A scotoma is a localized area of reduced or absent vision within the overall visual field, surrounded by normal vision. The physiological blind spot (corresponding to the optic disc) is the only normal scotoma. Pathological scotomas can be central (affecting fixation, seen in macular disease or optic neuritis), paracentral (near fixation, common in early glaucoma), or peripheral (from retinal or nerve fiber damage).

What visual field defect pattern is characteristic of glaucoma?

Glaucoma produces arcuate (nerve fiber bundle) defects that follow the arc of the superior or inferior retinal nerve fiber bundles from the optic disc to the horizontal meridian. Early glaucoma may show a nasal step or isolated paracentral scotoma. As the disease progresses, a full Bjerrum scotoma forms, and advanced glaucoma leaves only a central island or temporal remnant of vision.

What does a bitemporal hemianopia suggest?

A bitemporal hemianopia (loss of the outer temporal half of the visual field in both eyes) indicates a lesion compressing the optic chiasm, where the crossing nasal retinal fibers meet. The most common cause is a pituitary adenoma, but other chiasmal lesions such as craniopharyngioma, meningioma, or glioma can produce this pattern.

How does a homonymous hemianopia differ from bitemporal hemianopia?

In homonymous hemianopia, the same half of the visual field is lost in both eyes -- for example, both right hemifields or both left hemifields. This indicates a post-chiasmal lesion (optic tract, optic radiations, or occipital cortex), most commonly a stroke. In bitemporal hemianopia, the outer halves of both fields are lost, which indicates a chiasmal lesion.

Why does a visual field defect in only one eye indicate a pre-chiasmal lesion?

At the optic chiasm, fibers from both eyes merge. Any lesion behind the chiasm affects visual information from both eyes simultaneously (producing bilateral field defects). Only a lesion anterior to the chiasm -- in the retina or optic nerve -- can produce a defect confined to a single eye, because at that point the visual pathway has not yet combined information from both eyes.

Visual Field Defect Patterns for the CPOA Exam

Why Field Defect Patterns Matter

The pattern of a visual field defect provides critical information about where along the visual pathway a lesion has occurred. The CPOA does not diagnose, but recognizing major defect patterns helps you understand why certain tests are ordered, communicate accurately with the doctor about findings, and recognize when a patient reports alarming symptoms that require urgent escalation.

The Visual Pathway: A Brief Map

Understanding defect patterns requires knowing the basic anatomy of the visual pathway:

Retina -- where light is detected
Optic nerve -- carries signals from each eye
Optic chiasm -- where the nasal fibers cross to the opposite side
Optic tract -- carries crossed and uncrossed fibers to the brain
Lateral geniculate nucleus (LGN) -- relay station in the thalamus
Optic radiations -- fan out from LGN to occipital cortex
Primary visual cortex (V1) -- in the occipital lobe

Lesions at different points produce characteristic defect patterns tied to which fibers are damaged.

Major Defect Patterns

Scotoma

A scotoma is an area of diminished or absent vision surrounded by normal field. Types include:

Central scotoma: Affects the central field (fixation). Causes include macular disease (AMD, macular hole), optic neuritis, and toxic optic neuropathies.
Paracentral scotoma: Adjacent to fixation. Common in early glaucoma and optic neuropathies.
Physiological blind spot (normal): Every eye has a blind spot corresponding to the optic disc. Enlargement of the blind spot suggests papilledema or optic disc drusen.

Arcuate (Nerve Fiber Bundle) Defect

An arcuate defect follows the arc of the superior or inferior retinal nerve fiber bundles from the optic disc to the horizontal meridian. It is the hallmark defect of glaucoma. Types:

Bjerrum scotoma: A classic arcuate scotoma of early-to-moderate glaucoma, running from the blind spot in an arc above or below fixation.
Nasal step: An arcuate defect that stops at the nasal horizontal meridian, creating a step.
Advanced glaucoma: Superior and inferior arcuate defects may both be present, leaving only a temporal island of central vision.

Altitudinal Defect

An altitudinal defect affects either the upper or lower half of the visual field while respecting the horizontal meridian. Causes include:

Anterior ischemic optic neuropathy (AION -- infarction of the optic nerve head)
Branch retinal artery occlusion
Advanced glaucoma (bilateral altitudinal defects)

Hemianopia

A hemianopia affects exactly half the visual field, respecting the vertical midline. It is almost always caused by a lesion at or behind the optic chiasm:

Homonymous hemianopia: The same half of the field is lost in both eyes (e.g., both right hemifields). Indicates a lesion in the optic tract, LGN, optic radiations, or occipital cortex -- i.e., post-chiasmal damage, often from a stroke.
Bitemporal hemianopia: The outer (temporal) halves of both visual fields are lost. Indicates compression at the optic chiasm where the crossing nasal fibers are damaged -- classic pattern for a pituitary tumor.

Quadrantanopia

Loss of one quadrant (quarter) of the visual field in both eyes (homonymous). Often results from optic radiation lesions: superior quadrantanopia ("pie in the sky") from temporal lobe lesions; inferior quadrantanopia from parietal lobe lesions.

Defect	Location Affected	Common Cause
Central scotoma	Fixation area	Macular disease, optic neuritis
Arcuate defect	Nerve fiber bundles	Glaucoma
Altitudinal defect	Upper or lower half	AION, branch RVO
Bitemporal hemianopia	Both temporal fields	Chiasmal compression (pituitary)
Homonymous hemianopia	Same half both eyes	Stroke, cortical lesion
Quadrantanopia	One quarter both eyes	Temporal/parietal lobe lesion

🔑 Key Point: If a field defect respects the vertical meridian (is hemianopic), the lesion is at or behind the chiasm. If the defect respects the horizontal meridian (is altitudinal), the lesion is more likely at the optic nerve or retina. If the defect is in one eye only, it is pre-chiasmal.

⚠️ Common Mistake: Confusing homonymous and bitemporal hemianopia. Homonymous = same side in both eyes (e.g., right field lost in both eyes). Bitemporal = outer (temporal) half lost in both eyes. Bitemporal points to the chiasm; homonymous points to behind the chiasm.

Key Takeaways

Visual field defect patterns map to lesion locations along the visual pathway.
Arcuate (nerve fiber bundle) defects are the hallmark of glaucoma.
Altitudinal defects (respecting horizontal meridian) suggest optic nerve or retinal ischemia.
Bitemporal hemianopia (both outer fields lost) = chiasmal compression, most often pituitary tumor.
Homonymous hemianopia (same half-field lost in both eyes) = lesion at or behind the chiasm, most often stroke.
Defects in only one eye are always pre-chiasmal (retina or optic nerve).