Blood Loss

Acute or chronic blood loss may also cause kiss of vision or field in one or both eyes. Acute blood loss rarely causes these symptoms unless chronic compromise of the ocular circulation already exists or the blood loss is severe (as in postpartum hemorrhage and massive gastrointestinal hemorrhage). A day or two after the hemorrhage, the patient may complain of loss of vision and show a decrease in vision or an altitudinal field loss, particularly in the lower field. The optic disc shows an ischemic edema.

Chronic Anemia in Pregnancy

Chronic anemia in pregnancy occurred in the past but is rarely seen today.

Vascular Hypotension

Vascular or any other kind of surgical procedure in which cardiac arrest or prolonged hypotension occurs can leave the patient with unilateral loss of vision or even cortical blindness. The degree and duration of hypotension and the status of the vessels in the affected area are factors that contribute to the degree of visual loss.

Blood Dyscrasias

Blood dyscrasias are of several varieties. Optic neuritis with a cecocentral field defect is well known in pernicious anemia. This disease usually occurs in persons over the age of 30, and it should be suspected in patients who have optic neuritis with evidence of glossitis and gastrointestinal symptoms. It is a macrocytic anemia with a defect in the gastrointestinal absorption of vitamin B₁₂, The Schilling test, the accepted confirmatory test, measures the body's ability to absorb radioactive vitamin B₁₂ over a 24-hour period. Another frequently used test involves the injection of histamine and the measure. ment of hydrochloric acid secretion in the stomacn. (This secretion is severely impaired in pernicious anemia.) The treatment of per. nicious anemia is administration of hydroxycobalamin rather than the standard form of vitamin B₁₂, thus bypassing the enzymatic defect that prevents proper absorption.

Another form of blood abnormality—and an all too common one—is central retinal vein occlusion. Patients with this abnormality complain initially of blurred vision; their visual acuity is usually 20/200 or better. The fundus shows hemorrhage, particularly along the veins, which are fat and sausage-like, with sludging of the blood column. Sometimes, such veins are seen with no hemorrhage, it condition referred to as impending venous occlusion. Patients with impending venous occlusion usually have no symptoms. Some have transient blurring of vision, which leads them to seek an evaluation.

Central retinal vein occlusion is caused by sludging of the blood column, which may be caused by an elevated pressure in the eye (glaucoma), slowing of the blood column, or a decrease in blood flow into the eye with normal intraocular pressure. It may also occur in known diabetes or as a premonitory symptom of early unsuspected diabetes. Central retinal vein occlusion is caused by n change in blood viscosity, and it can also be seen in the rarer hyperviscosity syndromes, such as Waldenstrom's macroglobulinemia. These hyperviscosity syndromes, which can be identified by paper electrophoresis of the blood proteins, can also occur as secondary manifestations of carcinoma. Polycythemia Vera can also cause sludging of the blood; it may occasionally be secondary to a hemangioblastoma of the cerebellum.

The diagnosis of central retinal vein occlusion usually presents no problem, since the appearance of the retina is dramatic and specific.

Transient Obscurations

Blurred vision from transient obscurations Is occasionally seen with increased intracranial pressure. Because the episodes last only 5 to 15 seconds and occur only a few times a day, the patient usually thinks they are insignificant and does not complain of them. I happen to have treated several patients who experienced transient obscurations frequently during the day and so did complain of them.

The obscurations differ from the amaurosis fugax of carotid artery insufficiency, which lasts anywhere from 5 to 25 minutes and which the patient notices and does complain about. Transient obscurations are usually bilateral, and they are always related to increased intracranial pressure. Other causes of disc edema do not cause transient obscurations; thus they are quite specific.

Cortical Blindness

The most common cause of cortical blindness is arteriosclerosis, and it obviously occurs in the older age group. In the young patient, the causes are more protean. They include trauma, poisoning from carbon monoxide and nitrous oxide, neoplasms, and infections (e.g., meningococcal, mumps, rubeola, and syphilitic meningitis). Cortical blindness also occurs after seizures and represents a form of Todd's paralysis.

In the days of ventriculography, it would occur occasionally with that procedure. The cause of the loss owing to ventriculography is only speculative. It was felt that it occurred more commonly with multiple passes of the ventriculography needle or was due to some shift of the brain when the ventricular fluid was withdrawn. Cardiac surgery with cardiac arrest or severe decreased blood pressure during the procedure is also implicated in cortical blindness.

Rare cases of cortical blindness have been reported with acute intermittent porphyria, blood transfusions, and temporal arteritis. Cortical blindness also occurs, although rarely, as the result of infectious disease of the CNS, Hemophilus influenzae is the usually reported agent, although the mechanism is not entirely clear. Pathologic descriptions have included occlusion, necrosis, and thrombophlebitis on the venous side of the circulation. There have also been findings of arteritis and subarachnoid exudate. Toxic factors have also been considered to play a role in these cases. Schilder's disease, which is quite rare, is also a cause of cortical blindness in infants.

The final visual results from cerebral infarcts that cause cortical blindness is difficult to predict, particularly in infants and the newborn. Severe cerebral ischemia tends to affect different areas In different age groups. In infants who have rubric cerebral palsy, the parasagittal area is a common target area This is the watershed zone between major cerebral arteries. In a study by Volpe et al. of asphyxiated infants, recorded blood flow in the parasagittal area was significantly lower than in other areas. PET studies demonstrated a 25 to 50% decrease in flow rate compared with that to the sylvian fissure; normally there is no more than a 10% difference. Cortical blindness is less common in premature infants because of meningeal anastomosis between the cerebral arteries. Premature infants develop infarcts in the periventricular area, which is seen on scans as prominent cortical sulci, decreased periventricular °white matter, and, later, enlargement of the ventricles. Some of these children recover useful vision because of a rewiring of neurons and neurochemical adaptations not available to older patients with mature brain tissue and arteriosclerosis. Infantile brains may also have collateral axonal sprouting and replacement by supernumerary neurons, which are known to exist during early neural development.

The more severe the infarct appears on CT scan, the worse is the prognosis for vision. For example, Lambert et al. found that in more than 30 cortically blind infants, the more widespread the infarction on CT scan, the worse the visual results were. However, if the hypodense areas were scattered, the visual results were better, and these areas were interpreted as representing incomplete myelinization rather than infarcts. This feature is not found in adult cases of cortical blindness (Fig. 13.14).

Figure 13.14. CT scan of an adult patient with bilateral occipital infarcts (arrows) and cortical blindness.

In adult patients with cortical blindness, measurement of the VEP is not very reliable. Vision is so poor, if present at all, that only flash VEP can be done because of the inability to fixate on a pattern-reversal stimulus. The variability in the latency and amplitude of the VEP makes it difficult to tell where the lesion is along the visual pathway. Sometimes it is even difficult to determine whether the patient can or cannot perceive light, However, opening and closing of eyes will affect the posterior dominant alpha rhythm if there is not complete cortical blindness. A few cases have been reported in which there was no light perception documented over a significant period of time and multiple examinations and yet normal VEPs were present. It is postulated but not proven that the responses in such cases are mediated by extras geniculocalcarine pathways from the association areas of 18 and 19.

Intracranial Aneurysm

Rupture of an intracranial aneurysm is els ther a lethal or, at the least, a devastating disease. Ninety-five percent of aneurysms occur in the anterior circle of Willis. One-third of these occur on the anterior communicating artery and from its junction with the anterior cerebral artery. Signs of impending rupture are not always easy to detect. Headache, though a common symptom, is infrequently of a severity or type that would suggest an aneurysm and the workup that it demands. Most aneurysms do not cause visual loss. There are spontaneous fluctuations in vision that are not entirely understood. The mechanism may be related to variation in the size of the aneurysm or associated arteriospasrn from subarachnoid hemorrhage. Visual deficits can range from total blindness in ono eye to field defects, which are variable from one examiner to another. The presence of this variability should alert an astute clinician to the possibility of an aneurysm rather than attributing it to the variable quality of the examiners.

In one retrospective survey of aneurysm patients, the highest incidence of warning signs was for those aneurysms located at the junction of the internal carotid and posterior communicating arteries, with those at the bifurcation of the carotid and the middle cerebral arteries running a close second. The older the patient who ruptures an aneurysm, the fewer warning signs usually occur. The three mechanisms for these premonitory symptoms and signs are (a) vascular disturbances, (b) minor leakage of blood, and (c) ischemic lesions.

Ninety percent of intracranial aneurysms are congenital and average between 0.5 and 1.5 cm in diameter. Those that enlarge to 3 cm are called giant aneurysms. Aneurysms cause symptoms either by rupture into the Hubarachnoid space or by slow expansion with compression of nearby structures such as in the cavernous sinus. Those that are less than 5 mm in diameter rarely bleed. This is fortunate, since aneurysms 5 mm or more can be seen on today's high-quality MR machines if cuts are made in the appropriate place. If they are not seen, MRA is not so accurate as to preclude arteriography if the suspicion of aneurysm is high enough.

Aneurysms appear in 4% of adult autop-sies and are multiple in 20% of cases. They usually become symptomatic between 40 and 65 years of age. They account for 10% of all fatal cerebral vascular accidents and for 50% of all fatal cerebral vascular accidents occurring in patients under 45 years of age. Eighty-five percent of aneurysms occur on the anterior circle of Willis. In the 15% that occur in the posterior circulation, most occur at the bifurcation of the basilar artery and posterior cerebral artery.

The symptoms of aneurysm can vary from mild to the most severe of headaches. Alterations in consciousness can vary from mild confusion to unresponsiveness even to painful stimuli. Meningeal irritation can be demonstrated by positive Kernig and Brudzinski signs. These patients may also demonstrate phofophobia, hyperacusis and hyperesthesia, mild fever from meningeal irritation, or high fever secondary to hypothalamic disturbances. There may be other hypothalamic dysfunctions, such as vomiting, sweating, chills, and irregular heart rate. Focal damage may give a clue as to which artery is affected. Weakness in one or both legs suggests hemorrhage from the anterior communicating artery. Weakness in an arm and the face suggests a middle cerebral artery location. If a dense hemiplegia occurs, its location is usually in the internal capsule. This involvement is from an aneurysm in the upward extension of the internal carotid or in a middle cerebral artery. Since any intracranial aneurysm can cause a significant and sudden rise in cerebral spinal fluid pressure, a preretinal hemorrhage may be seen. If the preretinal hemorrhage is in front of the optic nerve, then no visual symptoms occur; however, if the hemorrhage occurs in a subhyaloid location in front of the macula, there may be a severe decrease in acuity. If the patient survives and the hemorrhage does not break out into the vitreous, the hemorrhage will absorb and good vision be restored.

Not all intracranial aneurysms present with the same signs and symptoms. The posterior cerebral artery passes around the cerebral peduncle medial to the temporal lobe, superior to the third nerve, and inferior to the optic tract. Aneurysms of this artery cause hemiplegia with involvement of the corticospinal tract in the cerebral peduncle, homonymous field defects, temporal lobe seizures, and third cranial nerve paresis.

The anterior communicating and anterior cerebral arteries are located above the optic nerve and chiasm and below the olfactory nerve. A giant: aneurysm of the anterior cerebral artery causes unilateral loss of vision and smell; that of the anterior communicating artery may give a bitemporal field defect.

Carotid ophthalmic aneurysms account for about 5% of the total number. They occur on the superior or medial surface of the internal carotid artery above the cavernous sinus. As they enlarge, they can erode the optic canal or anterior clinoid bone, which can be seen even on routine tomograms, and cause associated visual loss. Supraclinoid aneurysms can also cause visual loss. They also develop from the internal carotid artery but distal to the origin of the ophthalmic artery.

Aneurysms located along the middle cerebral artery occur in the sylvian fissure and cause hemiplegia, focal seizures, homonymous field defects, and speech problems.

Aneurysms of the posterior communicating artery occur near the junction of the internal carotid artery, usually between the anterior choroidal and posterior communicating arteries, and rarely, at the junction of the two. These aneurysms present with classic third cranial nerve palsy with pupillary involvement. These aneurysms and those of the cavernous sinus are more fully described in the section on third cranial nerve paralysis in Chapter 6, Diplopia.

Aneurysms occurring on the basilar artery, particularly in the interpeduncular fossa, produce third cranial nerve paresis and headaches that are not necessarily caused by rupture of the aneurysm but by the obstruction of the sylvian aqueduct that then causes increased intracranial pressure and autonomic disturbances with hypothalamic pressure. Pressure on the fifth nerve can cause a ticlike syndrome, and pressure on the facial nerve in particular can cause hemifacial spasm. Aneurysms located on the vertebral artery and posterior inferior cerebellar artery cause apraxia and bulbar involvement. If they occur on the anterior inferior cerebellar artery, they may cause hemifacial spasm and can mimic a cerebellopontine angle tumor or Ménière's disease.