Kernicterus
Kernicterus is a bilirubin-induced brain dysfunction. The term was coined in 1904 by Schmorl. Bilirubin is a naturally occurring substance in the body of humans and many other animals, but it is neurotoxic when its concentration in the blood is too high, a condition known as hyperbilirubinemia. Hyperbilirubinemia may cause bilirubin to accumulate in the grey matter of the central nervous system, potentially causing irreversible neurological damage. Depending on the level of exposure, the effects range from clinically unnoticeable to severe brain damage and even death. When hyperbilirubinemia increases past a mild level, it leads to jaundice, raising  the risk of progressing to kernicterus. When this happens in adults, it is usually because of liver problems. Newborns are especially vulnerable to hyperbilirubinemia-induced neurological damage, because in the earliest days of life, the still-developing liver is heavily exercised by the breakdown of fetal hemoglobin as it is replaced with adult hemoglobin and the blood–brain barrier is not as developed. Mildly elevated serum bilirubin levels are common in newborns, and neonatal jaundice is not unusual, but bilirubin levels must be carefully monitored in case they start to climb, in which case more aggressive therapy is needed, usually  via light therapy but sometimes even via exchange transfusion. Classification. Acute bilirubin encephalopathy (ABE). ABE is an acute state of elevated bilirubin in the central nervous system. Clinically, it encompasses a wide range of symptoms. These include lethargy, decreased feeding, hypotonia or hypertonia, a high-pitched cry, spasmodic torticollis, opisthotonus, setting sun sign, fever, seizures, and even death. If the bilirubin is not rapidly reduced, ABE quickly progresses to chronic bilirubin encephalopathy. Chronic bilirubin encephalopathy (CBE). CBE is a chronic state of severe bilirubin-induced neurological lesions. Reduction of bilirubin in this state will not reverse the sequelae. Clinically, manifestations  of CBE include: These impairments are associated with lesions in the basal ganglia, auditory nuclei of the brain stem, and oculomotor nuclei of the brain stem. Cortex and white matter are subtly involved. Cerebellum may be involved. Severe cortical involvement is uncommon. Subtle bilirubin encephalopathy (SBE). SBE is a chronic state of mild bilirubin-induced neurological dysfunction (BIND). Clinically, this may result in neurological, learning and movement disorders, isolated hearing loss and auditory dysfunction. Causes. In the vast majority of cases, kernicterus is associated with unconjugated hyperbilirubinemia during the neonatal period. The blood–brain barrier is not fully functional in  neonates and therefore bilirubin is able to cross into the central nervous system. Moreover, neonates have much higher levels of bilirubin in their blood due to: Bilirubin is known to accumulate in the gray matter of neurological tissue where it exerts direct neurotoxic effects. It appears that its neurotoxicity is due to mass-destruction of neurons by apoptosis and necrosis. Gilbert's syndrome and G6PD deficiency occurring together especially increases the risk for kernicterus. Diagnosis. In neonates with kernicterus, the Moro reflex may be absent or symmetrically reduced. Prevention. Measuring the serum bilirubin is helpful in evaluating a baby's  risk for developing kernicterus. These numbers can then be plotted on the Bhutani nomogram. In neonates with hyperbilirubinemia, light therapy may be effective in reducing the serum bilirubin level. More severe cases may require the use of exchange transfusion. Treatment. Currently no effective treatment exists for kernicterus. Future therapies may include neuroregeneration. A handful of patients have undergone deep brain stimulation, and experienced some benefit. Drugs such as baclofen, clonazepam, gabapentin, and artane are often used to manage movement disorders associated with kernicterus. Proton pump inhibitors are also used to help with reflux. Cochlear implants and hearing aids have also  been known to improve the hearing loss that can come with kernicterus (auditory neuropathy – ANSD).