Is a visual field defect with nasal loss in the right eye and temporal loss in the left eye a heteronymous (bitemporal) defect, and what are the appropriate next diagnostic steps?

This is NOT a Heteronymous (Bitemporal) Defect

The visual field pattern described—nasal loss in the right eye and temporal loss in the left eye—represents a homonymous hemianopia (specifically left homonymous hemianopia), not a heteronymous bitemporal defect. This is a critical distinction that fundamentally changes the diagnostic approach and localization.

Understanding the Pattern

A heteronymous bitemporal defect would show:

• Temporal loss in BOTH eyes (respecting the vertical midline)
• Indicates chiasmal pathology (e.g., pituitary adenoma, craniopharyngioma)

Your described pattern shows:

• Right eye: nasal loss
• Left eye: temporal loss
• This means both defects are on the LEFT side of the visual field
• This is a LEFT homonymous hemianopia indicating RIGHT-sided retrochiasmatic pathology 1

Anatomical Localization

The retrochiasmatic visual pathway includes 1:

• Optic tracts
• Lateral geniculate nucleus
• Optic radiations
• Primary visual cortex (occipital lobe)

Visual acuity typically remains relatively intact with retrochiasmatic lesions 1.

Most Likely Etiologies (in order of frequency)

1. Ischemic stroke (most common cause) 1

   • Posterior cerebral artery territory → occipital lobe
   • Middle cerebral artery territory → optic radiations

2. Parenchymal hemorrhage

3. Brain tumor

4. Trauma

5. Less common: venous sinus thrombosis, demyelinating disease, neurosurgical complications 1

Immediate Diagnostic Steps

Acute presentation (hours to days):

• CT head without contrast initially to rule out hemorrhage
• MRI brain without and with contrast is the definitive study 2
• Consider stroke protocol if symptom onset is acute

Subacute/chronic presentation:

• MRI brain without and with contrast is the preferred initial imaging 2
• No need for dedicated orbital imaging since the lesion is retrochiasmatic 2

Critical Pitfall to Avoid

Do not confuse this with bitemporal hemianopia and order pituitary-focused imaging. The pattern you described localizes POSTERIOR to the chiasm, not AT the chiasm. Ordering sellar/suprasellar imaging would be inappropriate and delay correct diagnosis 1, 2.

Additional Considerations

If vascular etiology is suspected based on imaging, consider:

• CTA or MRA for vascular characterization
• Evaluation per stroke protocols if acute 2

The acuity of symptom onset is crucial—patients may not immediately recognize a hemianopia, so careful history about when the deficit was first noticed versus when it actually occurred is essential 1.