Hemianopia

Hemianopia (also spelled hemianopsia) is a neurological visual disorder characterised by the loss of vision in half of the visual field of one or both eyes. The condition results from damage to the visual pathways in the brain rather than to the eyes themselves. Depending on the site of the lesion, vision loss may affect the same side of both eyes or different sides, leading to various forms of field defects. Hemianopia is commonly associated with stroke, brain injury, or tumours that affect the visual cortex or optic pathways.

Anatomy of the Visual Pathway

To understand hemianopia, it is important to consider the anatomy of the visual system. The visual pathway begins with the retina in each eye, where light is converted into neural signals. These signals travel through the following structures:

1. Optic nerve — carries visual information from each eye.
2. Optic chiasma — the point where fibres from the nasal (inner) half of each retina cross to the opposite side, allowing visual information from the right visual field to be processed by the left hemisphere and vice versa.
3. Optic tract — carries information from the optic chiasma to the lateral geniculate nucleus (LGN) of the thalamus.
4. Optic radiations — transmit signals from the LGN to the primary visual cortex in the occipital lobe of the brain.

Damage to any part of this pathway can result in characteristic patterns of visual field loss.

Types of Hemianopia

Hemianopia is classified according to the portion of the visual field affected and the anatomical site of the lesion.
1. Homonymous Hemianopia

• Loss of the same half (right or left) of the visual field in both eyes.
• Caused by lesions posterior to the optic chiasma (optic tract, optic radiations, or occipital cortex).
• Example: A lesion in the left occipital lobe causes right homonymous hemianopia.

2. Heteronymous Hemianopia

• Loss of opposite halves of the visual field in each eye.
• Results from damage at the optic chiasma, where nasal fibres cross.
• Subtypes include:
  • Bitemporal hemianopia: Loss of the outer (temporal) halves of both visual fields, typically caused by a pituitary tumour compressing the centre of the optic chiasma.
  • Binasal hemianopia: Loss of the inner (nasal) halves of both visual fields, usually rare and associated with bilateral lateral compression of the optic chiasma by carotid artery aneurysms or lesions.

3. Quadrantanopia (Quadrantanopsia)

• Loss of vision in one-quarter of the visual field of each eye.
• Caused by partial lesions of the optic radiations or occipital cortex.
  • Superior quadrantanopia (“pie in the sky”) results from temporal lobe lesions affecting Meyer’s loop.
  • Inferior quadrantanopia (“pie on the floor”) arises from parietal lobe lesions.

Causes of Hemianopia

Hemianopia can arise from a variety of neurological and vascular conditions that disrupt the visual pathways. Common causes include:

• Cerebrovascular accidents (stroke): The most frequent cause, especially when the occipital lobe or posterior cerebral artery is involved.
• Traumatic brain injury (TBI): Damage to the occipital or parietal lobes may lead to partial or complete field loss.
• Brain tumours: Particularly pituitary adenomas, craniopharyngiomas, or occipital lobe tumours.
• Multiple sclerosis: Demyelination of the optic pathways can produce field defects.
• Infections and inflammation: Encephalitis or abscesses may affect the visual cortex or optic radiations.
• Surgical or vascular lesions: Aneurysms, arteriovenous malformations, or postoperative complications can disrupt visual processing areas.

Symptoms and Clinical Presentation

Patients with hemianopia may experience a range of symptoms depending on the extent and location of vision loss:

• Loss of vision in one half or quadrant of the visual field.
• Difficulty reading, navigating, or recognising objects on the affected side.
• Bumping into obstacles or failing to notice people or objects.
• Visual neglect (inattention to one side of space), particularly when the lesion is in the right hemisphere.
• Impaired depth perception and reduced awareness of surroundings.

In some cases, individuals are unaware of their field defect — a phenomenon known as anosognosia.

Diagnosis

Diagnosis of hemianopia involves a combination of clinical evaluation and neuroimaging.
1. Visual Field Testing:

• Confrontation test: A simple bedside screening method.
• Perimetry (Automated or Goldmann): Quantitative mapping of visual field defects, which helps identify the exact pattern and side of vision loss.

2. Neuroimaging:

• Magnetic Resonance Imaging (MRI): Detects structural lesions such as infarcts, tumours, or demyelinating plaques.
• Computed Tomography (CT) scan: Useful in acute cases, especially for detecting haemorrhage or trauma.

3. Neurological Assessment: A full neurological examination helps localise the lesion and determine associated deficits (e.g., speech or motor impairments).

Management and Rehabilitation

While the underlying cause of hemianopia determines specific treatment, vision restoration is often limited, as brain tissue does not regenerate easily. However, various strategies can improve functional vision and quality of life.
1. Treatment of Underlying Cause:

• Stroke management: Use of anticoagulants, thrombolytics, or rehabilitation therapy.
• Tumour or aneurysm treatment: Surgical removal, radiation therapy, or endovascular repair.
• Inflammatory or demyelinating conditions: Corticosteroids and disease-modifying therapies.

2. Visual Rehabilitation:

• Compensatory strategies: Training to increase scanning and head movements toward the blind side.
• Prism lenses: Optical devices that shift images from the blind to the seeing field.
• Vision restoration therapy (VRT): Computer-based programmes that stimulate neural plasticity at the visual field border.
• Occupational therapy: Helps patients adapt to everyday tasks and regain independence.

3. Lifestyle Adjustments:

• Marking obstacles and doorways clearly in the home.
• Using audio signals or tactile markers for navigation.
• Avoiding driving until vision has been fully assessed and legal requirements are met.

Prognosis

The prognosis for hemianopia depends on the extent of brain damage and timing of intervention. Some patients show partial or complete recovery of vision, particularly within the first six months following a stroke or injury, due to neuroplastic adaptation. Others may experience permanent visual deficits requiring lifelong compensation strategies.
Early diagnosis, neuro-ophthalmologic assessment, and targeted rehabilitation significantly enhance functional outcomes and help patients regain independence.

Clinical and Neurological Significance

Hemianopia provides important diagnostic clues to localise brain lesions within the visual pathway. The pattern of field loss often correlates precisely with the anatomical site of damage. For instance,

• Bitemporal hemianopia suggests a lesion at the optic chiasma.
• Homonymous hemianopia points to post-chiasmatic lesions (optic tract or visual cortex).
• Quadrantanopia indicates partial involvement of optic radiations.

Understanding these correlations aids clinicians in early detection of neurological disorders such as tumours, aneurysms, or demyelinating diseases.