Over-ordering tests

Continued developments in ancillary testing, particularly in the field of neuro-imaging, have greatly enhanced the prompt and accurate diagnosis of a variety of neuro-ophthalmic disorders. As impressive as this technology is, there are still conditions in which information obtained from the history and physical examination is more valuable than the results of ancillary investigations. Unnecessary testing is to be discouraged for several reasons. Medical testing entails additional expense which can be considerable. Some forms of testing involve risk to the patient and certainly should be avoided if not truly necessary. Finally, the addition of extra testing can create a delay in diagnosis which may affect the outcome.

Testing should be limited to those conditions which could actually be responsible for the condition under investigation. While this may sound obvious, in clinical practice testing is often ordered as part of a “shotgun” approach to diagnosis that does not adhere to this principle. For example, a serum B12 level is sometimes obtained for patients with acute monocular visual loss. While B12 deficiency can indeed cause optic neuropathy, the clinical syndrome is that of symmetric, bilateral, subacute, central visual loss (see Chapter 10, Unexplained visual loss). Deficiency of vitamin B12 can cause a range of neurologic deficits but acute unilateral visual loss is not among them, and so this test should not be part of the work-up. In this chapter we highlight several examples of excessive ancillary testing.

Isolated unilateral mydriasis

Case: A 30-year-old paralegal developed blurred near vision and a mild ache behind the left eye. The next day she noticed that her left pupil was larger than the right pupil. She was otherwise healthy with no headache, diplopia, ptosis, other neurologic deficits or recent systemic symptoms. Physical examination in a local emergency room was unrevealing except for a large, unreactive left pupil. A head CT was normal as were a subsequent MRI and MRA. A cerebral arteriogram was recommended but the patient was sent first for neuro-ophthalmic consultation.

Examination one week after symptom onset showed uncorrected visual acuity of 20/20 at distance in each eye. Near vision tested with Jaeger acuity was decreased to J8 OS compared to J1 OD, but could be improved to J1 using a +2.00 lens. Eye movements were full with brisk and accurate saccades, she was orthophoric in all fields of gaze and there was no ptosis. The right pupil measured 6 mm in dim room light and constricted briskly to 3 mm with direct light stimulation. The left pupil was 7 mm in dim room light and showed no apparent reaction to direct or consensual light stimulation. Pupillary reactivity in the left eye was not better when the patient viewed a near target. Highmagnification slit-lamp examination of the left eye showed paralysis of most of the iris sphincter except for two focal segments that were briskly reactive to light stimulation.

This patient’s acute unilateral mydriasis raised the possibility of third nerve palsy due to a posterior communicating artery aneurysm, and the work-up was directed with this possibility in mind. Can isolated mydriasis, in fact, be a sign of a posterior communicating artery aneurysm?

There are no documented cases of a posterior communicating artery (pCOM) aneurysm that presented as isolated pupillary dilation. The fibers that subserve pupillomotor innervation travel on the dorsal medial surface of the oculomotor nerve and in this location are vulnerable to compression by an aneurysm at the junction of the posterior communicating and the internal carotid arteries. As a result of this anatomic relationship, pupillary dilation can be an early sign of aneurysmal compression. However, in such cases there is virtually always involvement of other third nerve motor fibers. The caveat in applying this “rule” is that one must perform an ocular motor examination that is adequate to rule out other elements of third nerve dysfunction. Specifically, it is not sufficient to simply ask the patient about diplopia or look only for ductional deficits on examination. Cross-cover testing should be performed, particularly on upgaze and with the suspect eye in adduction to rule out subtle underaction of the elevators and of the medial rectus muscle, respectively. In addition, external examination should include careful inspection of lid position and function. If other evidence of third nerve palsy has been excluded in this way, one can be confident that the unilateral mydriasis is an isolated finding and is not due to compression by a pCOM aneurysm.

If an isolated, enlarged and poorly reactive pupil is not a sign of a pCOM aneurysm, what other causes should be considered?

There are other causes of third nerve palsy that can produce isolated pupillary dilation, but they are rare. For example, a small mesencephalic hemorrhage affecting the fascicular pathway of the third nerve can cause isolated mydriasis. In transtentorial herniation, pupillary dilation may be the first sign

of third nerve compromise, but in such cases alteration of consciousness is also present.

More commonly, the site of pathology is peripheral: injury to the short ciliary nerves in the orbit (an Adie’s tonic pupil), pharmacologic blockade of the iris or direct damage to the iris sphincter muscle. Sphincter muscle damage may result from trauma, inflammation or ischemia. Evidence of previous trauma or inflammation can generally be seen with slit-lamp examination. Causes of iris ischemia include giant cell arteritis, high-grade carotid stenosis and angle closure glaucoma. An iris sphincter that is pharmacologically blocked by an atropinic substance is usually widely dilated and fails to constrict to full-strength (1% to 4%) pilocarpine.

This patient’s slit-lamp examination showed no structural abnormalities of the iris, leaving tonic pupil and pharmacologic blockade as the primary considerations. Following instillation of dilute ( 18 %) pilocarpine, the left pupil constricted from 7 mm to 4 mm, consistent with cholinergic denervation supersensitivity. The right pupil was unchanged. The patient received a diagnosis of Adie’s tonic pupil and was managed expectantly. Examination three months later showed no improvement of the pupillary response to light in the left eye but there was now a slow, sustained constriction during near effort. One year later the left pupil was 1 mm smaller than the right in dim light but still showed minimal response to light stimulation.

Discussion: The pathophysiology of an Adie’s tonic pupil is a two-fold process, acutely reflecting the effects of denervation, and later those of reinnervation. In the acute stage of denervation, the iris sphincter and ciliary muscle are paralyzed. The patient complains of photophobia and blurred near vision. The pupil is dilated and unresponsive to both light stimulation and near effort, and accommodation is lost. Denervation is typically incomplete, manifest as incomplete or sectoral palsy of the iris sphincter (see Chapter 9, Acute tonic pupil vs. pharmacologic mydriasis).

Supersensitivity of the iris sphincter to weak cholinergic agonists develops within several days to