Comment on: Unique case of midbrain tuberculoma presenting as isolated inferior rectus palsy with nystagmus

Sir,

In a case of midbrain tuberculoma, Kowsalya et al. conflate a ‘unique’ presentation that raises many quintessential issues.[1] The association with nystagmus by itself renders inferior rectus palsy (IRP) as nonisolated. Subsequently, a definitive diagnosis of pupil involving partial third nerve palsy with downbeat nystagmus was made abandoning even IRP. Crucial inputs are woefully lacking leading to lot of speculation including primary position (PP) deviation, deviation in 9 cardinal gazes fixing either eye, primary and secondary deviation. Right hypertropia (RHT) of 10 and 16 prism diopters (PD) on right and left tilt with unknown PP deviation need not be extrapolated as positive head tilt test. Abnormal head posture (AHP) could be adopted in third nerve palsy depending on involvement of individual muscles and has no diagnostic significance, same is true about head tilt test, further both AHP and head tilt test results are known to be unreliable in IRP.[2] Hess and diplopia charts are confusing, former reveals approximately 10 PD of hypotropia and 28 PD of hypertropia in left and right eye, respectively, in PP (secondary and primary deviations), which does not conform to laws of ocular motility. Diplopia chart betrays an undocumented horizontal crossed diplopia due to exotropia likely emanating from unilateral/bilateral internuclear ophthalmoparesis (INO), it is not due to medial rectus under-action due to third nerve palsy as it is comitant, both are thus in conflict.[1] The 9 gaze montage neither betrays muscle sequalae consistent with acute onset IRP, nor is there any a pattern strabismus, AHP is sans face turn and chin depression, frequently seen in IRP.[2]

The causative lesion reported is 1.4 × 1.2 cm tuberculoma on magnetic resonance imaging involving subthalamic region, periaqueductal gray, and crus cerebri, without corresponding neurological involvement. A lesion less than one-tenth of this size would be enough to cause fascicular third nerve palsy; the lesion is crossing the midline, hence bilateral involvement is inevitable. The third nerve nucleus and fascicles are located in the tegmentum of the midbrain ventral to periaqueductal gray juxtaposed to rostral interstitial nucleus of medial longitudinal fasciculus (riMLF), ventrally with interstitial nucleus of Cajal (INC) lying close to rostral visceral motor nucleus of oculomotor nucleus.

Since fascicles for inferior rectus and pupil sphincter are located adjacently in rostral and medial part of oculomotor fascicles, it was once suggested that oculomotor fascicular infarcts may co-involve or co-spare fibers for pupillary sphincter and inferior rectus.[3] However, this postulate has not been supported as many cases of pupil sparing IRP exist from ischemic fascicular infarcts.[4,5] All emanate from ischemic microinfarcts; accordingly, authors contention that a huge tuberculoma led to such an isolated involvement is untenable.[1]

The RHT likely is an incomitant left tonic ocular tilt reaction (OTR) with a lesion (bilateral asymmetrical) implicating otolithic pathways close to right riMLF and INC.[6] The OTR will be contraversive in midbrain with RHT, left hypotropia and conjugate torsion to left.[6] Bilateral asymmetrical injury close to riMLF is also reflected in bilateral INO giving rise to exotropia. Impaired vertical pursuit and decreased vertical vestibulo-ocular reflex (VOR) gain would have offered necessary insight regarding medial longitudinal fasciculus involvement but were not assessed. OTR may have three components, perceptual (tilt in the subjective visual vertical), ocular motor (ocular torsion and skew deviation), and postural (head tilt) in various permutations and combinations.[6] Head tilt may thus be absent in OTR. Patient should have adopted left head tilt in left OTR but it may have been adopted to right side for some other reason like torsional nystagmus from involvement of INC. The head tilt test is usually negative in OTRs but may be positive in incomitant OTRs creating diagnostic confusion with cyclovertical muscle palsies.[7]

Nystagmus is not downbeat as contended since it is absent in PP.[1] Gaze evoked vertical conjugate nystagmus militates against a diagnosis of third nerve palsy and further attests to involvement of INC, a structure crucial for vertical gaze holding and reflecting a leaky brainstem integrator. Pupil involvement, INO, and nystagmus have been reported with skew deviations[8] and are likely fellow travelers in this case.

Assessment of subjective and objective conjugate fundus torsion with intorsion of hypertropic eye and extorsion of hypotropic eye is essential part of work up and change in vertical deviation and torsion from erect to supine position would have settled the issue but authors chose not to include both as 4^th and 5^th step after Parks 3 step test.[7,9] Prompt recovery within a month courts the diagnosis of a skew. Skew deviations can skew many a presentation and should be kept in mind.

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