Chiasmal disorder is a general term for conditions in which slowly progressive visual acuity and visual field defects occur due to compressive lesions of the optic chiasm. Ischemia as well as compression is suggested to be involved. In adults, pituitary adenomas are common; in children, craniopharyngiomas are common.
The optic chiasm is located in the suprasellar cistern, almost directly above the pituitary gland. The left and right optic nerves merge, and fibers from the nasal half of the retina cross to the contralateral optic tract. Fibers from the temporal half of the retina do not cross and travel to the ipsilateral optic tract. Therefore, damage to the optic chiasm results in a characteristic visual field pattern (bitemporal hemianopia).
Slow progression is typical, but acute onset can occur in pituitary apoplexy or trauma. Early recognition of chiasmal damage and appropriate treatment according to the cause are important for preserving visual function.
A chiasmal lesion is a general term for a group of diseases in which the optic chiasm is damaged by compression, inflammation, trauma, ischemia, etc., resulting in decreased visual acuity and visual field defects (typically bitemporal hemianopia). Causes are diverse and include compressive (pituitary adenoma, craniopharyngioma, aneurysm), inflammatory (optic neuritis of the chiasm, lymphocytic hypophysitis), and traumatic types.
Chiasmal lesions are broadly classified into three categories based on their cause.
| Classification | Representative Diseases |
|---|---|
| Compressive | Pituitary adenoma (most common in adults), craniopharyngioma (most common in children), internal carotid-ophthalmic artery bifurcation aneurysm (middle-aged women), optic glioma (associated with NF1) |
| Inflammatory | Optic neuritis at the chiasm (associated with MS), lymphocytic hypophysitis, chiasmatic arachnoiditis |
| Traumatic | Traumatic chiasmal syndrome |
Compressive lesions often progress slowly. Pituitary adenomas compress the crossing fibers of the optic chiasm from below. Craniopharyngiomas are often located above the optic chiasm and may cause different patterns of visual field defects. Giant aneurysms at the internal carotid-ophthalmic artery bifurcation compress the optic chiasm from the lateral side, causing junctional scotomas or paracentral scotomas.
Inflammatory lesions are typically represented by chiasmal optic neuritis, which is associated with demyelinating diseases (MS, NMO, MOGAD). Chiasmal lesions are found in approximately 20% of AQP4 antibody-positive NMOSD and about 16% of MOG antibody-positive optic neuritis. MOG-related cases are reported to often present with longitudinally extensive optic nerve lesions1. Lymphocytic hypophysitis is an autoimmune disease accompanied by enlargement of the entire pituitary gland.
Traumatic chiasmal syndrome occurs after head trauma and develops due to direct damage to the optic chiasm or indirect blood flow disturbance. It is often seen in cases with frontal bone trauma or anterior skull base fractures, and visual prognosis varies greatly depending on the case2.
Pituitary adenoma is the most common cause of chiasmal compression in adults. Non-hormone-producing tumors are often discovered due to visual impairment, while prolactin-producing adenomas tend to be detected earlier due to hormonal symptoms (amenorrhea, galactorrhea, hypogonadism).
They are broadly classified into three categories: compressive (pituitary adenoma, craniopharyngioma, internal carotid artery aneurysm, optic glioma), inflammatory (chiasmal optic neuritis, lymphocytic hypophysitis, chiasmal arachnoiditis), and traumatic (traumatic chiasmal syndrome). In adults, pituitary adenoma is the most common, while in children, craniopharyngioma is relatively more frequent.
Bitemporal hemianopia respecting the vertical meridian is the characteristic visual field pattern of chiasmal lesions. However, complete symmetric bitemporal hemianopia is rare; asymmetric incomplete hemianopia is more common.
Initial visual field changes begin with bitemporal superior quadrantanopia respecting the vertical meridian. This is because when a pituitary adenoma compresses the optic chiasm from below, it first affects the crossing fibers running inferiorly (corresponding to the superior temporal visual field).
Differences in visual field patterns by causative disease are as follows:
Pituitary adenoma (typical)
Bitemporal hemianopia: Starts from the superior temporal quadrant and progresses.
Junctional scotoma: Lesion at the anterior angle of the optic chiasm causes a central scotoma in one eye with a contralateral superior temporal defect.
Internal carotid artery aneurysm
Junctional scotoma: Caused by compression of the optic chiasm from the temporal side.
Paracentral scotoma: May begin as a slowly progressive paracentral scotoma.
Optic chiasm neuritis
Bitemporal hemianopia: Caused by inflammatory lesions of the optic chiasm.
Junctional scotoma: Anterior chiasmal optic neuritis may present with a junctional scotoma.
In the chronic phase, a characteristic fundus finding called bow-tie atrophy is observed. The optic disc becomes pale in the temporal and nasal sectors, while the superior and inferior portions are relatively preserved. This pattern results from selective damage to crossing fibers, leading to atrophy of the corresponding ganglion cell axons.
Papilledema is rare. If there is a difference in visual field defects between the two eyes, a relative afferent pupillary defect (RAPD) may be positive.
When hemorrhage or necrosis occurs in a pituitary adenoma, it can present acutely. Severe headache, rapid vision loss, and ocular motor disturbances (diplopia due to cranial nerve III, IV, and VI palsies) appear acutely. Rarely, it may be accompanied by impaired consciousness and requires urgent management.
Damage to the anterior angle (junction) of the optic chiasm causes junctional scotoma (central scotoma in one eye + contralateral superior temporal defect). Internal carotid artery aneurysms may begin with paracentral scotoma. Lateral chiasmal lesions rarely cause homonymous visual field defects (nasal field defects) due to involvement of non-crossing fibers. Visual field patterns vary depending on the site and direction of compression.
Using a Goldmann perimeter or Humphrey automated perimeter, confirm visual field defect patterns that respect the vertical meridian. A sharp vertical border of the visual field defect is a diagnostic feature of chiasmal lesions and is important for differentiation from occipital lobe lesions (homonymous hemianopia).
Regular visual field testing is essential for assessing disease progression and evaluating treatment efficacy.
Contrast-enhanced MRI is the most important test for confirming a diagnosis of chiasmal lesions. It evaluates the direction of tumor extension, the positional relationship with the optic chiasm, and signal characteristics. Coronal imaging is particularly useful for understanding anatomical relationships.
CT is performed as an adjunct. In craniopharyngiomas, calcification may be observed, and CT can be useful in such cases.
OCT can detect mild optic atrophy with higher sensitivity than fundus examination. It allows quantitative assessment of localized thinning of the inner retinal layers (RNFL, GCL) and can detect patterns corresponding to band atrophy at an early stage. It has been reported that the pattern of GCC thinning corresponds to visual field defects, and that preoperative GCC thickness correlates with postoperative visual field prognosis 3. Furthermore, loss of asymmetry in macular GCC between the eyes may be an early sign suggesting chiasmal compression before obvious thinning appears 4. OCT is also useful for pre- and postoperative monitoring, and retinal thickness measurements by OCT help estimate prognosis after treatment.
Endocrine testing is essential for differentiating hormone-producing adenomas. Measure prolactin (PRL), GH, IGF-1, ACTH, cortisol, TSH, FT4, and LH/FSH. In prolactinomas, PRL is markedly elevated (over 200 ng/mL), which helps distinguish it from mild elevation due to pituitary stalk compression.
OCT can quantitatively evaluate thinning of the retinal nerve fiber layer (RNFL) and ganglion cell layer (GCL), detecting mild optic atrophy earlier than fundus examination. It is also useful for confirming the band atrophy pattern (selective thinning of the temporal and nasal quadrants) characteristic of chiasmal disorders. Additionally, serial evaluation before and after surgery helps estimate the prognosis of visual function recovery after decompression.
Except for prolactinomas, surgery is the first-line treatment.
Surgical Therapy
経蝶形骨洞手術（transsphenoidal surgery）が標準アプローチである。鼻腔から蝶形骨洞を経由して腫瘍に到達し、視交叉への圧迫を解除する。近年は内視鏡下手術も広く行われる。圧迫解除により視力・視野の改善が期待できる。ただし、すでに明らかな視神経萎縮が確立している場合は視機能予後が不良となる。RNFL厚が正常〜境界域で視野障害が半盲以内にとどまっているうちに早期減圧することが、十分な視野回復のために重要である5。
薬物療法（プロラクチノーマ）
ドパミンアゴニスト内服が治療の中心である。ブロモクリプチン（開始量 1.25 mg/日、維持量 5〜7.5 mg/日）またはカベルゴリン（週 0.25〜2 mg）を使用する。80〜90%のプロラクチノーマで腫瘍縮小とホルモン正常化が達成可能とされる。
放射線療法
手術後残存・再発腫瘍に対する補助治療として定位放射線治療（ガンマナイフ等）が選択肢となる。
Surgery ± radiotherapy is the mainstay. Craniopharyngiomas often adhere strongly to the optic chiasm, making total resection difficult, so postoperative radiotherapy is frequently combined. A meta-analysis in adults reported that while total resection significantly reduces recurrence risk, it increases the risk of panhypopituitarism and permanent diabetes insipidus, and there was no significant difference in visual improvement rates between total and partial resection 6.
Endovascular treatment (coil embolization) or clipping is performed by neurosurgeons.
Steroid pulse therapy (methylprednisolone 1,000 mg/day intravenously for 3 days) is the first-line treatment. In anti-aquaporin-4 (AQP4) antibody-positive cases unresponsive to steroids, plasma exchange may be performed. Long-term management based on the diagnosis of the underlying disease (MS, NMO, MOGAD) is important.
In the acute phase, steroid administration and observation are the mainstays. While expecting spontaneous recovery of visual acuity and visual field, the indication for neurosurgical intervention is determined.
Pituitary apoplexy is an emergency condition.
For prolactinomas, oral dopamine agonists (bromocriptine, cabergoline) are highly effective, achieving tumor shrinkage and prolactin normalization in 80-90% of cases. Since equivalent or better therapeutic effects can be obtained while avoiding surgical risks, medical therapy is the first choice. Other adenomas have lower responsiveness to drugs, so surgery is prioritized.
At the optic chiasm, fibers from the nasal half of each retina cross to the opposite side (crossing fibers). Fibers from the temporal half do not cross and remain on the same side (non-crossing fibers).
Pituitary adenomas compress the optic chiasm from below, selectively damaging the crossing fibers that run in the lower portion. Damage to crossing fibers causes temporal visual field defects (bitemporal hemianopia). Since fibers corresponding to the upper temporal visual field are affected first, visual field loss often begins in the upper temporal quadrant.
When the internal carotid artery compresses the optic chiasm from the lateral side, non-crossing fibers may be damaged, resulting in nasal visual field defects (rare).
When crossing fibers are persistently damaged, the temporal and nasal central quadrants of the optic disc, through which these fibers pass, selectively atrophy. The superior and inferior quadrants (which contain more non-crossing fibers) are relatively spared. This asymmetric atrophy forms the pattern of bow-tie atrophy.
In chiasmal optic neuritis (demyelinating), an autoimmune reaction against the myelin sheath damages nerve fibers at the optic chiasm. In AQP4 antibody-positive NMOSD, astrocytic aquaporin-4 channels are targeted, and inflammatory lesions tend to form in the posterior optic nerve, including the chiasm.
Ischemia is also suggested as a contributing factor in chiasmal damage, and compression of microvessels by tumors is thought to promote nerve injury.
When compression of the optic chiasm is relieved, visual acuity and visual field often improve. However, if optic atrophy is already evident, the visual prognosis is poor. This is due to irreversible loss of nerve fibers, and early treatment intervention is key to preserving visual function.
Retinal thickness measurement by OCT is useful for estimating prognosis after treatment. If GCL and RNFL thinning is severe before surgery, postoperative visual function recovery tends to be limited, and preoperative OCT findings help predict prognosis.
In chiasmal optic neuritis, idiopathic cases often have good visual recovery (most recover to visual acuity of 0.5 or better), but 40% reportedly transition to MS within 3 years, so long-term collaboration with neurology is important. See the article on “Chiasmal Optic Neuritis” for details.
In chiasmal compression due to pituitary adenoma, visual field improvement can be expected after transsphenoidal surgery, but if postoperative hypopituitarism occurs, hormone replacement therapy may be necessary.
If compression is relieved before optic atrophy is established, improvement in visual acuity and visual field can be expected. However, if band atrophy or severe GCL thinning is present preoperatively, visual function recovery is limited. Quantitative assessment of preoperative retinal inner layer atrophy by OCT is useful for estimating postoperative prognosis, and early diagnosis and treatment are most important for preserving visual function.
