Have you ever evaluated a patient who described the sensation of having one eye lag behind the other when looking to the side, only to “find nothing wrong”? They may actually have had ONM.
Ocular Neuromyotonia, or ONM, is a condition that was first named in 1970. It is an ophthalmic clinical observation patterned after the more generalized condition of neuromyotonia in the body, which is convulsant-like activity in the motor system that affects normal gait. When involving the extraocular muscles, it is a spastic action of the EOM in its primary field of action, which resulting in a transient paresis when returning to primary gaze. Here is an example that comes form a nice review paper on the subject in Current Opinion in Ophthalmology:
The clinical key is to make sure that the patient gazes as far horizontally as possible, for as long as possible, before coming back to primary gaze. The condition is most often reported in adults who have undergone radiation therapy, particularly in the sella region. There are five main differential diagnoses:
- Superior oblique myokymia (SOM) – a monocular condition associated with brief episodes of oscillopsia and diplopia that can range range from seconds to hours and can occur multiple times a day. While these symptoms mimic ONM, eye movements in SOM have a high frequency and low amplitude nystagmoid motion distinguishing them from the paresis in ONM, and likely stems from damage to the trochlear nerve.
- Ocular myasthenia gravis (the original OMG!) – should always be considered in patients presenting with intermittent diplopia. As a subtype of the generalized disease of the neuromuscular junction, OMG can result in EOM weakness and fatigue that causes ptosis and diplopia. However, diplopia consistently following sustained eccentric gaze does not commonly occur in OMG, distinguishing it from ONM.
- Cyclic oculomotor paresis and spasms (COPS) is a unilateral condition presenting with a paralytic phase of ptosis, mydriasis, reduced accommodation, and ophthalmoparesis, accompanied by a spastic phase of eyelid elevation and pupillary constriction. Most cases are congenital and occur in the setting of an oculomotor palsy, resulting in abnormal ocular motility between episodes. Conversely, ONM usually has normal ocular motility between episodes. Like ONM, COPS can occur after radiation to the skull base, but ONM usually results in more random episodes, compared with the continuous and predictable cycles in COPS.
- Convergence spasm, or spasm of the near reflex – commonly presents with blurred vision, asthenopia, and intermittent diplopia. Similar to other conditions on this differential, it does not typically occur after sustained eccentric gaze, and so can be distinguished from ONM. Convergence spasm also presents with bilateral miosis. With most cases arising from functional causes, management typically consists of convergence exercises and symptom control. Convergence exercises have been described as treatment for ONM … and then the authors say something curious: “though it is unclear whether it would have resolved on its own in that case”. I describe this as curious because a) they don’t say that about any other proposed treatment including medication or surgery and b) PT has been proposed to aid generalized neuromyotonia, so it would be logical that VT would be considered for ONM.
- Giant cell arteritis (GCA) – a systemic vasculitis that can have serious ocular manifestations, may cause EOM ischemia or cranial nerve dysfunction thereby mimicing ONM. GCA typically presents with symptoms beyond intermittent diplopia including headache, jaw claudication, and scalp tenderness.
One of the references cited in this review paper is an interesting case of bilateral ONM. Take a look at the photographic documentation supplied in that article:
So the next time you have someone in your exam chair who either volunteers that:
- they sometimes see double when looking to the side, but it is variable or
- they have the feeling that one eye lags behind the other eye when they look to the side
… the patient may be experiencing ONM (ocular neuromyotonia) or subthreshold ONM. Chances are you won’t see anything on routine testing, but if you have the patient look in extreme gaze to alternate sides for 30 seconds (that’s a long time, but you have to prolong it to evoke it), you may very well see a carry-over strabismus in primary gaze such as IXT or IET, or a noticeably higher phoria, any of which will lessen over the course of a few minutes. There can be a role for prismothherapy or active vision therapy, particularly in cases that are unresponsive to medication.
Keep this in mind as well for children who have seemingly inexplicable postural skew, with their head habitually turned even subtly toward one side. And of course, keep it in mind for brain injury patients who tend to follow Murphy’s Law as it is (that’s a non-ophthalmic law).