COMANAGEMENT

Motility Testing Key To Diagnosing FNP

Edited by
Paul C. Ajamian, O.D.

Q. How do you evaluate a fourth nerve palsy, and what tests should you perform?

A.Fourth nerve palsy (FNP) affects the superior oblique muscle, which is responsible for depression, intorsion and abduction of the eye. Paralysis of this muscle will result in relative elevation, extortion and an eso posture of the in-volved eye. Patients commonly complain of vertical diplopia and often adopt a head tilt to compensate for the double vision. The majority of FNPs are congenital, but some are acquired, mainly through head trauma, vascular diseases or neoplasms. They are the most common of the extraocular palsies.

Diagnosing FNPs can be tough because they sometimes mimic other muscular defects, says Michelle Caputo, O.D., optometric director of the neuro-ophthalmology service at the Bascom Palmer Eye Institute.

Likewise, the various patterns that are possible can be confusing. For example, with time, there can be a spread of comitance in which the motility pattern appears equal in almost all gazes. Variations can occur in the synergistic and antagonistic muscles, such as inhibitional palsies of the contralateral antagonist. Also, in some patients the underlying etiology can be a life-threatening tumor or aneurysm, although this is rare.

Key to making a successful diagnosis is to confirm the involvement of the superior oblique muscle and to rule out other conditions.

“Patients typically present with diplopia that worsens in the gaze opposite the involved eye, and have a head tilt on the same side as the involved eye,” Dr. Caputo says. Also, determine if the diplopia worsens as the day progresses, as this might be important in ruling out neuromuscular conditions like myasthenia gravis.

You’ll also have to determine if the origin of FNP is congenital or acquired. In both, diplopia and a head tilt are the most common symptoms. Often, adults with the congenital form have compensated for the diplopia throughout their lives. In later years, they may lose their fusional ability and begin to “decompensate,” thus triggering an acute onset of FNP.

If you suspect this to be the case, ask to see old photographs of the patient to look for the presence of a head tilt, and question family members as well. It’s critical to confirm the longevity of the tilt, because in acquired lesions, patients also adopt a head tilt to relieve symptoms of diplopia.

Note if there are any other neurological or systemic conditions, such as diabetes or hypertension, and ask if the patient has had a history of trauma. Keep in mind that FNP associated with mild trauma may be the first indication of a neoplasm.

Next, begin a series of diagnostic tests to identify the type of palsy and to rule out other etiologies. These include duction and version tests and the three-step test.
When testing versions, ask the patient to follow a target in all nine positions of gaze (see photo) while observing the range of movement between each eye.

The three-step test is valuable in identifying the superior oblique as the paralytic muscle and in confirming FNP. Each step consists of an alternate cover test measurement for various gaze positions:

Step 1: Determine if there is a right or left hypertropia in primary gaze.
Step 2: Determine which gaze—right or left—worsens the hypertropia.
Step 3: Determine the direction of the head tilt that worsens the hypertropia.

For example, a patient with a right FNP will present as a right hypertropia that worsens on left gaze and on right head tilt, and vice versa.

Also measure the amount of cyclorotation and the fusional amplitudes. Cyclorotation can be measured by the double Maddox rod test and is especially useful in identifying a FNP when it’s associated with other palsies. Large fusional amplitudes can ascertain whether the FNP is congenital or acquired. Note that in slow-growing neoplasms, patients can also develop large fusional amplitudes.

Be sure to review other cranial nerve functions to rule out associated neurologic conditions. Consider doing a forced duction test when you think there might be a muscle restriction, as experienced in thyroid eye disease.

Q. How should you manage a patient with fourth nerve palsy?

A. The type of treatment depends on several conditions: whether the FNP is congenital or acquired, the age of the patient, the presence or absence of systemic diseases or neurological symptoms, and the etiology of the palsy.

Since most fourth nerve palsies are congenital, brain scans are not usually necessary. But in younger, non-vasculopathic patients (less than 55 years of age) who present with no history of trauma, or in elderly patients with neurological symptoms, Dr. Caputo suggests ordering an MRI. Also order blood tests for vasculopathic patients.

Most acquired FNPs resolve on their own. With the congenital form, try prism treatment to stablize the patient’s symptoms. If the patient is unhappy with the prism, and depending on the size of the deviation, strabismus surgery might be necessary.

R. Michael Siatkowski, M.D., assistant professor of pediatric and neuro-ophthalmology at Bascom Palmer, says of those needing surgery: “The success rate is about 80 percent. Of the remaining 20 percent, some will need further surgery, and others can be managed with prism lenses.”
Dr. Caputo adds: “You can manage this on your own, but beware of underlying etiologies.”

Send your comanagement questions to Review of Optometry, 201 King of Prussia Road, Radnor, PA 19089; fax them to 610-964-2959; or e-mail them to reviewofoptometry@jobson.com.

[]   [Optometric Study Center]   [Managed Care Library]  
[]  [Optometric Resources]  [About RO]  [Search]