Understanding Papilledema
Is it always present in intracranial hypertension?
Papilledema, or swelling of the optic nerves, 
is often considered a hallmark sign of intracranial hypertension. But what does your doctor see when he looks into your eyes? Read on to find out what papilledema is, why it develops, what a venous pulsation is and whether papilledema is necessary for an IH diagnosis.
How does papilledema occur?
The optic nerve is actually an extension of brain tissue, connecting the inner lining of the retina from the eye to the visual cortex in the brain. The nerve is comprised of axons, or fibers of nerves cells, that carry signals from light sensitive cells in the retina to the visual cortex where the signals are converted into formed images.
The drawing below depicts a cross-sectional view of the optic nerve and its three covering membranes, which are the same membranes that envelop the brain. The first and innermost membrane, which is in intimate contact with both the brain surface and the optic nerve, is called the pia mater (shown in pink). The second membrane is known as the arachnoid (yellow). The outermost and third membrane is the dura mater (purple).
Between the arachnoid and the pia mater is a space known as the subarachnoid space, which surrounds the brain, as well. The subarachnoid space surrounding the brain is normally filled with cerebrospinal fluid (CSF). The subarachnoid space around the optic nerve is not normally filled with CSF, but with small fibrous strands. However, when intracranial pressure is high, this area—the subarachnoid space surrounding the optic nerve—can become inflated, as pressurized CSF fills it. 
As a result, high intracranial pressure will compress the optic nerve and the central retinal vein (not shown) which travels along with the central retinal artery through the middle of the optic nerve and normally drains blood from the inside of the eye. This compression of the central retinal vein and the optic nerve is believed to be the cause of papilledema.
Compression of the central vein also causes internal venous pressure to rise. Moreover, normal arterial compressions transmitted from the central retinal artery to the adjacent central retinal vein—which are known as venous pulsations and are seen in approximately 80% of normal patients without IH—disappear. Doctors often consider an absence of venous pulsations at the optic nerve head as a sign of IH.
Is it possible to have IH without papilledema?
While many physicians believe papilledema must be present in order to confirm an IH diagnosis, others argue that papilledema does not always have to be present. In some instances, a patient with intracranial hypertension may not show papilledema because of a time lag between the initial rise in cerebrospinal fluid (CSF) pressure and the development of papilledema. Therefore, at the time of diagnosis, papilledema is not seen.
Another theory is that the actual increase in CSF pressure may produce headache but is not high enough to produce papilledema.
Anatomy may also play a role. Variations in the micro-anatomy of the optic nerve can restrict CSF and prevent it from entering the optic nerve’s subarachnoid space. Or CSF pressure may be directed to a point of lesser resistance, such as the sella turcica (pituitary gland fossa) which then produces the appearance of an empty sella on imaging studies.
Other evidence of intracranial hypertension without papilledema is found in the fact that it’s possible to have asymmetric papilledema, in which the optic nerve swelling is either worse in one eye or only develops in one eye, rather than both.
To Learn More:
For more information on papilledema, please see:
Wraige E, Chandler C, Pohl KR. Idiopathic intracranial hypertension: is papilloedema inevitable? Arch Dis Child. 2002 Sep; 87(3):223-4.
Marcelis J, Silberstein SD. Idiopathic intracranial hypertension without papilledema.
Arch Neurol. 1991 Apr; 48(4):392-9.
Brazis PW, Lee AG. Elevated intracranial pressure and pseudotumor cerebri.
Curr Opin Ophthalmol. 1998 Dec; 9(6):27-32. Review.