The value of VEP in the diagnosis and post-operative monitoring of meningioma.

Meningiomas are the most frequently reported primary intracranial neoplasms. At first they usually cause unilateral visual loss but eventually result in a bilateral loss of vision. Depending upon the size and location of the tumour, the ocular signs and symptoms of meningiomas may include visual field abnormalities, optic atrophy, papilledema, diplopia and proptosis. This case report highlights the value of visual evoked potentials (VEP) in a patient with unexplained bilateral optic atrophy and a progressive loss of vision over 2 years. As a result of a delayed response in the VEP recorded from the right eye, a compressive lesion of the optic nerve was suspected. That prompted the referring ophthalmologist to request a MRI scan which led to the diagnosis of meningioma. Following the subtotal removal of the suprasellar meningioma, the remaining vision in the right eye improved and the latency of the VEP returned to the normal range. However, the VEP from the blind eye (left) did not show any measurable response either pre- or postoperatively. Experience with this patient suggests that early recognition of optic nerve compression is vital to an optimal outcome and the VEP technique, which is much more cost-effective than MRI, is clinically useful for detecting such compressive lesions.

EOG as a monitor of desferrioxamine retinal toxicity.

Iron overload caused by blood transfusion-dependent anaemia usually results in lethal cardiac toxicity unless treated by iron-chelation therapy. Chelation therapy with desferrioxamine (DFO) is well established and widely used to remove excess iron. Unfortunately, visual disorders have been recorded after DFO infusion. In this investigation, a 61-year-old Caucasian female received DFO for her autoimmune haemolytic anaemia. Prior to starting with the DFO treatment, her baseline ophthalmic screening and electrooculogram (EOG) were completely normal. Two years later she noticed a grey scotoma in her right eye. Visual acuity in this eye was reduced from 6/5 to 6/9 and funduscopy revealed evidence of non-specific mottling of the retinal pigment epithelium of both retinae. The EOG was flat (106%) in the right eye and subnormal in the left (155%). The lower limit of our EOG Arden Ratio for normal subjects is 180%. After her DFO treatment was stopped, her right visual acuity returned to 6/5, her field tests showed progressive improvement bilaterally and the EOG went back to the normal range. While waiting for splenectomy, the patient was restarted on a lower dose of DFO and EOG measurements were carried out every two (or three) weeks to monitor for DFO toxicity. The EOG varied during this period indicating some deterioration of function in the retinal pigment epithelium. However, normalisation of the EOG values (right = 217%, left = 217%) occurred after splenectomy and cessation of DFO therapy. Her visual function was normal and her visual acuity 6/4 bilateral when she was discharged from our outpatient clinic. On reviewing her history it was apparent that the EOG was the most sensitive indicator of DFO toxicity.

Normalisation of visual evoked potentials after optic neuritis.

Normalisation of the visual evoked potential (VEP) in patients with optic neuritis (ON) appears to be a rare phenomenon. However, although several workers have indicated that it can happen, they have not followed up with subsequent VEP tests to confirm how long the VEP latency of the affected eye remains in the normal range. To resolve this, 18 patients with a clinical diagnosis of acute unilateral ON were followed for 5 years with repeated VEP tests to determine if the latency of the P2 wave from affected eye could return to the normal range. Furthermore, in cases where the latency returned to normal, the length of time that it remained so was also assessed. The normal range for the latency of the P2 wave was determined by measuring VEPs from a group of 18 healthy control subjects with a similar age distribution to the patients. This established an upper limit of 115.9 ms. At presentation the mean P2 latency of the affected eyes of the patients was 140 ms with a standard deviation of 16 ms. In general, the VEP latencies remained constant over the period of the investigation. However two patients demonstrated a return to normal latencies but this was only temporary. Their latencies become prolonged again within 2 years. These results provide evidence that the delayed P2 latency observed in patients with ON can return to the normal range in a small percentage of cases. However, this improvement may spontaneously deteriorateonce more as a result of further episodes of subacute demyelination.