Edrophonium

CLINICAL CHALLENGES

Do you recognize the many disguises of myasthenia gravis?

Like most of you, I'm confident in my diagnostic skills. But some cases make me wonder, "Where was I when they taught this?" Here's one such case; maybe you'll learn from it as I did.

Detailing Dora

Dora was a 75-year-old African American whom I first saw 1 year ago. Her internist referred her to me because her OS had been running water for 1 week and it felt "weak." Dora also pointed out that she hadn't had her eyes checked in more than 4 years. Dora repeated that her OS felt irritated, adding that she didn't see well at distance or near.

Dora took loratadine and pseudoephedrine (Claritin D) for allergies, celecoxib (Celebrex) for arthritis and losartan (Hyzaar) for hypertension. Her visual acuity (VA) with her spectacles was 20/50 OD, which improved to 20/40 +2 with pinhole occlusion, and 20/50 OS with no improvement using the pinhole. A slightly lower hyperopic refraction yielded 20/40+2 VA OD and 20/50 +2 OS.

Dora's visual fields were full to confrontation. Her pupils were 5 mm, round and reactive with no afferent pupillary defect. Physiological H testing of the extraocular muscles showed no restrictions, but cover testing revealed a 15-prism diopter (pd) left exotropia (LXT). Dora had mild nuclear sclerotic and posterior subcapsular cataracts, which contributed to her poor VA.

The remainder of the right anterior segment was normal. The OS showed mild bulbar conjunctival injection intrapalpebrally and a mild dellen on the limbus at 9 o'clock. A dilated fundus exam revealed a cup-todisk ratio of 0.4/0.4 in each eye, with no disk edema and retinas free of hemorrhages, vascular changes, edema or drusen.

I attributed the irritation to ocular surface disease (based on the dellen) and the decreased VA in her right eye to the cataract. I felt that Dora had a strabismic amblyopia OS. Dora couldn't confirm or deny having had an eye turn in the past, and I didn't have old records on hand to show a pre-existing exotropia.

Dora decided to wait on cataract surgery. I prescribed artificial tears OU q.i.d. and asked her to return in 6 weeks because the exotropia troubled me.

Dora drops in

Dora returned unexpectedly 25 days later. She said that for the past 4 days, her left eyelid drooped. She felt the eye was "pulling" but denied irritation.

Her VA OS had decreased to 20/60, the pupils weren't changed and the extraocular muscles showed no restrictions. Cover testing revealed a subtle 4 pd intermittent LXT. Red lens testing revealed no diplopia, although Dora noted a ghost image at times throughout the test.

The left upper eyelid was indeed ptotic (see photo, page 67). The palpebral aperture measured 10.5 mm OD and 8 mm OS. The lid crease was present OU. Dora could voluntarily raise both of her eyelids. The dellen was gone OS and nothing else had changed from the previous exam.

Dora's blood pressure (BP) was 186/86 mm Hg. Now I diagnosed a partial 3rd nerve palsy OS. I felt that a vascular etiology was most likely, given her age, so I sent her back to her medical doctor for a vascular workup. I specifically ordered an erythrocyte sedimentation rate (ESR), a fasting blood sugar (FBS) and a complete blood count (CBC).

All results were normal. Dora's doctor told me that a carotid Doppler ultrasound was negative but that her BP was 182/86 mm Hg, so he added a second BP medication. Both of us felt that the elevated BP was the most likely culprit. He scheduled Dora to see me again in 1 week.

Dora's diagnostic dilemma

One week later, the VA had improved to 20/40 OU. Dora exhibited a 15 pd LXT. Her OS didn't adduct completely, but I saw no other extraocular muscle restrictions. She felt that the OS was less droopy and I measured only a 1 mm ptosis.

Dora exhibited spurious results with the red lens test, inconsistent with any pattern or lesion. My diagnosis remained a partial third nerve palsy, but despite her M.D.'s assurances, I was still uneasy. I ordered a magnetic resonance imaging (MRI) of the head with specific instructions to rule out an aneurysm.

The MRI was negative for an aneurysm or mass, but revealed fairly extensive white matter ischemic disease. Could a series of mini-strokes cause partial 3rd nerve palsy?

Diplopia debuts

Ten days after the MRI, I saw Dora again. Her vision was worse, the eyelid still drooped and she occasionally saw double.

I measured the VA as 20/50 OD, 20/40 OS. Her pupils were equal and reactive OU. Extraocular muscles showed a mild adduction deficit OS. Cover testing now revealed a constant 20 pd LXT with a 4 pd left hypertropia. The lid droop measured 2.5 nun OS.

Red lens testing showed that the double vision was vertical and horizontal and worse when Dora looked to her right. This was consistent with a third nerve problem. I resummarized my findings; a worsening third nerve palsy that changed appearance; normal carotid Doppler, ESR, FBS and CBC; high blood pressure. To me, the key was the variable presentation. This pointed to myasthenia gravis (MG).

A variable enemy

Weakness and skeletal muscle fatigue characterize MG. It affects any age, and the cause is impaired transmission of acetylcholine in the neuromuscular junction. Double vision and ptosis that vary, even from hour to hour, characterize the ocular form. Extraocular muscle involvement doesn't follow any set pattern and may shift to different muscles or eyes. This variability is key to making a diagnosis. Most (90%) of MG patients show ocular symptoms.

Another finding that's almost pathognomonic for MG is a patient's inability to sustain an upgaze or to hold the arms up for a normal period. There's also a 5% incidence of associated dysthyroidism and an increased incidence of thymoma.

A Tensilon test, whereby edrophonium chloride (Tensilon) is injected intravenously, will confirm the diagnosis. In patients with MG, Tensilon improves the lid droop or diplopia within a matter of minutes.

Treatment ranges from patching and monitoring the affected eye to (more likely) prescribing oral prednisone. Some physicians prescribe pyridostigmine, an anticholinesterase, for patients with moderate cases.

Dora's improvement

I referred Dora to a neuroophthalmologist, who confirmed MG with the Tensilon test. She placed Dora on 30 mg of prednisone and titrated the dosage over 5 months.

The last time I saw Dora (2 months ago), her diplopia had resolved but a mild 1-mm ptosis remained OS. This didn't bother her, and she said that it no longer changed throughout the day. Dora remains on 5 mg oral prednisone; the neuro-ophthalmologist and I will watch her for signs of worsening MG and for potential side effects of the steroid.

with Eric Schmidt, O.D.

Contributing Editor Eric Schmidt, O.D., is director of the Bladen Eye Center in Elizabethtown, N.C. E-mail him at KENZIEKATE@aol.com.

Copyright Boucher Communications, Inc. Apr 2002
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