Basic Neurosciences · Paper A
Visual field defects
The pattern of visual field loss localises a lesion along the visual pathway, from the optic nerve at the front to the occipital cortex at the back. For example, imagine a patient who loses the outer half of vision in BOTH eyes, that specific bitemporal pattern points to compression at the optic chiasm, classically a pituitary tumour, not a problem in either eye alone. In practice, decide whether the defect is monocular, bitemporal, or homonymous before naming the lesion site.
An optic nerve lesion causes monocular visual loss, affecting only the eye on that side. A chiasm lesion causes bitemporal hemianopia, because the nasal (crossing) fibres from each eye are compressed together at the chiasm. An optic tract lesion, beyond the chiasm, causes a contralateral homonymous hemianopia instead. For example, imagine identical right-sided field loss in both eyes: this cannot come from one optic nerve and instead localises behind the left side of the chiasm. In practice, this means the field defect's SHAPE, monocular, bitemporal, or a full homonymous half-field, tells you exactly where along the pathway the lesion sits. A separate, non-lesional cause of field loss is drug toxicity. Vigabatrin, an anti-epileptic used for refractory partial seizures and infantile spasms, is a well-recognised cause of irreversible peripheral visual field constriction, distinct from any single lesion site above. Patients on long-term vigabatrin need baseline and periodic visual field testing to detect this.
Vision starts at the retina in each eye and travels backward through a defined pathway: the optic nerve, the optic chiasm, the optic tract, the optic radiations, and finally the occipital cortex at the very back of the brain. The pattern of visual field loss a patient reports or demonstrates on examination reliably localises exactly where along this pathway a lesion sits, making visual field testing one of the most useful bedside tools in neurology. Imagine a patient who loses the outer half of their vision in both eyes simultaneously: that specific bitemporal pattern points to compression at the optic chiasm, classically caused by a pituitary tumour, rather than to a problem confined to either eye alone. The core skill in this topic is deciding whether a defect is monocular, bitemporal, or homonymous before you ever name a lesion site.
A visual field defect is simply a gap or area of loss in what a person can see, mapped out as a shape rather than described as vague "blurriness." Because the visual pathway is a long, ordered relay from eye to brain, the SHAPE of that gap acts like a signature for WHERE along the relay the damage sits. Learn the shapes, and you can localise a lesion before a scan is even ordered.
The trap most people fall into is treating "loss of vision" as one symptom with one cause. It isn't. The single most important discriminator is whether the loss affects one eye alone (monocular) or a matching pattern across both eyes (binocular, either bitemporal or homonymous), because that single distinction tells you whether the lesion is in front of the chiasm or behind it. Get this step wrong and every subsequent localisation collapses.
Each optic nerve carries signal from only one eye. At the optic chiasm, the nasal fibres (from the inner half of each retina) cross to the opposite side, while the temporal fibres (from the outer half of each retina) stay on the same side. Beyond the chiasm, everything is reorganised by which SIDE of the world the information came from, not which eye it came from. That's why a lesion before the chiasm gives an asymmetric, one-eye pattern, while a lesion at or after the chiasm gives a matched, both-eyes pattern.
A lesion of the optic nerve, between the eye and the chiasm, causes monocular visual loss (loss of vision affecting only the eye on the same side as the lesion), because that nerve has not yet joined the crossing point. Causes include optic neuritis, often investigated with MRI and associated with demyelinating conditions such as multiple sclerosis, and direct compressive lesions.
At the optic chiasm, a lesion compressing it from below or behind, classically a pituitary adenoma, selectively damages the crossing nasal fibres from both eyes, producing bitemporal hemianopia (loss of the outer, temporal, half of the visual field in both eyes).
Beyond the chiasm, in the optic tract, a lesion causes a contralateral homonymous hemianopia (loss of the same side, left or right, of the visual field in both eyes, on the side opposite the lesion). Consider a patient with identical right-sided field loss in both eyes: this cannot come from one optic nerve, since a nerve serves only one eye, so it localises to the left optic tract or further back on the left.
Further back still, the optic radiations split into two limbs. The portion, Meyer's loop, carries fibres from the LOWER visual field, so a temporal lesion here produces a superior quadrantanopia (loss of the upper quarter, opposite side). The portion carries fibres from the UPPER visual field, so a parietal lesion produces an inferior quadrantanopia (loss of the lower quarter).
Finally, a lesion of the occipital cortex, typically a posterior cerebral artery (PCA) stroke, produces a homonymous hemianopia much like an optic tract lesion, but classically with macular sparing (preservation of central vision), because the occipital pole receives a dual blood supply from both the posterior and middle cerebral arteries.
Several conditions mimic a "true" neurological visual field defect and need to be told apart at the bedside. Glaucoma causes a slowly progressive, patchy peripheral loss (arcuate scotomas expanding to tunnel vision) from raised intraocular pressure damaging the optic nerve head, not the visual pathway further back, and is picked up on fundoscopy and tonometry rather than by the hemianopia patterns above. Retinal detachment causes sudden monocular field loss, often described as a curtain coming down, and can be mistaken for an optic nerve lesion until fundoscopy shows the detached retina itself. Migraine with aura produces a transient, evolving visual disturbance (classically a spreading, shimmering scintillating scotoma) that resolves within an hour, unlike the fixed defects above. Functional (non-organic) visual loss produces a field defect that doesn't respect the expected anatomical boundaries, for example a "tunnel vision" pattern that stays the same size regardless of testing distance, which is a physical impossibility for a true organic lesion and a useful bedside clue.
Diagnosis starts with confrontation visual field testing at the bedside (comparing the patient's field to the examiner's, one eye at a time) as a quick screen, then formal automated or Goldmann perimetry (a device that maps the exact boundary of the visual field, quadrant by quadrant) to confirm and document the precise shape of any defect. Fundoscopy examines the optic disc and retina directly to exclude a purely ocular cause such as glaucoma or retinal detachment before assuming a neurological pathway problem. Once the shape of the defect points to a likely site, MRI imaging of the brain (or the pituitary region specifically, for a suspected chiasmal lesion) confirms the structural cause, following NICE-aligned referral pathways for suspected pituitary tumours or stroke.
Management always targets the underlying cause rather than the visual field defect as an isolated finding. A pituitary adenoma compressing the chiasm needs endocrine investigation, and depending on size, function, and growth, medical treatment, or surgical (transsphenoidal) referral, managed by a neuro-endocrine multidisciplinary team. A PCA stroke causing occipital field loss is managed with the standard acute stroke and secondary prevention pathway, in line with NICE guidance on stroke, since the visual defect itself typically cannot be reversed once established, though patients are supported with visual rehabilitation and safety advice, for example around driving. Optic neuritis is typically managed by treating any underlying demyelinating process and monitoring recovery, since vision often improves over weeks.
The single highest-yield fact is macular sparing: if a homonymous hemianopia SPARES central vision, the lesion is occipital (dual blood supply protecting the pole), almost always a PCA stroke; if central vision is also lost, think optic tract instead. Pair that with "bitemporal = chiasm = think pituitary" and you can answer most exam stems from the shape of the defect alone.
The shape of a visual field defect is a map of the visual pathway: monocular means optic nerve, bitemporal means chiasm (think pituitary), homonymous without macular sparing means optic tract, and homonymous WITH macular sparing means occipital cortex (think PCA stroke).