Author Response: Neurological Hemifield Test in Binasal Defects

We appreciate the interest Alvarez and colleagues¹ have shown in the neurological hemifield test (NHT).² In their letter, they present two patients with binasal patterns of visual field loss and “positive” NHT results (≥70), suggesting a neurological cause of visual field loss. In the first case, the patient did, in fact, have chiasmal pathology responsible for field loss, namely, a dolechoectatic right internal carotid artery causing compressive right optic neuropathy. In the second case, the patient had NHT scores above 70 in both eyes in the setting of glaucoma, and no chiasmal or postchiasmal disease was present. The authors concluded that a detailed evaluation by a trained ophthalmologist is essential to reach an accurate diagnosis, and we could not agree more. 

Our intention in creating the NHT was to alert physicians to the presence of unsuspected neurological disease, not to substitute for clinical judgment. Just as the glaucoma hemifield test (GHT) should never be used by itself to diagnosis glaucoma, similarly the NHT should be considered in the context of the patient's symptoms, visual acuity, color vision, presence or absence of RAPD, and optic nerve pallor or cupping. In the first case presented by Alvarez et al.,¹ the right RAPD, temporal optic nerve pallor, and lack of glaucomatous cupping add to clinical suspicion for chiasmal or postchiasmal pathology, which is borne out on neuroimaging. In the second case, the cupped appearance of the optic nerves is consistent with the nasal field loss and diagnosis of glaucoma; no additional investigation appears to be warranted. 

As stated in our manuscript,² “while some glaucoma eyes were above the NHT score criterion as neurological, it is reassuring that more than 60% of these were binasal defects. There are occasional neurological diseases, such as dolichoectatic carotid arteries that give binasal defects, but glaucoma is the overwhelmingly dominant reason for such defects.” The NHT was designed to exclude from analysis the points in the nasal-most field that are typically affected in glaucoma, to reduce the incidence of false-positives. However, case 2 illustrates that some glaucoma patients have nasal steps extending more centrally, which would be falsely “interpreted” by the NHT as suspicious for neurological disease. We considered using the strong association between a binasal pattern of field loss and glaucoma as an additional input to our NHT algorithm. However, ultimately we opted against this because we did not want to miss the rare case of disease such as dolichoectatic carotid arteries, so nicely illustrated in case 1. Thus, an NHT score greater than 70 should not by itself prompt neuroimaging, particularly for a binasal pattern of visual field loss in a patient with an otherwise typical clinical presentation of glaucoma. 

In summary, the NHT is a tool to be used in conjunction with clinical judgment. A positive NHT result should be corroborated by clinical suspicion for disease at or posterior to the chiasm in order to prompt neuroimaging, particularly in the setting of binasal defects, for which the most common cause is glaucoma. We thank the authors for bringing up this important issue and giving us a chance to clarify the role of the NHT in clinical diagnosis.