Left Ponto-Mesencephalic Infarcts Causing an Ocular Tilt Reaction With Ipsilesional Torsional Nystagmus [TN] and Lateral Alternating Skew Deviation [LASD].

Sir,

The complete ‘pentad’ of an Ocular tilt reaction [OTR] consists of Skew deviation, head tilt, ocular torsion, deviation of the subjective visual vertical [SVV] and torsional nystagmus [TN]. More often, varying combinations of the above signs are encountered.

A 51-year-old diabetic and hypertensive man presented with sudden onset of inability to walk and diplopia. Examination revealed a left sided limb ataxia, right sided head tilt with Skew deviation, ocular torsion [OT] [right eye hypotropic and excyclotorted, left eye hypertropic and incyclotorted], a deviation of the subjective visual vertical [SVV] was 4.50 to the right. Additionally he had a left Internuclear ophthalmoplegia [INO] and ‘Counterclockwise’ beating TN [from the patient's point of view]. Additionally, he had ‘Lateral Alternating Skew Deviation [LASD], which was more prominent on looking down and to the sides [Figure 1]. On both sides, the adducting eye was more hypotropic compared to the abducting eye. [Supplementary data and Video 1] The OT was determined as the mean of 4 fundus photographs taken with the head upright, with the patient in the sitting position. The head was adjusted to true upright as per convention. After pharmacologic dilatation bilateral fundus photos were taken. The degree of torsion [in roll plane] was calculated as the angle between a straight line through the papilla and fovea and the horizontal line. MRI showed a left rostral pontine infarct as well as a caudal mesencephalic infarct that extended along the paramedian line to the midline. MRA showed a top of the basilar occlusion and thrombecomy was deferred as he was well preserved. [Figure 2] He was discharged with minimal symptoms after a week.

Figure 1

Eyes in the nine cardinal positions of gaze. Panel A shows the left INO. Panel E shows the skew. LASD is more prominent in the adducting eye in panels G & I

Figure 2

MRI images. Panel a and b showing the rostral left pons and linear left midline approaching left caudal mesencephalic infarcts. Panel C shows a top of the basilar occlusion

The “Ocular tilt reaction” [OTR] is a disorder of the ‘roll’ plane. Four clinical signs are commonly encountered in varying combinations, although the OTR actually consists of a ‘pentad'. Skew deviation, head tilt, ocular torsion and deviation of the subjective visual vertical [SVV]. The causative lesion is ipsilateral if it is located caudal to the decussation of the otolithic pathway in the mid-pons and contralateral if is located rostral to this decussation.

The 5th sign of the ‘roll plane’ is torsional nystagmus [TN]. Acquired central nystagmus is usually caused by a central vestibular tone imbalance in one of the three major planes of action of the vestibular ocular reflex (VOR): yaw, pitch, and roll. ‘ Yaw’ plane signs include horizontal nystagmus, horizontal past pointing, lateral body falls to either side and deviation on the Subjective straight ahead test [SSA] {horizontal deviation of perceived straight ahead}.

Signs indicating pitch plane involvement are upbeat or downbeat nystagmus, forward or backward body tilts and falls, and deviations of the Subjective visual horizontal [SVH] {up or down deviations of the perceived horizontal]. When the ‘Roll plane’ is involved, signs include torsional nystagmus [TN], skew deviation, ocular torsion and tilts of head, body, and perceived subjective visual vertical [SVV].

With low medullary lesions the TN is often a combined horizonto-rotary type due to an overlap of areas of the Yaw plane and Roll-plane. Pure TN is more common above the pons, where selective involvement of the anterior and posterior SCC central fibers alone can occur.[1] An MLF lesion can inactivate the ispilateral interstitial nucleus of Cajal [INC] inducing a contralesional slow phase.[2] If there is a corrective ipsilesional quick phase, then the rostral interstitial nucleus of the MLF [riMLF] is intact.[3] Usually combined lesions of the riMLF and INC or riMLF lesions alone show a contralesional TN. Isolated small INC lesions are rare and show ipsilesional TN.[4] Sometimes this can be combined with a vertical seesaw nystagmus. [Figure 3]

Figure 3

Diagram showing the OTR at different levels of the brainstem. Note the ‘discordant’ TN from the medulla upto the INC. The TN becomes ‘concordant’ at the riMLF. Panel on the right; Diagram of the brainstem showing the various structures involved in vestibule-ocular pathways and areas of lesions

Thus, a general rule can be postulated in that the fast phase of TN is opposite to the side of the OTR from the medulla upto the INC [Discordant TN]. Above this level, when the lesion involves the riMLF, the TN becomes concordant with the OTR [Concordant TN].

In skew deviation, the lower eye is ipsi-lesional with a “low lesion” affecting the otolith organs, vestibular nerve, or lateral medulla [below the decussation of the graviceptive pathway fibers in the pons. Likewise the higher eye is ipsi-lesional to a higher lesion above the decussation [rostral pons, MLF or midbrain] with a contralateral OTR. The ‘higher eye’ is also often seen on the side of an INO and TN is also ipsilesional [caused by inactivation of the INC] as the MLF is affected after its decussation.[5]

Three classic types of skew deviation are proposed: Comitant, Incomitant and Lateral alternating skew deviation [LASD]. [Table 1] LASD is seen with lesions affecting bilateral graviceptive pathways. Although, the MLF is the principal tract by which vestibular signals are conveyed to the ocular motoneurons, the differential circuits for angular motion of the head [semicircular canal pathways] and for linear acceleration of the head [otolith organ pathways] accounts for the fact that LASD can be seen without bilateral INO.[6]

Table 1

Different types of skew deviation

Type	Incomitant Skew deviation	Comitant Skew deviation	Lateral alternating skew deviation
Eye deviation with gaze direction	Skew hyperdeviation worsens on looking to one side	Skew deviation remains the same in all directions of gaze	Reversing hypertropia that is present in both lateral position of gaze.
			Most commonly the abducting eye is higher, but sometimes the adducting eye is higher.
Pathway lesion	Asymmetric & Unilateral Partial injury to graviceptive pathway of either the ASCC or PSCC	Asymmetric Unilateral complete central graviceptive dysfunction affecting both ASCC and PSCC pathways	Bilateral Symmetric graviceptive pathway lesions anywhere from the medulla to the midbrain.

ASCC; Anterior semicircular canals. PSCC; Posterior semicircular canals

Phenomenologically, there are two types of central Skew; Mesencephalic skew with conjugate cyclotorsion of both eyes [in the absence of damage to ocular motor nuclei] and Medullary dysconjugate skew with monocular excyclotorsion in the hypotropic eye. Thus, in our patient, the left rostral pontine lesion affecting the MLF produced the ‘Pentad’ of OTR along with a left INO. The caudal mesencephalic lesion which extended to the midline might have involved bilateral graviceptive pathways and accounted for the LASD.

Although an INO with an ipsilesional TN has been described earlier, this is the first report of this unusual combination of the complete ‘Pentad’ of OTR along with an INO and LASD with a ponto-mesencephalic lesion.[7]

Declaration of patient consent

The authors certify that they have obtained all appropriate patient consent forms. In the form the patient(s) has/have given his/her/their consent for his/her/their images and other clinical information to be reported in the journal. The patients understand that their names and initials will not be published and due efforts will be made to conceal their identity, but anonymity cannot be guaranteed.

Financial support and sponsorship

Nil.

Conflicts of interest

There are no conflicts of interest.