The Axenfeld nerve loop is an anatomical variation in which the long posterior ciliary nerves form a loop-like anastomosis within the sclera. It was first reported by Axenfeld in 1902.
It is also referred to as “intrascleral nerve loops.” It is not a disease with pathological significance but is considered a natural anatomical landmark.
It is a relatively common incidental finding on slit-lamp examination, and because it can be mistaken for an intrascleral foreign body, it is important to accurately understand its characteristics.
It is an incidental finding on slit-lamp examination and is not necessarily rare. However, because it can resemble intrascleral foreign bodies, nevi, malignant melanomas, or cysts, accurate identification is clinically important.
Usually, there are no subjective symptoms. It is often asymptomatic and discovered incidentally during an ophthalmic examination.
However, when this loop or the conjunctiva covering it is manipulated during examination of trauma, the patient may feel pain. This is thought to be because nerve tissue is directly involved.
Macroscopic Findings
Color: Observed as a gray to white nodule.
Pigmentation: Especially in individuals with dark iris color, pigment may be present around the loop.
Location: Located under the bulbar conjunctiva. Often found within 3–4 mm of the limbus.
Morphological Features
Shape: Observed as a small nodular elevation.
Borders: Relatively well-defined borders.
Mobility: May have some mobility depending on its relationship with the conjunctiva.
Histopathologically, findings consistent with peripheral nerve are observed. The structure consists of neural tissue within the emissary canal that penetrates the sclera.
In individuals with dark iris pigmentation, surrounding pigmentation may occur, but this finding alone does not suggest malignancy. However, differentiation from nevus or malignant melanoma is important, and observation of morphology and course is necessary. See the “Diagnosis and Examination Methods” section for details.
Axenfeld nerve loop is a normal anatomical variant and has no specific cause or risk factor.
The long posterior ciliary nerves penetrate the sclera near the optic nerve and run within the sclera and the suprachoroidal space. They follow this path until they branch at the level of the ciliary body.
A nerve loop is an anastomotic site where the long ciliary nerve once turns into the sclera and then reverses to head back to the ciliary body. An emissary canal penetrating the sclera and a structure located above the pars plana are observed.
Diagnosis is primarily based on visual inspection using slit lamp examination. The characteristic finding is a gray to white small nodule observed under the bulbar conjunctiva within 3–4 mm of the corneal limbus.
Clinically important is the differentiation from intrascleral foreign bodies, nevi, malignant melanoma, and cysts.
Differentiation from the following diseases is necessary.
| Disease | Key Differentiating Points |
|---|---|
| Scleral foreign body | History of trauma, opacity of foreign body |
| Nevi | Uniform pigmentation, border shape |
| Malignant melanoma | Enlargement, irregular pigmentation, elevation |
| Cysts | Translucent, fluid content |
In suspicious cases, it is important to confirm morphological changes through follow-up observation. If enlargement, color change, or irregular margins are observed, further examination is required.
Histopathological examination reveals findings consistent with peripheral nerves, which is useful for definitive diagnosis. Nerve fibers and myelin components are confirmed.
If typical findings (gray-white small nodules within 3–4 mm of the corneal limbus) are present, a clinical diagnosis is possible. If malignancy or foreign body cannot be ruled out, histopathological examination is useful for definitive diagnosis.
Axenfeld nerve loop is a normal anatomical variant and does not require treatment.
If found, observation is the basic policy. When explaining to the patient, it is important to convey that this variant is a benign anatomical finding that does not require treatment, avoiding unnecessary anxiety.
The long posterior ciliary nerve enters the sclera near the optic nerve, travels within the sclera toward the ciliary body. During its course, the nerve may change direction anteriorly within the sclera and exit the sclera again, forming a loop structure. This is the anatomical basis of the Axenfeld nerve loop.
This loop lies within an emissary canal that penetrates the sclera, located above the pars plana of the ciliary body. Due to individual variations in nerve course, the size and position of the loop differ among cases.
Several case reports exist of anterior nerve sheath tumors arising from intrascleral nerve loops. Chang and Glasgow (2009) reported evidence that anterior scleral nerve sheath tumors originate from the Axenfeld nerve loop. This suggests that this site, where nerve sheath tissue is present, can rarely become a starting point for tumor formation.
