Ophthalmology – Foveal Hypoplasia

Basics

Description

Foveal hypoplasia is a congenital developmental abnormality in which the fovea fails to develop normally. Because the fovea is responsible for high-resolution central vision, incomplete foveal specialization can result in reduced visual acuity and nystagmus.

Funduscopic examination typically demonstrates an absent or poorly developed foveal pit and foveal light reflex. The normal foveal avascular zone may be absent, allowing retinal vessels to pass unusually close to or directly across the expected center of the fovea.

The remainder of the retina and the optic nerves may appear normal, particularly in isolated foveal hypoplasia.

Visual acuity is variable and depends on the severity of the developmental abnormality and associated ocular conditions. Historically, acuities around 20/100 to 20/200 have been described in more significant cases, although milder forms can have substantially better vision.

Epidemiology

The true incidence and prevalence of foveal hypoplasia are unknown. The condition is uncommon and may occur either as an isolated ocular abnormality or as part of a genetic or developmental syndrome.

Risk Factors

Foveal hypoplasia is strongly associated with albinism and aniridia. A positive family history of foveal hypoplasia or an associated inherited disorder increases the likelihood of the diagnosis.

Genetics

The genetic basis depends on whether foveal hypoplasia occurs in isolation or as part of another disorder.

When associated with aniridia or albinism, the inheritance pattern generally follows that of the underlying condition.

Mutations involving PAX6, located on chromosome 11p13, can cause aniridia-associated foveal hypoplasia and some forms of isolated foveal hypoplasia.

Autosomal recessive forms have also been described, including foveal hypoplasia associated with anterior segment dysgenesis.

Foveal hypoplasia has additionally been reported in association with Axenfeld–Rieger spectrum disorders and other genetic conditions.

Prevention

Because foveal hypoplasia results from abnormal development of the fovea, there is no method for preventing the ocular abnormality after development has occurred.

When a causative mutation or inherited syndrome is identified, genetic counseling can help affected individuals and families understand inheritance patterns and recurrence risks. Prenatal or other genetic testing may be available for selected known mutations.

Pathophysiology

Foveal hypoplasia results from incomplete differentiation and specialization of the central retina during development.

Normal foveal development involves displacement of the inner retinal layers away from the center of the macula, formation of the foveal pit, specialization and elongation of the photoreceptors, and development of a foveal avascular zone.

In foveal hypoplasia, these developmental events are incomplete. Consequently, the inner retinal layers may persist across the foveal center, the foveal pit may be shallow or absent, and retinal vessels may cross the area where the normal foveal avascular zone should be located.

In albinism, deficient melanin within the retinal pigment epithelium is associated with abnormal foveal development.

In PAX6-related disorders, including aniridia, disruption of developmental gene regulation interferes with normal macular differentiation.

Etiology

The condition is believed to result from incomplete embryologic and postnatal development of the fovea.

Normal foveal maturation progresses from an initially indistinct central retinal area toward formation of the annular reflex, development of the foveal pit, and finally appearance of the mature foveal reflex.

Interruption or failure of these developmental processes produces varying degrees of foveal hypoplasia.

Associated Conditions

Important associated disorders include albinism, aniridia, achromatopsia, and congenital nystagmus.

Less commonly, foveal hypoplasia may occur with Axenfeld–Rieger spectrum abnormalities and other developmental or genetic ocular disorders.

Reduced visual acuity is common. Strabismus and amblyopia may develop, particularly when visual function is asymmetric between the two eyes.

Diagnosis

History

Patients usually present with reduced vision beginning in infancy or childhood. Parents may first notice abnormal visual behavior or involuntary eye movements.

Infantile nystagmus is a frequent presenting feature.

Some patients are diagnosed during evaluation for an associated disorder such as albinism or aniridia. Mild isolated foveal hypoplasia may remain undetected until later childhood or adulthood.

A family history of poor vision, nystagmus, albinism, aniridia, or similar ocular abnormalities should be obtained.

Physical Examination

A complete ophthalmologic examination should be performed, with particular attention to the macula.

Funduscopic examination may reveal absence or reduction of the normal foveal light reflex and foveal pit.

The normal foveal avascular zone may be poorly developed or absent. Retinal vessels may extend unusually close to or directly across the presumed foveal center.

The examiner should assess visual acuity, fixation behavior, refractive error, ocular alignment, and nystagmus.

The iris, anterior segment, optic nerves, and peripheral retina should also be examined carefully for evidence of an associated condition such as aniridia or albinism.

Diagnostic Testing

Optical Coherence Tomography

Optical coherence tomography (OCT) is the most useful imaging technique for confirming and grading foveal hypoplasia.

OCT can demonstrate characteristic persistence of the inner retinal layers through the foveal center and absence or reduction of the normal foveal depression.

Other findings can include abnormal retinal thickness, reduced or absent widening of the outer nuclear layer, and incomplete specialization or elongation of the photoreceptor outer segments.

Modern OCT-based grading systems can classify the severity of foveal hypoplasia according to the degree of structural development. In general, more severe structural abnormalities are associated with poorer visual acuity.

Laboratory and Genetic Evaluation

Routine laboratory investigations are generally not required for isolated foveal hypoplasia.

Molecular genetic testing should be considered when there is suspicion of an inherited disorder, particularly in patients with aniridia, albinism, anterior segment dysgenesis, achromatopsia, a positive family history, or apparently isolated congenital foveal hypoplasia.

Referral to ocular genetics may help determine the most appropriate testing strategy.

Additional Testing

Visual-field testing may occasionally be useful in cooperative patients, particularly when another optic nerve or retinal disorder is being considered.

Additional electrophysiologic or retinal testing may be appropriate when a retinal or cone dystrophy is part of the differential diagnosis.

Pathological and Structural Findings

The principal structural abnormality is failure of normal foveal differentiation.

The ganglion cell and other inner retinal layers may continue across the macular center rather than being displaced centrifugally as they are in a normally developed fovea.

The normal foveal avascular zone may be absent.

Developmental abnormalities of the visual system beyond the retina have also been reported, including changes involving the optic pathways and visual cortex, particularly in syndromic conditions.

Differential Diagnosis

Foveal hypoplasia should be differentiated from other disorders that alter the appearance or position of the macula.

Important considerations include epiretinal membrane, macular dragging from retinopathy of prematurity or familial exudative vitreoretinopathy, high myopia, retinal vascular anomalies, and retinal or macular dystrophies.

A lightly pigmented or blonde fundus can make identification of the fovea difficult but does not necessarily indicate foveal hypoplasia.

Optic nerve hypoplasia and optic atrophy can also produce reduced vision and nystagmus but have different structural findings.

OCT is particularly valuable in distinguishing true developmental foveal hypoplasia from these conditions.

Treatment

There is no medical or surgical treatment capable of creating a normally developed fovea once foveal development is complete.

Management therefore focuses on maximizing the patient’s existing visual potential and treating associated ocular abnormalities.

Optical Correction

Refractive errors should be corrected accurately with spectacles or contact lenses.

Even when central vision is limited by foveal hypoplasia, correction of refractive error can produce meaningful functional improvement.

Amblyopia Treatment

If visual acuity is asymmetric and amblyopia is suspected, appropriate amblyopia therapy should be instituted during the period of visual development.

Early treatment is important because amblyopia represents an additional potentially preventable source of visual loss.

Photophobia

Patients with albinism, aniridia, or other conditions associated with significant photophobia may benefit from tinted or light-filtering lenses.

Low-Vision Rehabilitation

Patients with significant permanent reduction in central vision should be referred for low-vision evaluation and rehabilitation.

Depending on age and functional requirements, magnification devices, electronic aids, educational accommodations, and other assistive technologies can improve reading and daily functioning.

Surgical Treatment

There is no surgical procedure for foveal hypoplasia itself.

Associated strabismus may be treated surgically when appropriate.

Selected patients with significant nystagmus and an abnormal head posture may undergo nystagmus-related surgery. Such procedures may improve head position or certain functional aspects of vision, but they generally do not restore normal foveal visual acuity.

Follow-Up

Follow-up depends on the patient’s age, visual function, and associated disorders.

Children require monitoring of visual development, refractive error, amblyopia, strabismus, and nystagmus.

Patients with associated aniridia or albinism require continued surveillance for complications related to those underlying disorders.

Low-vision needs should be reassessed as educational, occupational, and daily visual demands change.

Patient Monitoring

Particular attention should be given to asymmetric visual acuity because amblyopia can coexist with structural foveal abnormalities and should not automatically be attributed to the foveal hypoplasia.

Ocular alignment and refractive error should also be monitored throughout childhood.

Prognosis

Visual prognosis depends primarily on the degree of foveal development and the presence of associated ocular or systemic abnormalities.

Mild foveal hypoplasia may permit relatively good functional vision, whereas severe developmental abnormalities can produce substantial lifelong reduction in central visual acuity.

Extensive abnormal macular vascularization, including vessels extending close to or across the presumed foveal center, has historically been associated with more severe hypoplasia and poorer visual function.

Because the structural abnormality is congenital, vision lost directly from severe foveal hypoplasia generally cannot be restored. However, maximizing refractive correction and treating amblyopia and strabismus can substantially improve the patient’s usable visual potential.

Complications

The major consequences are low vision and nystagmus.

Patients may also develop amblyopia and strabismus, particularly when the two eyes are affected unequally.

Associated disorders such as albinism and aniridia may produce additional ocular complications that require separate long-term surveillance and treatment.