Aetiology

There are many causes of visual field loss. Some more common ones are included here.

Central field loss occurs with:

• Age-related macular degeneration.
• Optic neuropathy.
• Macular holes.
• Cone dystrophies.^[2]
• A number of rare conditions like Best's disease, Stargardt's disease and achromatopsia.

Peripheral field loss occurs with:

• Glaucoma (angle-closure glaucoma and open angle glaucoma).
• Retinal detachment.
• Retinitis pigmentosa.
• Chorioretinitis.
• Leber's optic atrophy.

History

The following need to be established:

• Was the onset sudden, rapid or slow?
• Where is the field loss? It is often helpful to say to the patient, "If what you see is like a television screen then where is the bit that is missing?"
• Does it affect one eye or both? If the patient says that it affects only one eye, it is worth asking them to close or cover the affected eye and to note again if there is any visual loss. If it is much more marked in one eye than the other, the loss in the less affected eye may be overlooked.
• Does the visual defect look like a black spot, a blur or does the picture look normal? If the lesion is cortical, the patient may fail to notice any defect. If the onset has been insidious, it may also have gone unnoticed.

Looking for evidence of 'asymptomatic' visual field loss

Examination

Visual acuity tests the eye's greatest power of resolution whereas visual field testing measures the peripheral sensitivity. It is important to remember that the image is projected on to the retina upside down and inverted. Hence, a lesion of the top right of the retina or in the pathway beyond will cause a defect in the bottom left of the visual field. Assessing for visual field defects can be via:

• Screening tests (easily carried out in a surgery) which include confrontational visual field testing and use of an Amsler grid.
• Quantitative measurements using manual or automated perimetry (specialist equipment is needed).

Terms which may be encountered include:^[3]

• Visual field defect - a portion of the visual field is missing. This may be central (eg, an optic disc or nerve problem) or peripheral (along the visual pathways from the optic chiasm back).
• Scotoma - this is a type of visual field defect. It is a defect surrounded by normal visual field.
  • Relative scotoma - an area where objects of low luminance cannot be seen but larger or brighter ones can.
  • Absolute scotoma - nothing can be seen at all within that area.
• Hemianopia - a binocular visual defect in each eye's hemifield.
  • Bitemporal hemianopia - the two halves lost are on the outside of each eye's peripheral vision, effectively creating a central visual tunnel.
  • Homonymous hemianopia - the two halves lost are on the corresponding area of the visual field in both eyes, ie either the left or the right half of the visual field.
  • Altitudinal hemianopia - refers to the dividing line between loss and sight being horizontal rather than vertical, with visual loss either above or below the line.
  • Quadrantanopia - is an incomplete hemianopia referring to a quarter of the schematic 'pie' of visual field loss.
  • Sectoral defect - is also an incomplete hemianopia.

Confrontation visual field testing

This is a simple (but approximate) method of assessing visual field loss. It is a qualitative measurement but is a good starting point and can easily be carried out in the surgery. Traditionally a hatpin has been used to define the visual field. A red or a white head is used and it may be moved across the visual field to ascertain where it disappears and hence to define a scotoma.

• Sit approximately 1 metre from the patient, facing the patient. Make sure that they have the acuity to see the intended target. Remove the patient's glasses if worn, as the rims get in the way.
• Look at the patient's nose and get the patient to look at yours.
• Test each eye separately; cover the other eye. You can ask the patient to cover or close one eye and you close the eye opposite.
• Make sure the target is equidistant between you and the patient.
• Starting at the top outer quadrant, move the target object (eg, fingers or hatpin) in from the side and ask the patient to tell you when they first see the object and, as you move towards the centre, whether they disappear.
• Repeat the process in each quadrant and for each eye separately.
• If you detect a defect, re-examine that area and define it further.
• Assess the blind spot (if you can't find it, then it's probably not enlarged).
• Repeat for the fellow eye.
• Then systematically assess:
  • Pupils and optic nerve function (visual acuity, relative afferent pupillary defect, colour impairment and brightness sensitivity).
  • Fundus.
  • Neurological examination (if a systemic problem is suspected).

Difficulties

• The most common is the patient's (or your) eye drifting away from the nose target. If this is happening, gently remind them throughout the test to keep fixing on your nose.
• This technique compares the patient's visual field with yours, so it does assume normal examiner visual fields!
• Visual fields in infants can be crudely assessed by making use of their involuntary fixational reflexes. First get the child's attention in a frontal gaze and while the child is watching your face, silently bring an interesting toy in from the periphery. This is a tricky process and eyes cannot be tested individually but, if you can manage it, valuable information will be gained.

Amsler grid testing

This assesses the central 10° the visual field. It detects central and paracentral scotomas. See the separate article on Examination of the Eye (go to 'Macular function' under 'Examination of function' heading) for details on performing this easy test.

Other examinations in a GP surgery

Visual field defects may stem from neurological or ophthalmic problems. Depending on what the history and findings so far suggest, a full neurological examination or further examination of the eye may be warranted.

To find details on how to test the optic nerve and use an ophthalmoscope, refer to the separate article Examination of the Eye.

Quantitative measurements

There are a number of techniques used in specialist practice. They fall into one of two categories: static or kinetic perimetry.

Static perimetry
This is the most commonly used assessment. An 'on/off' light signal is presented throughout the patient's potential visual field and the patient clicks every time they see the signal. These automated machines can assess various amounts of the visual field (10° to full field).

These are sensitive tests but are difficult to perform in that the patient has to be able to understand the instructions and respond appropriately. The tests may take time and can be very tiring for the patient. Thus, frail individuals who may tire easily, patients unable to sit still for long or unable to follow the instructions will get unreliable results. Even if the patient can follow what is going on, a temporary loss of concentration can affect the results. Pupil size, refractive error and artefacts (drooping eyelid, thick spectacles' rim) may also affect the result if not taken into consideration. To some extent, this can be detected by the machine but if the defect is very subtle, it may take a few readings to confirm its presence.

A few different types of static perimetry tests can be used of which Humphries' is the most common.

Kinetic perimetry
This presents a moving stimulus from a non-seeing area to a seeing area. It is repeated at various points around the clock and a mark is made as soon as the point is seen. These points are then joined by a line (an isoptre). The process is repeated with a point of lesser luminescence and another isoptre is created. Thus, a number of isoptres are plotted and you have a chart of the maximal peripheral vision for each (decreasing) level of brightness.

The most commonly used kinetic test is Goldmann's perimetry. It tends to be used for more neurological conditions, although not exclusively so. It is also used where there is suspicion of functional rather than organic problems, as a characteristic pattern of spiralling isoptres may be seen. Goldmann's perimetry also has its limits and can be affected by ptosis, refractive errors, tremor and inadequate operator skills.

Interpretation of your findings

Having elicited a visual defect, the patient needs referring on for further confirmation of the defect and investigation. However, you may find the following useful for your own investigative work.

Visual pathway^[4]

• The visual image is projected through the lens on to the retina upside down. The macula is responsible for central vision. Fibres from the macula feed into the temporal part of the optic nerve at the level of the retina, and gradually migrate to the central part of the optic nerve at the optic chiasm.
• The information from each eye is split at the chiasm, with the medial fibres (lateral visual field) crossing to the opposite side and the lateral fibres (nasal visual field) passing to the ipsilateral optic tract.
• Thus, information from both eyes concerning the same part of the visual field passes to the same part of the visual cortex (left half of visual field in right optic tract) via the optic tract, geniculate body and optic radiation.

Lesions at the level of the retina

These affect one eye only.

• Retinal detachment and occlusion of blood vessels at a level smaller than the central retinal artery or vein, give defects with boundaries in the horizontal meridian.
• Retinal detachment tends to be fairly rapid in onset. It may follow trauma or there may be predisposing factors. It may be preceded by floaters and flashes before what the patient describes as "a curtain" coming across the visual field. A crescentic red or orange slip of detachment may be apparent at the periphery of the retina.
• Central retinal artery occlusion tends to be a sudden and complete loss of vision in one eye but, if occlusion is at the level of one of the four arteries to the retina, there will be loss of just a quadrant of field. The affected area will look pale and poorly supplied with blood vessels. One of the four arteries will not be seen. Central retinal vein occlusion presents in a fairly similar way to arterial occlusion but the retina looks very different. Haemorrhages are scattered throughout the fundus in a typical blood-storm pattern with cotton-wool spots. With less complete blockage, sparse scattered haemorrhages occur.
• Age-related macular degeneration which affects the macular area and the periphery is spared until very late.

Drugs can cause disturbance of vision (tend to be bilateral); chloroquine can cause a classic bull's-eye maculopathy affecting the centre of the field. The Amsler grid can be used as a screening tool.^[5] Vigabatrin can cause field defects in up to 40% of those who take it.^[6]

Lesions before the chiasm

These will produce a field deficit in the ipsilateral eye. Field defects from damage to the optic nerve tend to be central, asymmetrical and unilateral. Visual acuity is often affected. Consider optic neuritis or optic atrophy, glaucoma and trauma (incomplete damage, transection or blunt trauma). Lesions just before the chiasm can also produce a small defect in the upper temporal field of the other eye as the decussating fibres loop back into the optic nerve after crossing (anterior chiasmal syndrome - eg, meningioma).

Lesions at the chiasm

These classically produce a bitemporal hemianopia.

• If they spread up from below (for example, pituitary tumours), the defect is worse in the upper field.
• If the tumour spreads down from above (for example, craniopharyngioma), the lesion is worse in the lower quadrants.

Lesions of the optic chiasm may show a phenomenon where two identical coloured objects are shown to one eye in the two vertical halves of the visual field, but one appears to be brighter and sharper than the other. For example, with a right hemianopia the left hemifield is brighter than the right.

Lesions after the chiasm

These produce homonymous field defects; a lesion in the right optic tract produces a left visual field defect. Fibres in the optic tracts gradually rotate until the fibres reach the geniculate body, so lesions in the tract before the geniculate body may produce incongruous defects.

• Lesions in the main optic radiation or optic peduncle cause complete (left or right) homonymous hemianopia without macular sparing. This is seen in stroke and middle cerebral artery lesions.
• Lesions in the temporal radiation cause congruous upper quadrantic homonymous hemianopia commonly with macular sparing - eg, tumours.
• Lesions in the parietal radiation (rare) cause inferior quadrantic homonymous hemianopia without macular sparing.
• Lesions in the anterior visual cortex (common) produce a contralateral homonymous hemianopia with macular sparing - eg, posterior cerebral artery occlusion.
• Lesions in the macular cortex produce congruous homonymous macular defect - eg, blunt injury to the occiput.
• Lesions of the intermediate visual cortex produce an homonymous arc scotoma, with sparing of both macula and periphery. This is seen in a distal posterior cerebral artery occlusion.

Occipital lobe lesions

• If both occipital lobes are injured then the patient is in a state of cortical blindness. The patient is unable to process visual information and behaves in a similar fashion to someone who suffers a peripheral blindness. However, some patients deny their blindness and attempt to behave as if they have vision. This state of denial of cortical blindness is called Anton's syndrome.
• Lesion of the primary visual perception area of the right or left occipital lobe will produce a clear loss of visual perception from the contralateral visual field. Patients are usually aware of the deficit and do not neglect that side of the visual field.
• Ventral stream damage; this area is involved with recognition, and damage here does not tend to produce visual field defects.

Safety to drive and DVLA

Patients with a newly diagnosed significant visual field defect should not drive until this is more formally assessed. Patients must inform the DVLA who will then organise for them to have a very specific visual field test (Estermann's visual field test) which will be carried out by one of their approved optometrists. The approved optometrist then reports back to their medical officers, who make the final decision regarding whether a person is safe to drive.