A Case of Superior Oblique Palsy without Brown Syndrome Induced by a Dog Bite

Background

Canine tooth syndrome is a rare strabismus caused by dog bites [1–4]. It was first reported by Knapp in 1974 as a variant of superior oblique palsy [1]. Of the 5 cases reported by Knapp, 2 occurred in association with dog bites; hence, the name “canine tooth syndrome” [1]. It is believed to be caused by direct trauma to the upper eyelid, with extensive disruption of the trochlear region, resulting in restriction of the superior oblique muscle either in the direction of extension or contraction [5]. Treatment of canine tooth syndrome is challenging, and Wise et al suggested no treatment in the acute phase unless there are symptoms in the primary position; if symptoms are present, depot steroids are administered around the trochlea [2]. If significant diplopia persists, removal of local scar tissue or strabismus surgery is performed [2,3,5]. In this report, we describe a case of superior oblique palsy without limitations in elevation during adduction, induced by a dog bite. Magnetic resonance imaging (MRI) allowed early diagnosis and therapeutic intervention. This is a variant of canine tooth syndrome that results from damage to the trochlear and superior oblique tendon due to dog bites around the eye.

Case Report

A 19-year-old male patient sustained a dog bite to the left side of his face from a German shepherd dog. He was treated with skin sutures on the head at the injury site. Notably, the dog had been vaccinated against rabies.

The patient experienced torsional diplopia after the injury, prompting a visit to his eye doctor 3 days later. During the first visit to the doctor, a sutured wound extending across the forehead and scalp was observed, along with bite wounds on the outer side of the left eyebrow, and nasal and temporal sides of the left upper eyelid (Figure 1). The left upper eyelid was swollen and lacked voluntary movement, accompanied by subconjunctival hemorrhage in the left eye. However, no abnormalities were observed in the eyeball. Visual acuity was 20/16 on the right and 20/20 on the left. Slit lamp and fundus examinations were normal. Antibiotics were initiated on the same day. At a follow-up visit 5 days after the injury, an alternate prism cover test (APCT) showed 6 prism diopters (Δ) exotropia and 5Δ left hypertropia in the primary position (Figure 2). Left hypertropia secondary to superior oblique palsy is classically worse in opposite gaze (10 Δ in right gaze and no deviation in left gaze) and better in opposite tilt (8 Δ in right-tile and no deviation in left-tilt). Although the Hess screen chart showed limitation of depression during adduction, ocular motility showed no significant limitation in elevation or depression in adduction. MRI performed on day 5 after the injury showed a continuous high-signal region in the nasal area of the upper eyelid to the trochlea and the belly of the superior oblique muscle of the left eye (Figure 3). Accordingly, acquired superior oblique palsy was observed in the left eye due to inflammation from the superior oblique tendon to the trochlea and superior oblique muscle belly, diagnosed as canine tooth syndrome without pseudo-Brown syndrome. Since the MRI showed inflammation, oral prednisolone (30 mg/day) treatment was initiated for 2 weeks.

During follow-up, his diplopia did not improve, and he was referred to our hospital 2 months after the injury. An APCT showed 6Δ exotropia and 6Δ left hypertropia at distance and 10Δ exotropia and 5Δ left hypertropia at near. Fundus photographs showed extorsion of the left eye (Figure 4) and 5 degrees of extorsion were measured using a cyclophorometer [6]. Since ocular alignment and diplopia did not improve, an inferior oblique myotomy of the left eye was performed 7 months after the injury. Torsional and vertical diplopia improved postoperatively (Figure 5).

Discussion

In this report, we discuss a case of canine tooth syndrome without pseudo-Brown syndrome. The patient presented with torsional diplopia and superior oblique palsy due to damage to the superior oblique tendon-trochlear complex following a dog bite around the left upper eyelid. MRI played an important role in early diagnosis, and early steroid administration may have prevented the development of acquired Brown syndrome.

Superior oblique palsy is a congenital or acquired abnormality of the superior oblique muscle or trochlear nerve that causes limitation of depression during adduction, hypertropia, and excyclotorsion of the affected eye [7]. Vertical and torsional diplopia can be treated with inferior oblique muscle weakening and vertical rectus muscle transposition [1,8,9]. In our case, the patient’s chief complaint was torsional diplopia, commonly caused by superior oblique palsy [7,10,11]. However, a Hess screen chart analysis performed 5 days after the injury initially showed a limitation of depression during adduction, which changed over time and deviated from the typical form of superior oblique palsy. Although the patient was aware of cyclovertical diplopia, and Parks’ three-step test was consistent with superior oblique palsy in the left eye, the ocular mis-alignment was not large, and the restriction of ocular motility was relatively mild. Therefore, it may have been difficult to identify the affected muscle through a routine strabismus examination. A short tau inversion recovery (STIR) magnetic resonance image revealed a high-intensity lesion in the nasal region of the upper eyelid, contiguous with the trochlea and superior oblique muscle belly. Scarring was evident on the nasal skin of the upper eyelid and the nasal bulbar conjunctiva of the left eye, suggesting that inflammation around the superior oblique muscle tendon or inflammation of the upper eyelid had extended to the trochlea and subsequently affected the superior oblique muscle belly. The high signal intensity of the superior oblique muscle on MRI facilitated an early diagnosis of superior oblique palsy, emphasizing the pivotal role of MRI in the diagnostic process.

This case presented as canine tooth syndrome without pseudo-Brown syndrome. Brown syndrome is a congenital condition characterized by an abnormality in the superior oblique tendon, leading to limited elevation in adduction and a positive traction test secondary to a taut superior oblique tendon [5]. In contrast, acquired Brown syndrome, or pseudo-Brown syndrome, is a secondary condition resulting from dysfunction in the superior oblique tendon-trochlear complex [5,12]. The trochlear tendon complex of the superior oblique muscle exhibits continuous and unrestricted movement with globe motion [13]. It has been reported that the central fibers of the tendons in the trochlear tendon complex are stretched by up to 16 mm when the globe is in adduction and move to their maximum extent from downward to upward rotation [14]. An abundant vascular sheath in the trochlea of the superior oblique muscle tendon helps to repair the wear and tear due to this dynamic motion and to dissipate heat [12]. This feature is not observed in other extraocular muscles. However, the abundant blood flow makes it prone to inflammation and scar formation, which contributes to acquired Brown syndrome [3], and extensive trauma around the trochlea results in canine tooth syndrome. Lee et al documented a case of canine tooth syndrome without pseudo-Brown syndrome resulting from a dog bite. In that case, despite extensive injury to the trochlea region, because of the loss of the superior oblique muscle due to the dog bite, there was no limitation of extension [4]. In our patient, the degree of superior oblique palsy was mild, and there was no limitation of elevation in adduction, ruling out pseudo-Brown syndrome. These findings suggested that the inflammation was extensive but mild; therefore, no scar formation or pseudo-Brown syndrome was observed. In canine tooth syndrome cases, depot steroids have been administered around the trochlea if diplopia is recognized in the primary position and within 2 weeks after injury [2]. In our patient, an early MRI facilitated an early diagnosis, enabling the initiation of oral steroids at an early stage. This timely intervention may have inhibited secondary scar formation due to the anti-inflammatory effect of steroids, preventing the development of pseudo-Brown syndrome. As the patient presented 2 months after the injury without improvement of diplopia, depot steroids were not administered, and the patient was followed up for observation. Given the persistent diplopia even 7 months after the injury, surgery was performed by myotomy of the left inferior oblique muscle, the antagonist muscle, which resulted in an improvement in the patient’s perception of cyclovertical diplopia.

Conclusions

This case illustrates that dog bites around the eye can result in abnormalities of the extraocular muscles. Early MRI may be useful for diagnosis and determining treatment strategies. This case report highlights the importance of rapid assessment and management of patients with dog bites involving the eye.