Megalopapilla

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Article initiated by:

Qiancheng Wang

All contributors:

Qiancheng Wang, Samantha Marie Arsenault, Adam.Cantor, Nagham Al-Zubidi, MD, Samantha Marie Arsenault, Mary Labowsky, MD

Assigned editor:

Amanda Ely, M.D., Mary Labowsky, MD

Review:

Assigned status Up to Date

 by Mary Labowsky, MD on October 22, 2024.

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Megalopapilla (MP) is a rare congenital anomaly that is characterized by an enlarged nerve head and abnormal disc shape. It is a benign condition but must be differentiated from other diseases of the optic nerve that can cause vision loss such as glaucoma.

Disease Entity

Megalopapilla

Disease

Megalopapilla (MP) is a rare congenital anomaly that is characterized by an enlarged nerve head and abnormal disc shape. MP can mimic glaucomatous changes of the optic nerve and can occur in one or both eyes. MP is a benign condition with unknown etiology, but it has been reported in individuals with congenital glaucoma, basal encephalocele, and pulverulent cataract.

MP can cause a physiologically enlarged blind spot in some individuals. While this entity may resemble glaucomatous neuropathy on fundoscopy, MP has no optic nerve fiber loss or retinal nerve fiber layer thinning. Therefore, MP is a pseudoglaucomatous condition and possibly a variant of normal, representing an extreme example of physiologic cupping.

The condition is described as an enlarged disc with surface area > 2.50 mm without any additional structural abnormalities that would be more consistent with an alternate diagnosis. There are two phenotypes:

1. Normal configuration with enlarged disc, high C/D ratio, disc surface and/or neuroretinal rim pallor.

2. An upwardly displaced cup that obliterates the neuroretinal rim.

Etiology

The mechanism behind MP has not been elucidated and it may be a variant of normal. A proposed mechanism is altered optic axonal migration in early embryogenesis in children with basal encephalocele. MP has been reported in a set of twins and their paternal grandfather, suggesting a possible hereditary component.

Risk Factors

There is no known epidemiological association or risk factors. However, a study of a genetically isolated Marshallese population found 22 out of 54 eyes to have an optic disc >2.10 mm and 36 patients with a cup to disc ratio of >0.6.

History

Typically, megalopapilla is an incidental finding on clinical exam without pathology. A thorough review of ocular history, past medical history and family history should be performed to rule out other causes of an anomalous appearing optic disc, such as glaucoma.

Physical examination

Megalopapilla should not cause visual impairment and the intraocular pressure should be in the normal range. Fundoscopy typically reveals an enlarged disc, high C/D ratio, disc surface and/or neuroretinal rim pallor, displaced cup with obliteration of the neuroretinal rim. These findings make it extremely difficult to distinguish MP from a glaucomatous optic nerve.

Diagnostic procedures

Heidelberg retinal tomography (HRT) is an excellent tool for differentiating megalopapilla from pathological causes of an anomalous optic nerve. Unlike glaucoma, MP eyes have preserved optic disc area, rim area, and rim volume. One study of 50 MP eyes and 80 normals eyes show rim area (mm²) of 1.96 ±0.36 in the megalopapilla group and 1.90 ± 0.22 in the normal group (p= 0.25) and rim volume (mm³) of 0.15 ± 0.07 in the megalopapilla group and 0.14 ± 0.07 in the normal group (p= 0.48).

Optical coherence tomography (OCT) of the peripapillary retinal fiber layer (pRNFL) is another tool that can assist in differentiating glaucoma from MP. OCT results in MP eyes demonstrate thickened pRNFL compared to normal eyes.

Studies show normal to increased thickness of the pRNFL in individuals with MP compared to normal. In one study of 50 MP eyes and 80 normal eyes in children, the average RNFL thickness(µm) is 117.34 ±11.88 (MP) versus 106.83 ± 13.48 (normal) (p <0.01; CI:-15.85 to -5.18). In children, the increased RNFL thickness is attributed to larger discs having more ganglion cells. .

MP patients typically do not present with peripheral field loss. Visual field testing can show an enlarged physiological blind spot in certain patients.

Differential diagnosis

• Glaucoma (Congenital; JOAG; POAG; secondary glaucomas)
  • Physicians should also be aware of the shared similarities between MP and glaucoma: increased cup area, cup volume, and cup shape.
• Morning glory syndrome
• Optic nerve coloboma
• Staphyloma
• Optic nerve glioma

Management

General risk to common eye diseases apply to individuals with MP. The anomalous optic nerve may provide a diagnostic challenge for other diseases that may affect the optic nerve such as chronic open angle glaucoma. Routine visual fields and OCT pRNFL could be considered in addition to eye exams.

Prognosis

There should be no progression of MP or visual changes beyond the natural loss of ganglion cells and rim area associated with normal aging. A study comparing 39 children with MP to 39 adults with MP found children with MP to have significantly greater optic rim area compared to adults with MP. These findings suggest individuals with MP loses optic rim area over time but their cup to disc ratio and cup volume remain greater than normal.