SHORT REPORT ABSTRACT: Oculomotor function has not been studied in patients withmyotonic dystrophy type 2 (DM2). We report the presence of reboundnystagmus in seven of eight patients with DM2 in the absence of a structuralbrainstem or cerebellar lesion. The rebound nystagmus observed in thesepatients is very suggestive of ocular myotonia, and examination of patientsusing video-Frenzel goggles may be a useful method for diagnosing myo-tonia of the extraocular muscles.

Muscle Nerve 38: 1326–1329, 2008

OCULOMOTOR INVOLVEMENT INMYOTONIC DYSTROPHY TYPE 2

SENDA AJROUD-DRISS, MD,1 ROBERT SUFIT,1 TEEPU SIDDIQUE,1

and TIMOTHY C. HAIN1,2

1 Davee Department of Neurology and Clinical Neurosciences,Feinberg School of Medicine, Northwestern University,710 North Lake Shore Drive, Abbott Hall 1426, Chicago,Illinois 60611, USA

2 Departments of Physical Therapy/Human MovementSciences and Otolaryngology, Feinberg School of Medicine,Northwestern University, Chicago, Illinois, USA

Accepted 24 June 2008

Myotonic dystrophy type 2 or DM2 is an autosomal-dominant multisystem disease caused by a CCTGrepeat expansion in intron 1 of zinc finger protein 9gene on chromosome 3q.8 DM2, like myotonic dys-trophy type 1 (DM1), is clinically heteroge-neous.1,2,9,11 Patients usually present with musclepain, stiffness, weakness, and myotonia. The weak-ness preferentially affects neck flexors, elbow exten-sors, thumb and deep finger flexors, and hip flexorsand extensors.1 As in DM1, systemic manifestationsof DM2 include cataracts and cardiac and endocrineabnormalities. Patients with DM2 can have specificcognitive abnormalities in visuospatial and frontallobe functions, but these are less severe than inDM1.3 Hearing loss is rare in DM2, and vestibularinvolvement was previously reported by Grewal et al.in 2004.4 Oculomotor function has not been studiedin DM2, and rebound nystagmus has never beenreported.

METHODS

We studied eight patients with DM2. Three patientswere male and five were female; ages ranged from 32to 69 years (mean 49 years). Five were studied usingboth video-Frenzel goggle examination and video-nystagmograms (VNGs), and three were examinedwith video-Frenzel goggles only. Detailed history andneurological examinations were obtained for all pa-tients. The diagnosis of DM2 was genetically estab-lished in seven patients. In one patient, genetic test-ing was not performed, as it was diagnostic of DM2 inher daughter.

RESULTS

Our first patient had a 2–3-year history of episodicdizziness, lasting 1–2 minutes, which he described asa feeling of imbalance. Episodes were triggered by achange of body position or head turning. The pa-tient did not complain of hearing loss, vertigo, ornausea during these episodes. A second patient alsocomplained of occasional dizziness with suddenchange in head position, but the remaining patientsdenied any dizziness or feeling of imbalance.

The neurological examination was significant forproximal upper and lower extremity weakness withpercussion myotonia in older patients. Younger pa-tients had myotonia but little or no weakness. Therest of the neurological examination was normal; in

Abbreviations: DM1, myotonic dystrophy type 1; DM2, myotonic dystrophytype 2; FLAIR; fluid attenuated inversion recovery; MRI, magnetic resonanceimaging; VNG, videonystagmogramKey words: myotonic dystrophy type 2; ocular myotonia; rebound nystag-mus; dizziness; oculomotor functionCorrespondence to: S. Ajroud-Driss; e-mail: s-ajroud@md.northwestern.edu

© 2008 Wiley Periodicals, Inc.Published online 21 September 2008 in Wiley InterScience (www.interscience.wiley.com). DOI 10.1002/mus.21113

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particular, there was no spontaneous or gaze-evokednystagmus or evidence of cerebellar signs. Magneticresonance imaging (MRI) of the brain was obtainedfor six of the eight patients and revealed mild non-specific areas of increased T2/FLAIR signal but nobrainstem or cerebellar lesions.

The first patient had a normal audiogram up to2000 Hz. The VNG revealed normal calorics, sac-cades, and pursuit, but gaze testing was markedlyabnormal. After eccentric gaze-holding for 10 sec-onds in the dark, a strong rebound nystagmus wasseen on taking up center gaze. The nystagmusreversed direction after gaze-holding to the op-posite side. Video-Frenzel goggles as well asVNG were used to observe the rebound nystagmus(Fig. 1).

VNG and video-Frenzel goggle examination werealso performed on three additional patients and, ongaze-holding, a strong rebound nystagmus identicalto the one observed in the first patient was detected.Three patients were examined with video-Frenzelgoggles only. They too had strong rebound nystag-mus, but the nystagmus was observed in only onedirection of gaze. In one of the patients, when weexamined the left eye, rebound nystagmus was ob-served after holding eccentric gaze to the left but notto the right. Examination of the right eye revealedrebound nystagmus after gaze-holding to the right

and not to the left. One patient had no findings onvideo-Frenzel goggle examination or on VNG. Noneof the patients had abnormal gaze-evoked nystag-mus.

DISCUSSION

Rebound nystagmus is a gaze-evoked nystagmus thatchanges direction with refixation to the primary po-sition.12 It occurs when the eyes return to the pri-mary gaze position following prolonged eccentricgaze.6 The rapid phase of the rebound nystagmus isopposite the direction of sustained gaze. Reboundnystagmus is most commonly elicited at the bedsidein the light by having the patient look to the side for10 seconds and then return gaze to the center. Re-cent advances in instrumentation, which allow videoobservation of the eyes in complete darkness, havemade it possible to improve on the rebound test.The test can be made more sensitive by testing pa-tients in complete darkness, as in this situation visualfixation and tracking mechanisms cannot be used tosuppress the nystagmus. Although rebound is gener-ally thought to be a specific sign for brainstem orcerebellar disease,7,10,15 it can be observed in normalpersons who hold gaze for long periods (e.g., 40seconds) and, in such cases, the subjects tend to haveendpoint nystagmus.13,14

FIGURE 1. Rebound nystagmus. All illustrations show horizontal eye position, recorded in complete darkness, following 10 seconds ofeccentric gaze-holding. (A) Patient with DM2. Left-beating nystagmus is observed following rightward gaze-holding, and right-beatingnystagmus following leftward gaze-holding. (B) Normal subject. No nystagmus is seen following eccentric gaze-holding.

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Seven of eight of our patients had strong re-bound nystagmus, but none had endpoint nystag-mus. In four patients, rebound nystagmus was ob-served in the setting of normal saccades and normalsmooth pursuit. There was no clinical evidence ofcentral lesions in any of the patients, which wasconfirmed by imaging in six of them.

We hypothesize that the rebound nystagmus wasdue to delayed relaxation or myotonia of extraocularmuscles. After sustained gaze to the left, for exam-ple, the left lateral rectus and the right medial rectusare contracted. When the patient shifts gaze to thecenter, the right medial rectus and left lateral rectusneed to relax; however, because of sustained con-traction of these two muscles, as the patient attemptsto look to the center, the eye drifts backward into theprevious position of gaze, causing the rebound nys-tagmus.

The rebound nystagmus observed in our patientsdiffers from that reported by Hood and associates5

in several ways. First, it was observed in the darkusing a video system rather than in the light wherevisual fixation was permitted. We suggest that, hadvision been permitted to allow for fixation, this smallnystagmus would not have been detectable in ourcases. Second, rebound nystagmus in our cases wasnot accompanied by impaired tracking or gaze-evoked nystagmus. This is evidence for a differentmechanism (i.e., ocular myotonia) than the one re-ported by Hood et al. Although this is the first reportof rebound in a myotonic disorder, on the basis ofthese data, it is reasonable to hypothesize that thereare two mechanisms for rebound nystagmus—mus-cular and central. The myotonic types can be distin-guished from the central variety by the lower ampli-tude of the nystagmus and by a lack of impairedpursuit and strong gaze-evoked nystagmus.

The presence of ocular myotonia has been de-bated in the literature. DM1 patients tend to haveslow saccades and abnormal smooth pursuit; how-ever, because of the diffuse central nervous systeminvolvement in DM1, the muscular origin of theseabnormal ocular movements is presently not estab-lished. Versino et al.16 presented evidence for ocularmyotonia in DM1. They used magnetic scleral searchcoil techniques to record small and large saccades,which were paced to two interstimulus intervals—one short and the other long—in 20 patients withDM1. They found that prolongation of the stimulusinterval led to an increase in saccade duration, with-out significant modification in saccade peak velocity.They concluded that these findings may be derivedfrom a delay in relaxing an extraocular muscle afterfixation and they hypothesized that the delay may be

considered an ocular myotonic phenomenon, al-though they could not exclude a myopathic origin.

The fact that three of our patients had reboundnystagmus in one eye in one direction of gaze fol-lowed by rebound nystagmus in the other eye in theother direction of gaze should eliminate any consid-eration of a central mechanism, because a centraletiology would be unlikely to cause dysconjugaterebound nystagmus.

Ocular myotonia has not been reported in DM2.A review of 379 individuals with genetically con-firmed DM2 by Day et al. in 20031 did not mentionextraocular movement abnormalities. However, re-bound nystagmus is a somewhat subtle ocular signand may have been missed. In our case, specialequipment that enabled viewing of the eyes in com-plete darkness (video-Frenzel goggles) was neededto observe this sign. It seems likely that in a personwith normal smooth pursuit, as was the case here,rebound nystagmus would be suppressed in thelight, and thus this sign would not be observablewithout special equipment. Although two of our pa-tients complained of mild dizziness, it is unlikely thatit was caused by the rebound nystagmus, becausemost patients were asymptomatic and the reboundnystagmus was small and only measurable with sen-sitive methodology.

In conclusion, oculomotor function should bestudied further in patients with myotonic dystrophy.Testing for rebound nystagmus may be a method ofdiagnosing ocular myotonia and, with the properequipment, it is easy to perform at the bedside.Demonstration of rebound nystagmus in additionalpatients with DM2 without cerebellar or brainstemlesion could further confirm the presence of myoto-nia of the extraocular muscles.

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