Visual Failure

Introduction

A patient with visual failure is a relatively common referral to neuroradiology from many clinical specialities. The site of the pathology affecting the visual pathway can be predicted from its anatomy and so an accurate description of the visual complaint and clinical findings are essential if imaging is to be optimized. Site in turn predicts the differential diagnosis. For example, monocular visual loss reflects retinal or optic nerve disease. Bitemporal hemianopia reflects pathology affecting the optic chiasm. Homonymous hemianopia is a disorder of the visual cortex.

Unfortunately we are not always given such detailed information and sometimes it is not possible to obtain it from the patient. The combination of visual failure and cranial nerve palsies 3, 4, or 6 will help to localize the pathology to the superior orbital fissure or cavernous sinus (Chapter 6). Colour vision abnormalities point to an optic nerve problem, while the onset of confusion can be due to bilateral occipital infarction causing cortical blindness.

It is possible to provide one single CT technique which will enable all parts of the visual pathway to be examined in sufficient detail that any causative structural pathology will be shown.

Opinion may be divided between those who advocate MDCT for orbital and parasellar pathology and those who advocate MR because of its superior soft tissue contrast. However, what cannot be denied is that if there is any urgency in the investigation of visual failure then a CT scan performed properly will exclude or identify any structural cause and any delay awaiting an MR is inappropriate. What is important is that the supervising radiologist is able to perform diagnostic studies in all patients.

Technique

The CT examination is a helical study of the entire visual pathway within the brain. It incorporates the anatomy of the orbital cavity, the optic chiasma, pituitary, and visual cortex. From the base data, multiplanar reformats of each of the above areas with appropriate reconstructive zooms can be created. This gives a high quality CT image of each area to diagnose pathology in the patient presenting with visual failure. Patient preparation is presented in Table 4.1 and a surview in figure 4.1. Protocol parameters are presented in Table 4.2.

Reconstruction and reformation

Presentation of images is done from multiplanar reformats of each area: orbital cavity, the pituitary/optic chiasma, and whole head to show the visual cortex.

Orbital cavity (4.1B)

The angulation is a line from the posterior clinoid through midglobe along the line of the optic nerve as shown in Chapter 3 (Orbital pathology) with 12 images. Slice width is 2 mm/interspace 2 mm. Note the enlarged left lacrymal gland.

Pituitary region and optic chiasma (4.1C)

The coronal images should show good arterial enhancement around the pituitary. 12 images with slice width 2 mm/interspace 2 mm.

Visual cortex (4.1D)

Whole head reformat to show most importantly the visual cortex. 12 images, slice width 6 mm/interspace 6 mm. These three main areas of interest are displayed in multiplanar reformations.

Pathology and illustrations

Globe

Retinal detachment

Vitreous/subhyloid haemorrhage

Trauma

Central retinal artery/vein occlusion

Optic nerve

Demyelination

Meningioma

Glioma

Trauma

Optic chaism

Pituitary tumour

Suprasellar meningioma

Aneurysm

Craniopharyngioma

Rathke cyst

Metastasis

Optic tract/radiation and visual cortex

Infarction

Demyelination

Malignant tumour

Extra-axial mass

Examples of a variety of pathology in all parts of the visual pathways are included.

Table 4.1 Patient preparation

The patient should be prepared as for CT angiography with an 18 or 20 gauge venflon in the antecubital fossa
The patient should be instructed to hold a downward gaze during the scan in order to ‘stretch’ the optic nerves

Table 4.2 Protocol parameters for visual pathway imaging

Patient position	Supine
Surview	Lateral
First slice	Bottom border of cavernous sinus
Last slice	To cover orbital roof and occipital cortex posteriorly
Field of view	˜250 mm
Slice width	0.9 mm
Slice increment	0.45 mm
Pitch	0.685
Collimation	64 × 0.625 mm
Rotation time	0.5 sec
kV/mAs	120 kV/300 mAs
Resolution	Standard
Filter	Soft tissue – special bone/brain interface filter if available
Reconstructive zoom	Zoom partial images to orbits/chiasma region initially (see below)
Windowing	WC 60
	WW 360
Contrast	High pressure pump injection 100 ml contrast media, 4 ml/sec, 60 sec scan delay