Youstina Metry

Ophthalmology department, Queen Elizabeth Hospital, Birmingham, UK

Neuropathy is a condition where nerve damage occurs due to a variety of reasons, leading to pain, weakness or altered sensation depending on the nerve injured (1). Optic neuropathy, or sometimes termed as optic atrophy (end stage of optic neuropathy), is the damage to the optic nerve anywhere along its path from the retina to the lateral geniculate (2).

Causes:

There is a diversity of causes leading to optic neuropathy (2,3). It is easier to remember these causes through the mnemonic VITAMIN CDEF (4) as shown in table 1.

	Causes	Examples
V	Vascular	Non-arteritic ischemic neuropathy (NAION), Arteric Ischennic Neuropathy (AION)
I	Infective	Toxoplasmosis, Syphilis, Parvovirus B19, Herpes Simplex-2 (HSV-2), Rubella, Cytomegalovirus, Herpes Zoster (HZV) (TORCH infections)
T	Traumatic	Direct or indirect traumatic optic neuropathy (TON or ITON)
A	Autoimmune	Multiple sclerosis, neuromyelitis optica, sarcoidosis, Bechet’s disease, systemic lupus erythromatosis (SLE)
M	Metabolic	Vitamin B1, B12 and folate deficiency
I	latrogenic	Radiation therapy
N	Neoplastic or compressive	Ocular or retrobulbar tumours, metastatic optic nerve infiltration, glaucoma
C	Congenital	Leber’s hereditary optic atrophy, autosomal dominant optic atrophy
D	Degenerative	Wernicke encephalopathy, excessive alcohol use
E	Endocrine	Thyroid eye disease, diabetes mellitus
F	Functional	Other causes ruled out

Clinical picture (2,3):

Due to damage of the optic nerve, the ability of perception of light and visual stimuli decreases. Accordingly, patients present with visual loss (onset varies depending on the cause), with or without subtle colour vision dysfunction as red colour desaturation “as if the red colour is faded”.

On examination of the optic nerve, multiple clinical signs can be detected such as, decreased visual acuity, decreased ability to read Ishihara plates for colour vision, relative afferent pupillary defect (RAPD) in unilateral optic neuropathy cases, and decreased visual field.

Fundal examinations only reveals retinal and optic disc changes of papilledema, pale optic disc, or retinal haemorrhages if the disease is causing intraocular changes, otherwise fundal examination can be normal. In inflammatory and infective causes, signs of uveitis can be present as well.

In addition to visual clinical picture, optic neuropathy can be associated with other systemic neurological features, signs of increased intracranial pressure, systemic signs of the causative autoimmune or infective disease.

Investigations (2,3):

To identify the cause of the optic nerve damage, investigations usually conducted are visual field testing, optical coherence tomography (OCT), electrophysiological studies, and neuroimaging such as MRI and CT.

Through kinetic or automated visual field testing, different pattens of scotoma can be detected depending on the extent and the site of the lesion. Some patterns, not all, of the visual field defects can be linked to a specific aetiology (table 2). OCT can show changes in the retinal thickness and any intra-retinal deposits. Electrophysiological studies are beneficial in identifying macular disorders, occult optic nerve dysfunction, early or subclinical lesions. With the evolution of the neuroimaging, MRI has shown promising approaches in the identification of demyelinating or malignant causes leading to optic neuropathy.

Laboratory work-up has a major role in detecting infective, metabolic, autoimmune, endocrine causes, etc. Hence, ESR, vitamin B12, folate, angiotensin converting enzyme (ACE), antiphospholipid antibodies, ANA, TORCH panel, Venereal disease research lab studies (VDRL), homocysteine, thyroid function test (TFTs) and HbAic are examples of useful laboratory studies that can help in the recognition of the cause for optic neuropathy.

Visual field defect pattern noted	Optic neuropathy cause
Altitudinal defect	Ischemic causes
Central scotoma ( from macular or papillomacular bundle lesion )	Metabolic Hereditary
Hemianopia defect	Posterior to the chiasm space occupying lesions
Junctional scotoma	Lesion at the junction of the optic nerve with the chiasm

Management (2,3):

There are different approaches of optic neuropathy management depending on the cause, for example, antiviral, immunosuppressants, nutritional replacement, or surgical intervention in space occupying lesions. The aim in optic neuropathy management is to prevent further nerve damage, as dead axons cannot be replaced. Hence, prognosis depends on the virulence of disease, the severity of optic nerve damage sustained and the time of intervention for management.

References:

1. Australia H. Neuropathy [Internet]. Healthdirect Australia; 2023 [cited 2024 Feb 3]. Available from: https://www.healthdirect.gov.au/neuropathy
2. Optic Atrophy – EyeWiki [Internet]. [cited 2024 Feb 3]. Available from: https://eyewiki.aao.org/Optic_Atrophy
3. Raed Behbehani (2007) Clinical approach to optic neuropathies, Clinical Ophthalmology, 1:3, 233-246, DOI: 10.2147/opth.s12160050
4. Jones J, Campos A, Bell D, et al. Surgical sieve (mnemonic). Reference article, Radiopaedia.org (Accessed on 05 Feb 2024) https://doi.org/10.53347/rID-10451