What happens when too much sodium flows in the cell due to a faulty voltage-gated sensor? What about too little?
To answer the OP directly: You may be interested in looking at diseases that abnormally increase or decrease the activity of sodium channel proteins, changing the degree to which sodium ions cross the cell membrane. Some diseases are genetic in basis, which means they are inherited. This lends them to characterization and study.

This review paper by George summarizes some of the genetic diseases associated with sodium channel mutations, which might give you some direction for further research:


Muscle sodium channelopathies (SCN4A)


Hyperkalemic periodic paralysis
Paramyotonia congenita
Potassium-aggravated myotonia
Painful congenital myotonia
Myasthenic syndrome
Hypokalemic periodic paralysis type 2
Malignant hyperthermia susceptibility

Cardiac sodium channelopathies (SCN5A)


Congenital long QT syndrome (Romano-Ward)
Idiopathic ventricular fibrillation (Brugada syndrome)
Isolated cardiac conduction system disease
Atrial standstill
Congenital sick sinus syndrome
Sudden infant death syndrome
Dilated cardiomyopathy, conduction disorder, arrythmia

Brain sodium channelopathies (SCN1A, SCN2A, SCN1B)


Generalized epilepsy with febrile seizures plus
Severe myoclonic epilepsy of infancy (Dravet syndrome)
Intractable childhood epilepsy with frequent generalized tonic-clonic seizures
Benign familial neonatal-infantile seizures

Peripheral nerve sodium channelopathies (SCN9A)


Familial primary erythermalgia



Generally, you might search on the term "sodium channel mutant" or "mutations" to see what kinds of health problems are caused by under- or over-activity of these proteins.