described in cases of compressive optic neuropathy at the orbital apex and also in patients with sixth nerve palsy. In the case under discussion, tumor regression was likely associated with the vascular and hormonal changes associated with the postpartum state.

Diagnosis: Optic nerve sheath meningioma

Tip: Radiographic evaluation of a progressive, unilateral optic neuropathy is incomplete without a post-contrast fat-suppressed orbital MRI that includes axial and coronal views.

Headache and papilledema

Case: A 25-year-old cashier presented to her local emergency room with a one-week history of severe headache and pulsatile tinnitus. She had been

previously healthy and was on no medications. She reported a 10 pound (4.5 kg) weight gain over the preceding year with current weight of 150 pounds (68 kg). Neurologic examination was normal except for bilateral disc edema. Visual acuity, pupillary responses and confrontational visual fields were normal. A CT scan including contrast infusion was unrevealing. Lumbar puncture demonstrated an opening pressure of 360 mm of water in the lateral decubitus position with normal cerebrospinal fluid (CSF) protein, glucose and cell count.

She received a diagnosis of idiopathic intracranial hypertension (IIH) and was started on oral acetazolamide. Neurologic follow-up was scheduled in two weeks, however one week later she returned to the emergency room because of a brief generalized seizure. MRI showed a venous infarct in the left parietal region and a CT venogram demonstrated thrombosis of the left transverse sinus (Figure 4.5).

The patient was anticoagulated and subsequently found to have anticardiolipin antibody syndrome.

Discussion: The criteria for the diagnosis of idiopathic intracranial hypertension (termed pseudotumor cerebri in the older literature) are: elevated intracranial pressure (>250 mm of water in adults and 200 mm in children) with normal CSF constituents, signs and symptoms limited to those of increased intracranial pressure (ICP), and no radiographic evidence of tumor, hydrocephalus or venous sinus occlusion. IIH usually affects otherwise healthy obese women of childbearing age. The mechanism by which obesity produces increased ICP is not completely understood but evidence suggests an abnormality of vitamin A metabolism. Cases fulfilling the diagnostic criteria but due

to a specific identifiable mechanism are sometimes referred to as secondary pseudotumor (see Table 4.1). With modern neuro-imaging, the process of ruling out a neoplasm or other mass lesion as the cause of increased ICP is usually straightforward. The identification of cerebral venous sinus thrombosis (CVT) may present more of a challenge.

The severity of clinical signs in patients with CVT depends in large part on the mechanism and the location of the occlusion. Signs and symptoms are usually more fulminant in patients with thrombosis compared to those with mass lesions causing compression of venous structures. Occlusion of the anterior portion of the superior sagittal sinus produces only mild symptoms, whereas involvement of the posterior portion results in severe clinical manifestations. Occlusion of the dominant

Table 4.1 Conditions associated with pseudotumor cerebri syndrome

Endocrine disorders obesity

hypoadrenalism (spontaneous or steroid withdrawal) hypoparathyroidism

growth hormone replacement

a thyroid replacement in children Toxins

excessive vitamin A (vitamin, liver, isotretinoin) tetracycline, minocycline

lithium chlordecone nalidixic acid

Increased cerebral venous pressure

sinus thrombosis (congenital or acquired coagulopathies)

compression of venous structures radical neck dissection arteriovenous malformation

Systemic conditions uremia iron-deficiency anemia

systemic lupus erythematosus

(usually right) lateral sinus causes markedly increased ICP, whereas occlusion of the nondominant side may be asymptomatic. Occlusion within the deep venous drainage system (straight sinus or the vein of Galen) usually presents with altered consciousness and long tract signs, and pursues a rapidly downhill course. In contrast, obstruction of the superior sagittal or transverse sinuses may cause signs and symptoms only due to increased ICP. In such cases with normal CSF constituents, the clinical picture may thus mimic IIH.

While CT scanning may reveal abnormalities in patients with CVT, its relative lack of sensitivity limits its usefulness in this condition. On a noncontrast CT, a thrombosed sinus may appear as an abnormally high density within the sinus. This abnormality, however, is identifiable in only 5% of cases. Cerebral edema and areas of hemorrhagic infarction may be seen but are often absent. Follow-

ing contrast administration, an “empty delta sign” may be seen, reflecting low-density clot surrounded by a border of enhancement of collateral veins in the sinus wall. While distinctive, this abnormality is found in only about one-third of cases.

In contrast, MR imaging is an extremely sensitive modality for the detection of CVT and has become the technique of choice for the diagnosis and followup of such patients. A thrombosed sinus appears hyperintense on T1and T2-weighted sequences of a routine brain MRI due to accumulation and degradation of hemoglobin. The pitfall of relying on MRI alone is the occurrence of false negative and false positive studies in certain circumstances. A falsely negative MRI, i.e. absence of bright signal in the affected venous structure, can occur if the study is performed very early or very late in the course of sinus thrombosis. In the first two or three days following acute thrombosis, the involved sinus is isointense to brain on T1-weighted images and remains hypointense on T2-weighted images. Later in the course, the sinus may lose its hyperintense signal and regain a more normal appearance, particularly if partial re-canalization has occurred. A false-positive MRI may occur when venous flow is slowed but not thrombosed. In either case, the addition of venography (MR or CT) should help clarify the diagnosis. Post-contrast MR and CT venography (MRV and CTV) are highly sensitive for demonstrating absent or decreased venous flow. These two modalities are comparable in terms of sensitivity.

In the evaluation of patients with increased ICP, CT scanning is usually adequate for excluding a mass lesion and hydrocephalus but is insensitive for the detection of venous sinus thrombosis. CT is also relatively insensitive for the detection of certain other conditions, such as gliomatosis cerebri and meningeal inflammation, that may cause increased ICP without ventriculomegaly or a mass lesion. Consequently post-contrast MRI has generally been considered a requisite for evaluating patients with suspected increased ICP. The addition of MRV or CTV is currently recommended to identify those patients with sinus thrombosis. In one study of patients with presumed IIH who had been

66Chapter 4: Radiographic errors

imaged only with MRI, the addition of MRV led to the identification of a cerebral venous thrombosis in 10 % of cases. Most of these patients, however, did not fit the typical demographic profile of IIH. Such “atypical” patients include men, non-obese women, prepubescent children and patients older than age 44 years.

Diagnosis: Cerebral venous sinus thrombosis

Tip: The investigation of patients with suspected idiopathic intracranial hypertension should include post-contrast MRI. The inclusion of venography is especially important in the evaluation of “atypical” patients.

Idiopathic ptosis and miosis

Case: A 26-year-old law student noticed drooping of his right upper lid for about one year. He had no eye pain, headache or visual symptoms but recently developed brief episodes of tingling in his right cheek, prompting him to seek medical attention. Examination showed 2 mm of right upper lid ptosis without definite lower lid ptosis (Figure 4.6A). There was no lid fatigue or twitch sign. Eye movements were full and saccades were brisk and accurate. In dim room light, pupils measured 4.5 mm OD and 6.0 mm OS. Both pupils constricted briskly to light stimulation but the right pupil exhibited dilation lag in darkness. Pharmacologic testing with 1% apraclonidine showed retraction of the upper lid and pupillary dilation on the right side, consistent with adrenergic denervation supersensitivity (Figure 4.6B). He returned one week later for hydroxyamphetamine testing. After instillation of two drops of 1% hydroxyamphetamine in each eye, there was dilation of the left pupil but no response in the right pupil (Figure 4.6C).

A diagnosis of postganglionic Horner syndrome was made. An MRI of the brain with views of the craniocervical junction was normal (Figure 4.7). The patient was satisfied with a diagnosis of idiopathic Horner syndrome, especially as it had been a year since onset and he continued to feel well in other respects. However his grandfather, who was

A

B

C

Figure 4.6 Young man with a right Horner syndrome.

(A)Baseline examination in dim illumination shows mild right upper lid ptosis and a smaller pupil on the right side.

(B)In room light, following instillation of topical apraclonidine in each eye, there is dilation of the right pupil and retraction of the right lid, consistent with adrenergic denervation supersensitivity. There is no appreciable effect of apraclonidine in the left eye. Note that the anisocoria now appears “reversed”. (C) Following instillation of topical hydroxyamphetamine in each eye (at a separate visit), there is pupillary dilation and lid retraction on the left side but no response on the right. The asymmetric response to hydroxyamphetamine localizes the patient’s right Horner syndrome to the postganglionic sympathetic fibers.

a retired radiologist, requested neuro-ophthalmic consultation.

Why is the current study incomplete?

The sympathetic pathway to the eye is an ipsilateral, three-neuron pathway (Figure 4.8). The

Figure 4.7 Axial non-contrast T2-weighted MRI of the above patient with a right postganglionic Horner syndrome of undetermined etiology. There is no abnormality of the carotid artery or brainstem. The remainder of the scan was also normal.

first-order (central) neuron descends from the hypothalamus to synapse in the intermediolateral gray column of the cervicothoracic spinal cord. The second-order (preganglionic) neuron exits with the ventral rootlets of C8–T2, passes across the apex of the lung and ascends in the neck via the sympathetic chain to synapse in the superior cervical ganglion. The third-order (postganglionic) neuron that supplies the pupillodilator and tarsal muscles travels with the internal carotid artery as a plexus around its wall and re-enters the intracranial space via the carotid canal and foramen lacerum. In the cavernous sinus, the oculosympathetic fibers briefly join with the abducens nerve and, once through the orbital apex, follow the nasociliary nerve to the eye.

This patient has a painless, postganglionic Horner syndrome and an unrevealing brain MRI. Keeping

Figure 4.8 Schematic diagram of the sympathetic innervation to the pupil and eyelids. (From G. T. Liu, N. J. Volpe, S. L. Galetta. Neuro-Ophthalmology, Diagnosis and Management (Philadelphia: W. B. Saunders, 2001), page 430, with permission.)

in mind the pathway of the sympathetic fibers to the eye, it should be clear that a complete radiographic investigation should include views down to the level of the superior cervical ganglion. A second MRI with neck images was obtained and revealed a large hypervascular lesion at the bifurcation of the right carotid artery (Figure 4.9). The patient underwent an excisional biopsy, which revealed the lesion to be a paraganglioma.

Discussion: Horner syndrome is caused by interruption of the sympathetic innervation to the head and eye producing miosis, ptosis and facial anhidrosis on the side of the lesion. Upper and lower lid ptosis results from superior and inferior tarsal muscle weakness. However, drooping of the lid is generally mild and about 12% of patients with

Horner syndrome do not have clinically apparent ptosis. Facial anhidrosis is seldom reported by patients. Thus, anisocoria is the most consistent sign of an oculosympathetic defect. The feature that best defines the anisocoria as oculosympathetic deficiency is dilation lag of the smaller pupil in darkness. When the room light is abruptly turned off, the Horner pupil shows slow and delayed dilation over 15–20 seconds compared to the normal pupil which promptly re-dilates back to baseline within 5 seconds. When dilation lag is absent, confirmation of Horner syndrome is provided by pharmacologic testing. The two most commonly used agents for this purpose are cocaine and apraclonidine (see Table 4.2).

Etiologies of Horner syndrome are varied, and their frequency depends on the location of the oculosympathetic defect. Central Horner syndrome

is usually due to stroke (e.g. Wallenberg lateral medullary syndrome) and is accompanied by other symptoms and signs of brainstem dysfunction. Preganglionic Horner syndrome is often due to a neoplasm of the pulmonary apex, mediastinum or neck, identified in 20–50% of cases. Postganglionic oculosympathetic defects are commonly accompanied by ipsilateral head/face pain. The most common causes are carotid artery dissection, skull base tumors, lesions in the cavernous sinus and cluster headache.

Any patient with an unexplained Horner syndrome should undergo neuro-imaging to rule out a structural lesion. Localization of the lesion using the hydroxyamphetamine test helps to direct the imaging studies appropriately. If both pupils dilate to hydroxyamphetamine, the Horner syndrome is central or preganglionic; if the Horner pupil does not