184Chapter 11: Over-ordering tests

significant blockage is found, it should be considered in the category of asymptomatic carotid stenosis.

The diagnosis of NAION is a clinical one; in most cases few ancillary tests are needed. In addition to the investigation of risk factors for vascular disease, the main focus of laboratory testing in such patients is the identification of those patients with the arteritic variety of AION (see Chapter 12, Evaluation and treatment of giant cell arteritis).

Diagnosis: Non-arteritic anterior ischemic optic neuropathy (NAION)

Tip: NAION is due to small vessel disease involving the optic disc. In typical cases, investigation for an embolic source is not indicated.

Acute isolated sixth nerve palsy

Case: A 69-year-old retired fireman with stable hypertension and hyperlipidemia noticed double vision and minor aching behind his left eye while at a family reunion. He mentioned his symptoms to his wife who noticed nothing unusual about his appearance. Over the next two days his left eye turned noticeably inward but he still had no other neurologic or systemic symptoms. Examination now showed a moderate esotropia and loss of abduction in the left eye (Figure 11.2). Left lateral rectus saccades were markedly slow. Optic nerve function and appearance were normal, as were pupils, lids and other cranial nerves, and there was no evidence of orbitopathy.

What is the most likely diagnosis and what evaluation would be appropriate?

An acute microvascular sixth nerve palsy was diagnosed. The natural course and excellent prognosis associated with a vasculopathic ocular motor neuropathy were explained to the patient and his wife. The option of ordering an MR scan was also discussed but not recommended by the treating physician. Instead the patient was advised to return for re-evaluation one week later. A CBC, ESR and C-reactive protein were obtained, results of which were normal. At follow-up one week later his periorbital ache was resolved, but the abduction deficit was unchanged. Other neurologic findings were again normal. Over the next eight weeks, the patient demonstrated spontaneous and complete recovery of his sixth nerve palsy.

Discussion: Etiologies of acute sixth nerve palsy include tumor, stroke, demyelinating disease, intracranial hemorrhage, meningeal disease, increased intracranial pressure, trauma and a variety of infectious, inflammatory and immunologic disorders. In many of these conditions there are other neurologic deficits and/or systemic symptoms. An acute, isolated sixth nerve palsy in adults older than age 50 years is most commonly due to microvascular ischemia of the peripheral portion of the nerve, commonly referred to as an ischemic or vasculopathic cranial neuropathy. Such palsies are usually accompanied by ipsilateral headache or retrobulbar pain that may precede the palsy by a day or two and usually remits in 7 to 10 days. The motor deficit in vasculopathic cranial neuropathies (third, fourth and sixth) often shows initial progression for

several days, occurring in about 50% of patients. It is helpful to recognize that this is a common feature of the natural history of this disorder and, as long as other features are typical, does not warrant additional investigation. While ancillary testing can be obtained to exclude other etiologies, the diagnosis of a vasculopathic sixth nerve palsy is a clinical one, based on the following criteria: age greater than 50 years, one or more vascular risk factors, absence of neurologic signs and symptoms other than initial ipsilateral headache, and spontaneous recovery which is usually complete within four months. In cases with typical clinical findings, little additional testing is needed. Assessment of vascular risk factors (blood pressure, serum glucose and lipid profile) should be undertaken. In patients older than 60 years of age, serologic tests for giant cell arteritis should be obtained. In patients without associated pain, a Tensilon (edrophonium chloride) test may be performed. Some clinicians obtain an MRI in such patients; others opt for observational management, as in the above case. More extensive testing, however (e.g. lumbar puncture, carotid imaging, evoked potentials and additional blood tests), is unnecessary and should be discouraged.

Serial examinations are critically important in the evaluation of an acute and isolated sixth nerve palsy. The specific goals in such evaluation are assuring that the palsy remains isolated, and documenting the expected spontaneous recovery. The determination of improvement should be based on examination findings, as patients are notoriously unreliable at assessing improvement of their motility deficit. The degree of sixth nerve dysfunction can be objectively assessed as degrees of abduction past midline, millimeters of scleral show on abduction compared to the fellow eye, or prism diopters of esotropia in primary position. Spontaneous recovery is the clinical feature that best defines this syndrome. Continued progression beyond two weeks, failure to show some improvement by eight weeks or significant residual nerve palsy after six months should cast doubt upon the diagnosis and prompt additional evaluation. In addition, any patient with

a history of cancer should undergo prompt evaluation, particularly neuro-imaging.

This restrained approach to the evaluation of a sixth nerve palsy applies to an acute event. In contrast, any patient with a chronic ocular motor palsy (usually defined as duration greater than six months) should be thoroughly evaluated for a compressive lesion at the skull base, meningeal infiltration and increased intracranial pressure (Figure 11.3). It should be noted that this principle is different from the approach used in many other medical situations in which chronicity confers a sense of the benign nature of the disease process.

The diagnosis of a vasculopathic third nerve palsy is somewhat more complex, relying on the pattern of motor and parasympathetic nerve involvement in addition to other clinical features as described above. Because the fibers that subserve pupillary constriction are located peripherally on the nerve, the pupil in vasculopathic third nerve palsy is often spared, due to vascular supply to this area by pial arterial branches. In contrast, in third nerve palsy due to a pCOM aneurysm, pupillary dysfunction is an early and frequent finding. The state of the pupil is therefore helpful in distinguishing vasculopathic from aneurysmal mechanisms, and has been summarized in the statement “a third nerve palsy with pupil sparing is not due to a pCOM aneurysm”. An important exception to this “rule of the pupil”, however, is the patient in whom the third nerve palsy is partial, i.e. in addition to pupil sparing, one or more extraocular muscles are spared as well. Such partial pupil-sparing third nerve palsy is indeed occasionally produced by aneurysmal compression. Thus, in cases of partial palsy, the presence of pupilsparing is less reassuring, and some form of neuroimaging is appropriate to rule out an aneurysm (see Chapter 10, Headache and third nerve palsy). In the adult patient with an otherwise complete but pupilsparing third nerve palsy with vascular risk factors, ancillary testing is usually considered optional.

There is no specific treatment for a vasculopathic cranial nerve palsy. Symptomatic relief of diplopia can be achieved by patching, occlusive tape on one lens or application of a paste-on prism. Insofar as