Phakomatous choristoma (PC) is a benign congenital tumor composed of ectopic lenticular tissue. It was first reported by Lorenz Zimmerman in 1971 and is also called “Zimmerman tumor.”
A choristoma is a mass of normal tissue in an abnormal location, distinct from a true tumor. It is congenital but becomes clinically significant when it causes symptoms as a space-occupying lesion. PC is a specific type of choristoma containing lens tissue.
PC is an extremely rare disease, with only about 20 cases reported to date 2). All cases have been discovered in infancy, with age at surgical resection ranging from 2 weeks to 13 months (mean approximately 5 months). It most commonly appears on the nasal side of the lower eyelid and extends inferomedially into the orbit. Occurrence in the upper eyelid has not been reported 1).
It is an extremely rare disease with only about 20 cases reported worldwide. All cases have been discovered in infancy (2 weeks to 13 months of age).
PC is recognized at birth as a hard palpable mass. Since the patient is an infant, it is difficult to assess subjective symptoms, but it is discovered when caregivers notice a eyelid mass or asymmetry.
The tumor can induce corneal astigmatism or anisometropia, leading to amblyopia. Early surgical intervention is recommended to avoid the risk of amblyopia.
PC arises from abnormalities in the lens formation process during embryonic development.
In normal development, the surface ectoderm invaginates to form the lens pit, followed by the lens vesicle. Abnormal migration of lens tissue during this process is thought to cause ectopic proliferation of lens tissue around the eye, forming PC. Since the eyelid skin is also derived from the surface ectoderm, it is similarly susceptible.
As a general mechanism of dermoid occurrence, it is known that dermoid cysts and dermoid tumors form from ectodermal tissue left behind in the mesoderm during the formation of the optic cup.
PC is a childhood disease, and all cases are discovered in infancy. No heritability has been reported.
A definitive diagnosis of PC requires histopathological examination after surgical excision.
Routine H&E staining (hematoxylin and eosin) reveals structures resembling the lens capsule, lens epithelium, and lens cortex. Characteristic findings include pseudoglandular structures and aggregates of cuboidal epithelial cells surrounded by a thick basement membrane that is positive for periodic acid–Schiff (PAS) staining, along with peripheral anterior synechiae 1).
Basic Staining
H&E staining: Confirms structures resembling the lens capsule, epithelium, and cortex. This is the basis of diagnosis.
PAS staining: Highlights the lens capsule and epithelial islands. Useful for evaluating basement membrane structures.
Immunohistochemistry
Vimentin, S-100, crystallin: Positive. Confirms proteins specific to lens tissue 3).
Cytokeratin: Negative. Useful for differentiation from other epithelial tumors 1).
NSE (neuron-specific enolase): Positive. An enzyme active during early lens epithelial development 4).
Immunohistochemistry (IHC) staining is usually not required for diagnosis but may be performed to demonstrate origin from the lenticular anlage. Staining for GFAP, HMB45, synaptophysin, chromogranin, etc., has also been reported.
MRI and CT are not essential for definitive diagnosis but are useful for differentiation from other orbital and periorbital lesions.
The characteristic imaging findings of PC are shown below.
| Finding | PC | Dermoid cyst | Hemangioma |
|---|---|---|---|
| Nature | Solid | Cystic | Vascular |
| Calcification | None | Present | None |
| Enhancement pattern | Homogeneous enhancement | Wall enhancement only | Vascular enhancement |
The following pediatric periorbital tumors are considered in the differential diagnosis of PC.
Definitive diagnosis of PC requires histopathological examination after resection; imaging alone cannot confirm it. However, MRI and CT are useful for differentiating from other orbital lesions such as dermoid cysts, hemangiomas, and rhabdomyosarcoma.
Surgical excision of the tumor is the only curative treatment. There are no reports of spontaneous regression, and no effective medical therapy exists.
Since surgery is performed on infants, general anesthesia is used. If the tumor induces corneal astigmatism, early surgical intervention is justified to prevent amblyopia. In cases of corneal astigmatism, treatment to improve visual acuity through astigmatism correction is also performed both before and after surgery.
The prognosis for PC is extremely favorable.
To date, no recurrences have been reported during follow-up periods of up to 7.5 years. Even in cases of incomplete resection, no recurrence has been observed.
The occurrence of PC is based on an abnormality in lens formation during the embryonic period.
In normal eye development, the surface ectoderm invaginates under the induction of the optic cup, first forming the lens pit. The lens pit then closes to form the lens vesicle, which eventually differentiates into the lens. PC is hypothesized to result from abnormal migration of lens tissue during this process, leading to ectopic proliferation around the eye, particularly on the nasal side of the lower eyelid.
Regarding dermoids in general, it is known that dermoid cysts and dermoid tumors can form from ectodermal tissue left behind in the mesoderm during optic cup formation, and PC is also positioned within the framework of similar developmental abnormalities.
Histologically, PC has structures resembling the following three lens components:
The lens primordium origin of these structures is supported by the expression of vimentin (an intermediate filament normally found in lens epithelium), S-100 protein, and α, β, γ crystallins (lens-specific proteins)3).
Inouye et al. (2023) reported a case where a lesion on the conjunctival surface of the upper eyelid after cataract surgery was misdiagnosed as PC 1). An 88-year-old woman developed a yellowish-white lesion on the upper eyelid two years after phacoemulsification. Histopathological examination revealed lens-like protein material and basement membrane fragments, initially suggesting PC. However, considering the patient’s age (PC is a disease of infants), the location (PC occurs on the nasal side of the lower eyelid, not the upper eyelid), and the history of cataract surgery, the final diagnosis was implantation of a lens capsule fragment into the conjunctiva during anterior capsulotomy.
This report indicates that the histopathological features of PC can resemble residual lens material after cataract surgery, emphasizing the necessity of integrating clinical information (patient age, location, surgical history) for diagnosis.
Jung et al. (2022) conducted a case report and literature review of PC, summarizing 20 previously published reports 2). They systematically organized the clinical features, histological characteristics, and immunohistochemical findings of PC, contributing to the establishment of a diagnostic algorithm for this disease.
