Sir,

Acquired nystagmus is often associated with oscillopsia and visual impairment. Therapeutic options for this incapacitating condition remain limited. We report the case of a young male with nystagmus and oscillopsia, treated successfully with retrobulbar botulinum toxin A.

Case report

A 39-year-old male, confined to a wheelchair due to multiple sclerosis, manifested bilateral visual impairment associated with distressing subjective perception of rhythmic movement of his surroundings.

Examination revealed fine pendular bilateral horizontal conjugate nystagmus in the primary position, changing to jerk nystagmus on lateral versions, without a definite null point, though vision was subjectively clearest in dextroelevation. Snellen acuity measured 6/36 and 1/60 in right and left eyes respectively. A left relative afferent pupillary defect corresponded with asymmetrical optic disc pallor, more pronounced in the left eye, from previous optic neuritis.

Right retrobulbar injection of botulinum toxin A (Dysport 40 units) was administered, 2 hours subsequent to which subjective visual improvement was first experienced. Later the same day, this evolved into diplopia, which however was clear, and anomalous spatial perception, his bed appearing to be inclined downhill. Vision continued to improve subjectively over the next week. However, by the 4th day the upper eyelid began to droop; the patient overcame this by taping it open, though this resulted in ocular irritation and watering. To alleviate this, he then employed tape to open the lid partially, and used his eyebrow to lift the lid enough to see, so that he could close his eye at will.

At review a month later, acuity had objectively improved to 6/12 in the right eye, which manifested total external ophthalmoplegia, with no nystagmus and profound ptosis with no levator function. Another month later, acuity had dropped to 6/36, with recurrence of fine nystagmus. The patient also reported bothersome oscillating images from his untreated left eye, which he actively ignored. At the patient’s request bilateral retrobulbar toxin injections were administered.

Vision has subsequently remained stable, at around 6/12 in the right eye and 6/60 in the left eye, on regular 2-monthly bilateral injections of botulinum toxin (Dysport 20–40 units), 28 in total over the past 5 years. The injections are often followed by nausea and headache for 2 days, which however, resolve as ophthalmoplegia sets in by the third day. The patient prefers to tape both eyes open, despite persistent diplopia, since images from the left eye are no longer distressing. On lower doses of toxin, ptosis is less profound, though the degree and duration of visual benefit remain unchanged.

Comment

Acquired nystagmus is often associated with impaired visual function, due to excessive movement of retinal images. Failure of the visual cortex to adapt to nystagmus results in oscillopsia, which may be incapacitating.1

Therapeutic options include correction of refractive errors,2 optical dampening of oscillopsia with contact lenses and spectacles,3 gabapentin,4 memantine5 and botulinum toxin, none of which are universally effective.

External ophthalmoplegia with retrobulbar botulinum toxin as a treatment for acquired nystagmus was first described in 1988,6 subsequent to which there have been few subsequent reports.7,8,9 Repeated injections are required to maintain therapeutic effect, and entail the risks of retrobulbar haemorrhage, damage to orbital contents and ptosis.7,9 Inadvertent intravascular injection of toxin however, appears to be of little significance, the dose being insufficient to produce systemic toxicity.6

Our patient experienced relief from intractable oscillopsia, with significant improvement of spatial acuity on therapeutic ophthalmoplegia, suggesting that nystagmus was the cause, rather than the effect of reduced vision. Subjective visual improvement 2 hours after treatment suggests an initial placebo effect. Subsequently the only side effect noted was ptosis of insufficient severity or permanence to warrant surgical intervention. Images from the weaker left eye are no longer bothersome after bilateral ophthalmoplegia.

External ophthalmoplegia entails loss of saccadic and pursuit movements, including those involved in the vestibulo-ocular reflexes, which may cause oscillopsia on head movement.8 This however appears of little significance in the context of visual improvement in our patient with debilitating neurological disease. We submit that retrobulbar injection of botulinum toxin is therefore an effective and relatively safe therapeutic option in the treatment of distressing oscillopsia in non-ambulatory patients.