Radiographic errors

Neuro-ophthalmology emerged as a subspecialty in the pre-scan era, when a great premium was placed on accurate localization of disease process without resorting to surgical exploration. One aspect of the field that draws many of us to this discipline is just this ability to localize a lesion based solely on the clinical findings. While sometimes viewed as obsolete in today’s era of sophisticated neuro-imaging, this skill is particularly valuable for the interpretation of radiographic studies.

In an ideal world, the clinician and the neuroradiologist would read each scan together. In every day life, we often settle for less but at the least, the clinician should always furnish the radiologist with a clear directive regarding the area of interest on the scan and the nature of the suspected disease process. The more confidently the clinician can establish the location of the lesion based on the clinical findings, the more attentive the radiologist can be to the area of interest. And when the scan interpretation is inconsistent with the clinical findings, the scan should be reviewed again.

In this chapter, we have divided some common radiographic errors into three groups and provide several case examples within each category. The first category consists of cases in which the wrong scan has been obtained, i.e. an error on the part of the clinician who ordered the study. In the next two groups, the study was the correct one but the critical radiographic abnormality was missed, i.e. an error by the interpreter of the scan. In the first of these the diagnosis was missed because the key finding was small and subtle. In the other, the finding was

not appreciated because it was either bilateral or involved a midline structure, making the interpretation more challenging. In each case, familiarity with the clinical features of the disorder points to the correct diagnosis.

Ordering the wrong scan

The value of any radiographic study is determined in large part by the quality of the scanner, the sophistication of the software and the expertise of the person reading the scan. Even the best quality scan may be inadequate if it is the wrong study for the clinical question at hand.

Progressive optic neuropathy

Case 1: A healthy 57-year-old homemaker described a three-year history of slowly progressive dimming of vision in the left eye. She denied eye pain, diplopia and other focal neurologic deficits. An eye examination early in her course was said to be normal but two years later, a follow-up examination showed left optic disc pallor. An MRI was unrevealing and she received a diagnosis of “chronic optic neuritis”.

Is there a problem with the diagnosis of “chronic optic neuritis”?

The answer to this question, and the key to the diagnosis in this case, rests on the time course of the visual loss. What we mean by “chronic” in this

context is not long-standing but showing continued slow progression. Most patients with demyelinating optic neuritis present with acute visual loss, usually progressing over just a few days. Occasional patients with multiple sclerosis develop a subclinical optic neuropathy, and in such cases the patient may not be able to pinpoint an exact onset of visual loss, but the course is not one of continued progression. Less common causes of optic nerve inflammation, such as granulomatous disease, may produce subacute optic neuropathy with progression over weeks to months. In rare cases of optic neuritis due to spirochetal disease, the time course may be more prolonged. In some patients, difficulty in getting a good handle on the temporal features of the visual loss makes the time course indeterminate. For example, we see patients who describe their decreased vision as progressive but actually mean that their visual loss is static but they are worrying about it more. In other cases, a patient with a residual optic neuropathy following a bout of optic neuritis subse-

quently develops another source of decreased acuity such as presbyopia or cataract which makes it appear that optic nerve dysfunction is worsening. Thus, establishing the time course of a patient’s optic neuropathy is sometimes challenging. But if the course is well defined and it is one of slowly progressive optic nerve dysfunction, a diagnosis of “chronic optic neuritis” is unlikely to be correct.

Continued progression of this patient’s visual loss prompted neuro-ophthalmic referral. Visual acuities were 20/20 OD and 20/70 OS with a moderate (2+) RAPD OS. She identified all 17 color plates in the right eye and just 5 plates in the left eye. Goldmann perimetry in the right eye was normal; in the left eye there was central depression and two small relative scotomas temporal to fixation (Figure 4.1A). The right optic disc was normal, the left showed temporal pallor (Figure 4.1B). Ocular motility was normal and there were no lid abnormalities, proptosis, chemosis or conjunctival injection.

What clinical features in this case suggest the likely mechanism of her chronic optic neuropathy?

In addition to the history of slowly progressive visual loss, the examination furnishes some important clues that point to the correct diagnosis. The earliest and most sensitive signs of optic nerve compression are decreased color vision and a prominent RAPD. Loss of visual acuity and optic disc pallor are later findings and visual field abnormalities are nonspecific. This patient demonstrates the classic clinical features of a compressive optic neuropathy and further work-up should be pursued accordingly.

What additional radiographic evaluation should be obtained?

Her MRI was repeated, this time with contrast, and revealed a small comma-shaped enhancing lesion medial to the left intraorbital optic nerve (Figure 4.2). A diagnosis of optic nerve sheath meningioma was made and the patient received

A

external beam radiation therapy consisting of 5400 cGy in 30 fractions. Subsequent follow-up visits showed progressive improvement of color vision and pupillary responses, with recovery of acuity to 20/20. Three years after radiation treatment she reported a persistently decreased sense of smell but no other adverse reactions. The patient remains stable clinically and radiographically 12 years later.

Case 2: A 34-year-old administrative assistant experienced a “film” over the vision in her left eye during her 29th week of pregnancy. Two previous pregnancies had been uneventful and she was in otherwise excellent health. Examination showed visual acuities of 20/15 OD and 20/20 OS with mild non-specific depression of visual field in the left eye. Color vision and optic disc appearance were normal but there was a small (1+) RAPD on the left side. An MRI was obtained but without contrast agent because of her pregnancy. There were two small, non-specific lesions in the cerebral white matter; the orbit views were normal. A diagnosis of

B

idiopathic (demyelinating) optic neuritis was made and she was followed expectantly. Following an uneventful delivery, the vision in her left eye improved spontaneously. Her clinical course was felt to be consistent with the working diagnosis and therefore no additional studies were undertaken.

She remained well until four years later when she experienced recurrent darkening of vision in her left eye. She had been warned of the risk of future multiple sclerosis and she took this recurrent visual loss as a sign of the disease. Based on her belief that nothing could be done, she did not immediately seek medical attention. Her visual loss continued to progress however, and six months later she returned for follow-up. Visual acuity OS was only slightly worse at 20/25 but there was now a large central scotoma and a moderate (2+) left RAPD. An MRI with contrast and fat-suppressed orbit views revealed a small intracanalicular meningioma (Figure 4.3). She received 5400 cGy of external beam radiation in 30 fractions and experienced progressive improvement of vision. Five years later, her vision is stable.

Discussion: Meningiomas are usually isointense to brain on both T1and T2-weighted images (Figure 4.4). Because of this radiographic charac-

teristic, detection of meningiomas was often difficult prior to the advent of gadolinium. It is important to be aware of this limitation of non-enhanced scans, particularly in the diagnosis of small tumors, whether they are located in the orbit or in the intracranial space. Inclusion of contrast infusion is also important for the detection of orbital inflammation.

In some cases, careful inspection of non-contrast MR images reveals enlargement of the optic nerve silhouette suggesting the presence of a mass lesion, but this is not true in all cases, particularly for intracanalicular lesions. Because there is scant additional room within the optic canal, even a small increase in the size of a tumor in this location can cause relatively rapid and profound loss of vision. In cases of unexplained optic neuropathy, especially those with continued progression, careful scrutiny of this area is crucial. Because the orbital fat normally appears bright on T1and T2-weighted images, enhancement around the optic nerve is only visible if the study includes dedicated orbital views with fat suppression.

Case 2 also highlights a curious feature of certain compressive lesions: spontaneous regression and resolution. This phenomenon has been previously