Megalopapilla (MP) is a rare congenital anomaly in which the optic disc is enlarged beyond 2.5 mm and exhibits an abnormal disc shape. It is a non-progressive benign condition that can occur unilaterally or bilaterally.
It is a diagnosis of exclusion, requiring the absence of other structural abnormalities. Because it resembles glaucomatous findings, it is also called a “pseudoglaucomatous condition” and is considered an extreme normal variant of physiologic cupping.
The normal optic disc size is 1.5–1.9 mm in diameter and 1.7–2.8 mm² in area. The DM/DD ratio (ratio of disc diameter to distance from the macula to the disc center) is used as an index for disc size. Normal values range from 2.1 to 3.2, and a DM/DD ratio of 2.2 or less suggests a large disc.
Type 1
Enlargement of the disc: The entire disc is enlarged while maintaining normal structure.
C/D ratio: Shows a high C/D ratio.
Appearance: Pallor of the optic disc surface and margin is observed.
Unilaterality/Bilaterality: Can be unilateral or bilateral.
Type 2
Cupping displacement: The cupping is displaced upward, and part of the disc margin appears to disappear.
Cilioretinal artery: High frequency of occurrence.
Unilateral: Usually occurs unilaterally.
There are also reports in patients with congenital glaucoma, basal encephalocele, and pulverulent cataract.
In type 1, the entire optic disc is uniformly enlarged and maintains normal structure, whereas in type 2, the cup is displaced upward and part of the disc margin appears to be missing. Type 2 is usually unilateral and is also characterized by a high frequency of cilioretinal arteries.
Usually asymptomatic. Some patients may notice enlargement of the physiological blind spot. Visual loss is rare; if present, other causes should be investigated.
Usually asymptomatic, but a high cup-to-disc ratio or large optic disc can be mistaken for glaucoma. Specialized tests are needed to definitively rule out glaucoma, and if incidentally noted, it is important to undergo a thorough ophthalmologic evaluation.
The etiology is currently unknown. The following hypotheses have been proposed.
Genetic factors have also been suggested. Reports of occurrence in monozygotic twins and a high prevalence in the Marshall Islands (22 of 54 eyes had a disc diameter >2.10 mm, and 36 individuals had a cup-to-disc ratio >0.6) suggest a genetic predisposition.
The most important aspect in diagnosing MP is reliable differentiation from glaucoma. For large optic discs, assessment of the cup-to-disc ratio (C/D ratio) must take disc size into account, and it is not appropriate to evaluate glaucoma risk based solely on the C/D ratio 1).
In quantitative optic disc evaluation using HRT (Heidelberg Retina Tomograph), the key differentiating point in MP is that the neuroretinal rim area and rim volume remain normal 2).
A comparative HRT study of 50 eyes with pediatric MP and 80 normal eyes yielded the following results.
| Parameter | MP group | Normal group | p value |
|---|---|---|---|
| Rim area (mm²) | 1.96±0.36 | 1.90±0.22 | 0.25 (not significant) |
| Rim volume (mm³) | 0.15±0.07 | 0.14±0.07 | 0.48 (not significant) |
The absence of significant differences in optic disc margin area and volume is an important basis for differentiating from glaucoma.
Measurement of peripapillary RNFL (pRNFL) thickness by OCT shows normal to increased values in MP 2).
In a comparison between pediatric MP and normal groups, the mean RNFL thickness was significantly thicker in the MP group (117.34±11.88 μm) than in the normal group (106.83±13.48 μm, p<0.01). The absence of RNFL thinning is evidence against glaucoma.
Additionally, a comparison of 39 pediatric MP patients and 39 adult MP patients showed that pediatric MP patients tend to have larger optic disc margin areas and smaller cupping than adult MP patients.
Enlargement of the physiological blind spot may be observed. In principle, peripheral visual field defects are not present; if glaucomatous visual field defects are found, the possibility of coexisting glaucoma should be considered.
It is important to differentiate MP from diseases that present similar findings.
The key differentiating points are that HRT shows normal rim area and rim volume, and OCT shows normal to increased RNFL. In glaucoma, rim area decreases and RNFL thins, so combining these tests is important for evaluation.
There is no specific treatment for MP. It is a benign, non-progressive disease that does not cause visual impairment, but continued differentiation from glaucoma is the mainstay of management.
Treatment for MP itself is not required. However, regular visits and examinations are necessary to continue differentiating it from glaucoma. If visual field changes or RNFL thinning occur during follow-up, management should be reconsidered considering the possibility of glaucoma.
The pathophysiology of MP is not fully understood. The following mechanisms have been proposed.
The finding that the optic disc rim area and rim volume are maintained normally indicates that MP is a condition without loss of optic nerve fibers, essentially different from glaucoma.
MP is non-progressive, and progressive loss of optic nerve fibers does not occur. Visual function is maintained in appropriately managed cases.
A study comparing 39 children with MP and 39 adults with MP showed that children tend to have larger optic disc rim areas and smaller cupping compared to adults. Although age-related morphological changes can occur, the condition is not considered to progress.
In long-term management, pay attention to the following points.
