic testing looking for mutations in the "IGHMBP2" gene.

The patient can be misdiagnosed if the respiratory distress is mistaken for a severe respiratory infection or DMSA1 can be mistaken for SMA1 because their symptoms are so similar but the genes which are affected are different. This is why genetic testing is necessary to confirm the diagnosis of DMSA.

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.

Canakinumab has been approved for treatment of HIDS and has shown to be effective. The immunosuppressant drugs etanercept and anakinra have also shown to be effective. Statin drugs might decrease the level of mevalonate and are presently being investigated. A recent single case report highlighted bisphosphonates as a potential therapeutic option.

Hyperkalemic periodic paralysis (HYPP, HyperKPP) is a genetic disorder. It occurs in humans, horses (where it is also known as Impressive syndrome, after an index case in a horse named Impressive, or possibly one of his ancestors), and perhaps other animals. It is an inherited autosomal dominant disorder that affects sodium channels in muscle cells and the ability to regulate potassium levels in the blood. It is most commonly associated with horses, but occurs in humans, where it may be called Gamstorp episodic adynamy. It is characterized by muscle hyperexcitability or weakness which, exacerbated by potassium, heat or cold, can lead to uncontrolled shaking followed by paralysis. Onset in humans usually occurs in early childhood, but still occurs with adults.

The mutation which causes this disorder is dominant on SCN4A with linkage to the sodium channel expressed in muscle. The mutation causes single amino acid changes in parts of the channel which are important for inactivation. In the presence of high potassium levels, including those induced by diet, sodium channels fail to inactivate properly.

Equine hyperkalemic periodic paralysis occurs in 1 in 50 Quarter Horses and can be traced to a single ancestor, a stallion named Impressive.

It can reduce the effectiveness of the sodium-channel blocker lidocaine in dental work, so the amide-type local anesthetic. articaine is used on victims instead.

Cerebrospinal fluid findings:

- Raised protein (25% cases)

- Negative for 14–3–3 protein

- May contain antithyroid antibodies

- Magnetic resonance imaging abnormalities consistent with encephalopathy (26% cases)

- Single photon emission computed tomography shows focal and global hypoperfusion (75% cases)

- Cerebral angiography is normal

Thyroid hormone abnormalities are common (>80% cases):

- subclinical hypothyroidism (35% cases)

- overt hypothyroidism (20% cases)

- hyperthyroidism (5% cases)

- euthyroid on levothyroxine (10% cases)

- euthyroid not on levothyroxine (20% cases)

Thyroid antibodies – both anti-thyroid peroxidase antibodies (anti-TPO, anti-thyroid microsomal antibodies, anti-M) and antithyroglobulin antibodies (anti-Tg) – in the disease are elevated but their levels do not correlate with the severity.

Electroencephalogram studies, while almost always abnormal (98% cases), are usually nondiagnostic. The most common findings are diffuse or generalized slowing or frontal intermittent rhythmic delta activity. Prominent triphasic waves, focal slowing, epileptiform abnormalities, photoparoxysmal and photomyogenic responses may be seen.

Weber's syndrome is the only form of alternating hemiplegia that is somewhat easy to diagnose beyond the general criteria. Although Weber's syndrome is rare, a child born with the disorder typically has a port-wine stain on the face around the eye. While the port-wine stain does not necessarily mean the child has Weber's syndrome, if the port-wine stain involves the ophthalmic division of the trigeminal nerve than the likelihood of it being weber's syndrome greatly increases. If a port-wine stain around the eye is found, the patient should be screened for intracranial leptomeningeal angiomatosis. Magnetic resonance imaging (MRI)can be used to determine the presence and severity while computed cranial tomography can be used to determine the effect. MRI is the preferred diagnostic test on children presenting with port-wine stain. Other imaging techniques can be used in addition to further determine the severity of the disorder. The initial diagnosis is made based on the presence of neurologic and ophthalmic disease but the disease progresses differently in each patient so after initial diagnosis the patient should be monitored frequently in order to handle further complications resulting from the syndrome.

THS is usually diagnosed via exclusion, and as such a vast amount of laboratory tests are required to rule out other causes of the patient's symptoms. These tests include a complete blood count, thyroid function tests and serum protein electrophoresis. Studies of cerebrospinal fluid may also be beneficial in distinguishing between THS and conditions with similar signs and symptoms.

MRI scans of the brain and orbit with and without contrast, magnetic resonance angiography or digital subtraction angiography and a CT scan of the brain and orbit with and without contrast may all be useful in detecting inflammatory changes in the cavernous sinus, superior orbital fissure and/or orbital apex. Inflammatory change of the orbit on cross sectional imaging in the absence of cranial nerve palsy is described by the more benign and general nomenclature of orbital pseudotumor.

Sometimes a biopsy may need to be obtained to confirm the diagnosis, as it is useful in ruling out a neoplasm.

Differentials to consider when diagnosing THS include craniopharyngioma, migraine and meningioma.

There is no diagnostic test for alternating hemiplegia, which makes it very difficult to diagnose. Also, because alternating hemiplegia is extremely rare, it is frequently missed and the patient is often misdiagnosed. Proper diagnosis, however, is critical for early treatment of the disorder. There are many criteria that can help in the proper general diagnosis of alternating hemiplegia.

CRMO/CNO is a diagnosis of exclusion. This means that other diseases must be ruled out before the diagnosis can be made. Generally, many tests are required, such as blood tests, x-rays, bone scans, MRI and often a bone biopsy.

Since interleukin 1β plays a central role in the pathogenesis of the disease, therapy typically targets this cytokine in the form of monoclonal antibodies (such as canakinumab), binding proteins/traps (such as rilonacept), or interleukin 1 receptor antagonists (such as anakinra). These therapies are generally effective in alleviating symptoms and substantially reducing levels of inflammatory indices. Case reports suggest that thalidomide and the anti-IL-6 receptor antibody tocilizumab may also be effective.