Introduction

What is foetal hydantoin syndrome?

Foetal Hydantoin Syndrome (FHS) is a congenital disorder that occurs as a result of prenatal exposure to phenytoin, an antiepileptic medication commonly prescribed for epilepsy.^1,2,3 This condition is characterised by a spectrum of physical and developmental abnormalities varying in severity among affected individuals. Not all infants exposed to phenytoin during gestation will develop FHS, with the risk of an infant expressing some effects of the syndrome estimated at 33%, while the risk of developing the syndrome itself is approximately 5-10%.³ 

Aetiology

The primary cause of the condition is maternal use of phenytoin during pregnancy. Phenytoin is a widely used anti-seizure medication, which can cross the placental barrier and affect foetal development.^1,2

FHS does not always occur in the offspring after phenytoin use during pregnancy, suggesting the influence of genetic susceptibility and other factors in its occurrence. The risk of FHS is highly dose-dependent, thus, higher doses of phenytoin are associated with an increased risk of the syndrome. Genetic factors also play a role, as some foetuses appear to be more susceptible to the teratogenic effects of phenytoin than others.^1,2 Phenytoin interferes with the body's ability to absorb folic acid, which significantly increases the risk of major birth anomalies.³

Disease pathophysiology

The exact mechanisms by which phenytoin causes FHS are not fully understood, but several theories exist:

• Epoxide intermediate: Phenytoin metabolism produces an epoxide intermediate, a compound in cholesterol biosynthesis, that may be teratogenic (an isoform resulting in reduced activity of the compound). Foetuses with inadequate epoxide hydrolase activity may be at higher risk for developing FHS¹
• Folate antagonism: Phenytoin interferes with folate metabolism, which is crucial for foetal development, particularly in neural tube formation and cellular division^1,3
• Oxidative stress: Phenytoin may increase oxidative stress in developing tissues, leading to cellular damage and developmental abnormalities¹
• Altered gene expression: Phenytoin exposure may affect the expression of genes critical for foetal development, particularly those involved in the formation of the skull and facial features, as well as digits^1,2

Hallmark symptoms of foetal hydantoin syndrome

The classic features of FHS include:

• Craniofacial abnormalities: Broad nasal bridge, hypertelorism (widely spaced eyes), short neck, ear malformations, and epicanthal folds (the upper eyelid covering the inner eye-corner, much like in Down’s Syndrome^1,2,3
• Growth deficiencies: Intrauterine growth restriction and postnatal growth delays^1,2,3
• Digital anomalies: Hypoplasia of distal phalanges, where the development of the at the tip of your fingers is incomplete. This particularly affects fingernails and toenails in FHS^1,2,3
• Developmental delays: Mild to moderate intellectual disability and motor delays. The risk of neurological impairment is estimated to be 1% to 11%, which is 2 to 3 times higher than in the general population³
• Occasional features: Cleft lip and/or palate (5 times higher risk), cardiac abnormalities (5 times higher risk), abnormally small head, also known as microcephaly, ocular defects, hypospadias (an elongated opening of the urethra in males), and umbilical hernias^1,2,3

The severity of these symptoms can vary widely among affected individuals, ranging from mild, barely noticeable features to more severe manifestations that significantly impact quality of life. It's also worth noting that there's an increased incidence of benign and malignant brain tumours suggested in individuals with FHS.³

Developmental and cognitive abnormalities associated with foetal hydantoin syndrome 

Foetal Hydantoin Syndrome (FHS) is a complex disorder resulting from prenatal exposure to phenytoin, an anticonvulsant medication commonly used to treat epilepsy. The syndrome is characterised by a spectrum of physical and neurodevelopmental abnormalities that can significantly impact a child's growth, cognitive function, and overall development.^1,2,3,4,5

The manifestations of FHS typically include three main categories of abnormalities: growth deficiencies, performance abnormalities, and dysmorphic features.^2,3,4 Growth abnormalities encompass both pre- and postnatal growth deficiency and microcephaly.^1,2,4 Performance abnormalities range from mild developmental delays to more severe cognitive impairments, including “dull mentality” as characterised by older papers^2,3,5,6, and easily recognisable cognitive deficiency. Dysmorphic craniofacial features, while not directly related to cognitive function, are often present and can include a short nose with a broad, depressed bridge, mild ocular hypertelorism, and other characteristic facial traits.^1,2,3,4

The severity and nature of cognitive and developmental abnormalities in FHS can vary significantly among affected individuals. Two primary factors influencing these outcomes are the dosage of phenytoin exposure and genetic susceptibility. The risk and severity of FHS appear to be dose-dependent, with higher doses of phenytoin associated with an increased risk of more severe outcomes.^1,2 However, it's important to note that there is no completely safe dosage, and effects can occur even at therapeutic levels. Genetic differences in maternal or foetal metabolism also appear to be important risk factors for congenital anomalies among children exposed to phenytoin prenatally.^1,2,3,4 For instance, variations in epoxide hydrolase activity have been associated with different manifestations of FHS, suggesting that this enzyme may play a crucial role in determining an individual's susceptibility to the teratogenic effects of phenytoin.¹

Developmental delay in children with FHS

Developmental delay is also a common feature of FHS, affecting various aspects of a child's growth and development. These delays can manifest in several physical and mental areas, including motor development, language development, social and adaptive skills, and cognitive development. Children with FHS may experience delays in reaching motor milestones, such as sitting up, crawling, or walking.^1,2,5 Speech and language delays, which can range from mild to severe, are often observed. Some children may show delays in developing age-appropriate social interactions and self-care skills.^1,2,6 Delays in cognitive milestones, such as problem-solving and abstract thinking, may also be present. It's important to note that the severity of these delays can vary widely among affected individuals, and some children may catch up in certain areas over time.^1,2

Cognitive abnormalities in FHS 

Cognitive abnormalities in FHS can range from mild to severe and may include intellectual disability, learning disabilities, attention and concentration issues, executive function deficits, and memory impairments. Intellectual disability can range from mild (described as "dull mentality" in some older literature) to more severe forms of mental deficiency.^1,2,5,6 Even in the absence of global intellectual disability, specific learning difficulties may be present. Some children may exhibit difficulties with sustained attention or concentration, while problems with planning, organisation, and problem-solving may be observed in others. Both short-term and long-term memory may be affected in some individuals. It's crucial to emphasize that cognitive outcomes in FHS exist on a spectrum, and not all affected individuals will experience significant cognitive impairment.^2,5,6

Challenges in disease diagnosis and management 

Diagnosing and managing FHS presents several challenges. The wide range of possible symptoms and their severity can make diagnosis challenging, especially in milder cases. Some features of FHS can be similar to other genetic or teratogenic syndromes, requiring careful differential diagnosis.^1,5,6 While some major malformations can be detected prenatally, many of the cognitive and developmental issues associated with FHS cannot be identified before birth. Due to the developmental nature of many FHS symptoms, long-term follow-up is crucial for proper management and support. 

Managing epilepsy during pregnancy while minimising risk to the foetus remains a significant challenge for healthcare providers, however, the emotional challenges mothers experience also should not be neglected. It is incredibly challenging for mothers to find a balance between their own health, by managing seizures, whilst also managing the health of their foetus. Finding the proper medication to ensure this balance is crucial, especially since untreated seizures can negatively affect the development of the foetus too. Effective management of FHS often requires a coordinated effort from various healthcare fields, including neurology, genetics, developmental paediatrics, and others.

Summary

Foetal Hydantoin Syndrome (FHS) is a congenital disorder resulting from prenatal exposure to phenytoin, an antiepileptic medication. This article explores the developmental delays and intellectual disabilities associated with FHS, a condition affecting approximately 5-10% of exposed infants, with milder effects seen in up to 33% of cases.

The syndrome's aetiology is primarily linked to maternal phenytoin use during pregnancy, with both dosage and genetic factors influencing susceptibility. The pathophysiology, while not fully understood, likely involves epoxide intermediates, folate antagonism, oxidative stress, and altered gene expression.

FHS manifests through a spectrum of abnormalities, including craniofacial features, growth deficiencies, and digital anomalies. Developmental delays in FHS affect motor skills, language, social abilities, and cognitive functions. Cognitive impairments range from mild learning difficulties to severe intellectual disabilities, impacting attention, memory, and executive functions.

Diagnosis and management of FHS presents significant challenges due to the variability in presentation, overlap with other conditions, and limitations in prenatal detection of cognitive issues. Long-term, multidisciplinary care is crucial for managing FHS, as is balancing maternal epilepsy treatment with foetal risk.