Pathologies of the optic nerve, even though not always detected on opthalmoscopic exam, 
 may  compromise its function  and  cause the following signs;

- reduced visual  acuity

- afferent pupillary defect

- visual field defects

- dischromatopsia ( impaired color vision) 

- diminished light brightness sensitivty

- diminished contrast sensitivity

Optic  disc changes on fundoscopy can  include disc edema,  hyperemia , paleness and atrophy. Many a times though the disc looks completely normal  yet it harbors a major underlying disease.

Congenital  anomalies 

1. coloboma :   is a defect  resulting from  malclosure of  the fetal  cleft .  Colobomas are usually inferior  and can involve the retina and choroid and  may be part of maldevelopment syndromes like CHARGE.                                                                                                                                                  

3. Optic disc pit : greyish depression usually inferior or temporal .

Anomalies associated with axial length/ refractive errors 

 Hypermetropic eyes are smaller in size  and their  discs are frequently  smaller , lacking  a physiologic cup, and have a crowded and full appearance mimicking   the swelling of papilledema.  Retinoscopy  gives the clue to the underlying  etiology.

Myopic  discs are larger and surrounded by a white crescent of  bare sclera. On the temporal side , the  cup to disc ratio is bigger and can mimick  glaucomatous cupping.

Myelinated nerve fibers:   The myelinating process which is completed by the ninth month  of gestation  sometimes extends  onto the surface of the disc or surrounding retina in a  radiating fashion causing feathery  white patches. The condition is usually benign.

Drusens : are yellow opalescent hyaline excrescences  derived from calcified axonal  debris  present on the surface  of the disc or burried in  it . The optic nerve head is typically   small and full  also mimicking papilledema. A clue to the diagnosis  is  their autofluorescence  when viewed  through a fundus camera  before injecting the dye in fluorescin angioraphy.( red- free image).
Drusens  are associated with  retinitis pigmentosa and angioid streaks

Optic nerve head swelling (diferential  diagnosis )

Active and passive swelling  of the optic disc provides a compelling objective evidence  of compromised distal optic nerve function but the appearance is rarely specific.
Common causes  of disc edema are the following:

-congenital :   ex; drusens

-systemic diseases :  ex;  anemia,  hypoxemia, uremia, hypertension....

-tumors : ex; disc hemangioma, orbital glioma, meningioma....

- infiltrative diseases:  ex; lymphoma

- vascular diseases : ex; anterior  ischemic optic neuropathy ( AION) , giant cell arteritis and other autoimmune vasculitides ,  diabetic proliferative retinopathy...

- ocular diseases  : ex;  uveitis,  hypotony , CRVO....

-inflammatory diseases : ex; optic neuritis ,papillitis and neuroretinitis...

-elevated ICP (intracranial pressure) : ex;   mass occupying lesions,  pseudotumor cerebri...



The  following table compares 3 common causes of optic nerve edema:

Papilledema	Optic neuritis	AION
Non inflammatory edema secondary to increased ICP	Inflammatory swelling	Vascular accident  (occlusion of   short posterior ciliary artery causing retrolaminar nerve infarction)
Brain tumors, abscesses, hematomas, meningitis might be underlying etiology	Multiple Sclerosis is highly associated	Hypertension, giant cell arteritis, hypercoagulable state are possible factors
Bilateral , maybe asymmetric	Unilateral	Unilateral
Headache, nausea, vomitting No visual loss usually,  only enlarged blind spot  and possible hyperopia	Retrobulbar pain, especially on ocular movement, early central scotoma,decreased acuity.impaired color vision, presence o APD	Acute painless visual loss,usually hemialtitudinal defect involving  the lower visual field
Variable degree of disc swelling, hemorrhages and cystoid infarcts	Fewer hemorrhages and cotton wool spots	Pale segmental swelling and splinter hemorrhages at its margins
Prognosis usually good if primary cause of increased ICP is treated	Vision usually returns to normal	Poorer prognosis with permanent . loss. Second eye  is ultimately  involved in one third of idiopathic cases.