Optic nerve hypoplasia (ONH) is a condition describing optic nerve underdevelopment during pregnancy. It is natural for the optic nerve fibers to die as a child develops in the womb, and ONH is an aggregation of the process. Magnus described the condition in 1884, although, at the time, the incidence was rare. Over time, the occurrence rate has been on the rise, presumably because of a better understanding of the condition and its associations.
ONH is one of the three predominant causes of visual impairment and blindness in children. The condition also includes an abnormal development or absence of corpus callosum (nerve fibers connecting the brain's hemispheres), cortical heterotopia ( an abnormal movement of neurons to the brain's surface) and septum pellucidum (a thin connective tissue centered in the brain's lateral ventricles). However, the septum pellucidum abnormality has no known functional consequence.
Many children with the condition also have involuntary eye movements (nystagmus) which in most instances is linked to significantly reduced visual sharpness in both eyes. ONH's impact on visual function may range from a normal visual acuity to absolute blindness.
Also Known As
- De Morsier syndrome
- Septo Optic dysplasia
Types
ONH classification is centered on the presence of neural complications:
- Type I - Optic nerve hypoplasia simplex
- Type II - Septo Optic dysplasia
- Type III - Septo Optic pituitary dysplasia
Causes & Risk Factors
The condition appears to occur spontaneously. Although research indicates that ONH is not inherited, there are reports of rare instances of families having more than one child with the condition, implying an autosomal recessive inheritance. Mutation of various genes has also been reported to be a cause.
The established maternal traits associated with ONH are gestational diabetes, maternal age and primiparity. Lifestyle factors and the role of nutrition are still on evaluation.
Signs & Symptoms
Most ONH symptoms become apparent during the childhood or adolescence years, even though a child is born with the condition. One noticeable indicator is involuntary, rapid eye movement, which is a result of the eye's inability to focus well. Patients may also have impaired vision in one or both eyes, ranging from mild to severe.
Due to the underdeveloped optic nerves, the optic disc may appear smaller than the typical size when viewed under an ophthalmoscope. The optic disc is where nerve fibers join to form the optic nerve. A child may also have problems with regulating some basic body activities such as sleep, thirst, body temperature and hunger. These problems arise because of an underdeveloped hypothalamus.
A fully developed hypothalamus comprises several parts that perform various roles. Among them is the coordination of pituitary gland function. The hypothalamus influences the glands to release certain hormones into the bloodstream. With ONH, communication between the hypothalamus and pituitary gland is inhibited, and so the glands fail to produce or release the expected levels of some hormones into the bloodstream. The growth hormone gets affected in most cases, hence the short stature and maturation delays in the child.
Some children may exhibit intellectual disabilities due to the absence of thyroid-stimulating hormone. The hormone is responsible for metabolizing energy. Patients may also develop diabetes insipidus, delayed sexual maturation, obesity, seizures and low glucose levels in the blood because of a deficiency in an antidiuretic hormone. Many children also have delays with acquiring skills that need mental and muscular activity coordination.
Diagnosis
Using an ophthalmoscope, a pediatric ophthalmologist will look at the eyes to identify whether the optic nerve's surface appears smaller than usual. The doctor may also carry out magnetic resonance imaging (MRI), and computerized tomography to examine optic nerves and corpus callosum.
The findings from MRI rarely provide a conclusive diagnosis since other anomalies can mimic the findings. Therefore, doctors use data on the levels of growth hormones and serum cortisol to confirm the diagnosis.
Treatment
No medical or surgical treatment can cure ONH, and so treatment aims at addressing specific symptoms. The treatments may require the collaborative effort of ophthalmologists, endocrinologists, neurologists, and other healthcare professionals.
For hormonal deficiencies, treatment may entail hormone replacement therapy. Since visual anomalies are not treatable, early interventions are vital in ensuring that affected children reach their potentials.
Various agencies offer special services such as occupational therapy, vision therapy and physical therapy, all of which may be beneficial to a child. ONH patients and their families can also receive genetic counseling services that aid in deciding on the best therapies as well as a better understanding of the condition's risk.
Prognosis & Long-Term Outlook
ONH is a stable condition. Therefore, it does not worsen over time. A child's vision may become slightly better, and nystagmus may also reduce.
Prevention & Follow Up
If a child demonstrates poor visual function, strabismus or nystagmus, a comprehensive eye exam is vital before s/he is three months old. Once a child is diagnosed with the condition, s/he should have routine eye exams to manage or treat present ocular conditions properly.
