Optic nerve hypoplasia

Optic nerve hypoplasia
	Classification and external resources
ICD-10	Q07.81
ICD-9	743.57-743.58
OMIM	165550
DiseasesDB	31311

Optic nerve hypoplasia (ONH) is a medical condition that results in underdevelopment of the optic nerve(s). This condition is the most common congenital optic disc anomaly in which the optic disc appears abnormally small, due to the fact that not all the optic nerve axons have developed properly.^[1]

Introduction to ONH

Optic nerve hypoplasia (ONH) is the underdevelopment of one or both optic nerves. It is often associated with endocrinopathies (hormone deficiencies), developmental delay, and brain malformations. The optic nerve, which is responsible for transmitting visual signals from the retina to the brain, has approximately 1.2 million nerve fibers in the average person. In those diagnosed with ONH, however, there are noticeably fewer nerve fibers, which range in number from patient to patient.

ONH is diagnosed by ophthalmoscopic examination. Patients with ONH exhibit an optic nerve that appears smaller than normal.^[2]

History and Epidemiology

ONH, also known as de Morsier’s Syndrome or septo-optic dysplasia, is a condition that involves multiple systemic problems stemming from miswiring of the brain and central nervous system, which manifests in the optic nerves, especially the hypothalamus. Because of this, all children with ONH are at risk for developmental delays and hormonal deficiencies, regardless of severity of ONH, or whether abnormalities are found by MRI.

Prevalence of ONH is increasing, although it is difficult to estimate the true prevalence. Between 1980 and 1999, the occurrences of ONH in Sweden increased four-fold to 7.2 per 100,000, while all other causes of childhood blindness had declined. ^[3] ^[4] In 1997, ONH overtook retinopathy of prematurity as the single leading cause of infant blindness in Sweden, with 6.3 in every 100,000 births diagnosed with ONH. The most recent prevalence report out of England in 2006 is 10.9 per 100,000. ^[5] ^[6]

Prenatal Risk Factors and Causation

Genetic risks:

A specific genetic causation has not been found to explain the majority of cases of ONH. ONH impacts all ethnic groups, although in the United States, occurrence is lower in persons of Asian descent.^[7] ^[8] ^[9]To date, there have been few reports of ONH occurrence in Asian countries, although it is uncertain whether this relates to relative genetic protection or differences in environment and/or diet related to culture.

Gestational and exposure history:

Although many perinatal and prenatal risk factors for ONH have been reported, the predominant enduring characteristics are young maternal age and primiparity (having given birth to only one child).^[10] ^[11] Increased frequency of delivery by caesarean section and fetal/neonatal complications, preterm labor, gestational vaginal bleeding, low maternal weight gain, and weight loss during pregnancy are also associated with ONH.^[12]

Symptoms

ONH may be found in isolation or in conjunction with a myriad of functional and anatomic abnormalities of the central nervous system. Nearly 80% of those affected with ONH will experience hypothalamic dysfunction and/or impaired development of the brain, regardless of MRI findings or severity of ONH.^[13]

Vision:

ONH can be unilateral (in one eye) or bilateral (in both eyes), although it presents most often bilaterally (80%). Because the unilateral cases tend to have better vision, they are typically diagnosed at a later age than those with bilateral ONH. Visual acuity can range from no light perception to near-normal vision.

Children diagnosed with ONH generally present with vision problems, generally involving nystagmus (involuntary movement of the eyes), which tends to develop at 1 to 3 months and/or strabismus (inability to align both eyes simultaneously), during their first year of life.

The majority affected experience improvement in vision during the first few years of life, though the reason for this occurrence is unknown. There have been no reported cases of decline in vision due to ONH.

Neuroradiographic Abnormalities:

Estimates of cerebral malformations vary from 39% to 90% of children with ONH. Neuroradiographic abnormalities include: hypoplasia of the corpus callosum, agenesis (absence or incomplete development) of the septum pellucidum, malformations of the pituitary gland, schizencephaly, cortical heterotopia, white matter hypoplasia, pachygyria, and holoprosencephaly. Hypoplasia of the corpus callosum, often in conjunction with other major malformations, is significantly associated with poor and delayed developmental outcome.^[14]

ONH is often diagnosed as septo-optic dysplasia, a term that refers to agenesis of the septum pellucidum. It is now clear that the absence of the septum pellucidum does not correlate with the associated symptoms of ONH.^[15]

Hypothalamic Dysfunction:

Dysfunction of the hypothalamus results in loss of regulation over behavior and function of the pituitary gland (master gland). The pituitary gland contributes to growth, metabolism, and sexual development. Hypopituitarism is present in 75% to 80% of patients with ONH. The most common pituitary endocrinopathies are: growth hormone (GH) deficiency (70%), hypothyroidism (43%), adrenal insufficiency (27%), and diabetes insipidus (5%).^[16] ^[17]

Absence of GH may often be indicated by short stature, although this is not always the case.4 Other indicators of GH deficiency may include: hypoglycemic events (including seizures), prolonged jaundice, micropenis in boys, and delayed dentition. Testing for GH may involve blood tests (IGF-1 and IGFBP-3), growth hormone stimulation test, or bone age x-ray of the hand or wrist (or body for children younger than 2 years).

A poorly functioning pituitary gland may also cause a lack of thyroid hormone, leading to central hypothyroidism. Thyroid hormone is critical for growth and brain development, especially during the first few weeks to months of life. Children with untreated hypothyroidism are at high risk of mental retardation; thus, early detection is crucial. Central hypothyroidism can be diagnosed by a low or normal thyroid-stimulating hormone (TSH) in the presence of a low level of free T4. Free T-4 should be checked annually for at least four years.^[18]

Cortisol is made in times of stress. Approximately one-quarter of patients with ONH have adrenal insufficiency, meaning they do not produce enough cortisol on a daily basis or in stressful situations.^[19]

Imbalances in sex hormone may result in a delay in sexual development (puberty) or precocious puberty. Sex hormones may be tested from birth to 6 months of age (during a mini-puberty).

Hyperprolactinemia (an excess of prolactin) often occurs in conjunction with ONH and indicates either dysfunction of the hypothalamus or a disconnect between the hypothalamus and pituitary gland.^[20] Hyperprolactinemia often correlates with development of obesity in children with ONH.^[21] ^[22]

The pituitary gland produces anti-diuretic hormone (ADH), which controls outflow of water from the body by urine. ADH deficiency, also known as Diabetes Insipidus (DI), results in dehydration and high sodium levels in the body from excessive urination. Testing for DI involves blood and urine testing, including water deprivation tests, to determine ADH creation levels by the body. DI may be treated with a medication called desmopressin acetate (DDAVP).^[23]

Hypothalamic dysfunction may also result in problems with feeding, sleep, and body temperature regulation. Feeding behaviors in children with ONH often include hyperphagia (overeating), resulting in obesity, or hypophagia (reduced food intake) with or without weight loss. Children also frequently experience aversion to specific textures of food. Disturbance of circadian sleep rhythm, resulting in abnormal sleep-wake cycles, is noted in one-third of children with ONH. This disturbance could result in behavioral problems and disruption of family life.^[24] ^[25]

Development:

More than 70% of children with ONH experience developmental delay, ranging from isolated focal defects to delay in all areas of development (global delay). Motor delay is most common (75%) and communication delay is least common (44%). Indicators of significantly delayed development include hypoplasia or agenesis of the corpus callosum and hypothyroidism. The absence of the septum pellucidum does not predict developmental delay. Delays may occur in unilateral (39%) as well as bilateral (78%) cases.^[26]

Therapeutic Options

There is no treatment for ONH; however, many therapeutic interventions exist for the care of its symptoms. These may include hormonal replacement therapy for hypopituitarism, occupational, physical, and/or speech therapy. The special emphasis of early development of oral motor skills and acclimation to textured foods for children with texture aversion, or who are otherwise resistant to eating, should be observed.^[27]

Sleep dysfunction can be ameliorated using melatonin in the evening in order to entrain the circadian clock.^[28]

Treatments of vision problems may include patching of the better eye, which may result in improved vision in the worse eye; however, this should be reserved for cases in which potential for vision improvement in both eyes is felt to be good. Surgical correction of strabismus should be used for children with symmetrical functional vision in both eyes, but otherwise should be deferred until it becomes a psychosocial issue.^[29]

Prognosis

The visual prognosis in optic nerve hypoplasia is quite variable. Occasionally, optic nerve hypoplasia may be compatible with near-normal vision; in other cases, one or both eyes may be functionally, or legally blind. Although most patients with optic nerve hypoplasia lead normally productive lives, those with septo-optic dysplasia may experience non-visual problems, for example, with growth retardation.

References

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