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Benign
intracranial hypertension is a condition wherein there is intracranial
hypertension with no demonstrable mass or any other abnormality. Three
names are in use today, pseudotumor cerebri, idiopathic intracranial
hypertension and benign intracranial hypertension.
It
is a diagnosis of exclusion. There is no single test or procedure leading
to the correct diagnosis of this syndrome.
Historically
BIH was referred to, as brain swelling of unknown cause, otitic or toxic
hydrocephalus, pseudoabscess, hypertensive meningela hydrops or serous
meningitis.
Epidemiology:
BIH
is a rare disease. There are no reliable epidemiological
data. It occurs in any age group including children and infants;
but is most common in obese young females between 25 and 35 years of
age. The mean age at onset of symptoms is about 30 years. 80-90% is
woman, and most of them are obese (mean weight 90 kg). Some studies
suggest an annual incidence rate of 1 per 100,000. In the high-risk
group of obese woman in the reproductive age-group, the annual incidence
rate is suggested to be about 20 per 100,000 people.
Etiology:
The
etiology of BIH remains obscure. Because of the different
pathophysiology and prognosis BIH is considered separate from conditions
producing symptoms identical to benign intracranial hypertension, such as
chronic meningoencephalitis venous sinus thrombophlebitis, congestive
intrathoracic lesions, polyradiculitis and spinal cord tumors.
The
most consistent finding in patients with BIH is obesity (40-80% of all
patients). Mostly a marked weight gain has been observed several months
before the symptoms occurred. Hormonal changing periods (menarche,
pregnancy, oestrogen therapy) and signs of hormonal dysfunction
(hypertrichiasis, sometimes galactorrhoea) are often associated with
BIH. Therefore it has been supposed that endocrine dysfunction may
be a cause of BIH but up to now there is little supporting
evidence. Occasionally drugs are suggested to cause BIH, e.g.
tetracycline, nalidixic acid or vitamin A (usually to treat acne in
adolescents or young women). Vitamin deficiency may also be
involved in the development of BIH, especially vitamin A and B12.
Pathophysiological
considerations:
We
know little about the pathophysiological mechanism leading to the
symptomatology.
The
pathophysiology of genuine BIH is still discussed controversially.
Increased brain water content, increased cerebral blood volume and
increased outflow resistance to CSF have all been suggested.
Increased
brain water content of about 4% was found in a study. It is
attributed to intracellular and/or extracellular swelling and leads to a
decrease in craniospinal compliance which counteracts ventricular
dilatation. The increased brain water content renders the brain
less compressible. Thus there will be in case of increased
resistance to CSF outflow a high pressure but no enlargement of the
ventricles. The result is a diminished total CSF volume and a
ventricular size that is even smaller than normal. In the
pathophysiological concept of Malm extracellular edema causes
partial compression of the major venous sinus with consequent rise in
sagittal sinus pressure as one mechanism for the development of increased
CSF pressure.
Increased
cerebral blood volume (CBV), according to the current knowledge, is found
in the majority of cases. It is the result of cerebral
vasodilatation or increased intracranial pressure, which leads to an
increase in pressure in the veins traversing the sub arachnoid space with
a consequent rise in total CBV.
Increased
resistance to outflow of CSF is according to the current pathogenic
hypothesis the basic defect in the development of BIH. The increased
resistance may be caused by congenitally fewer arachnoid absorptive
channels or acquired structural changes in the arachnoid villi. Now
the symptoms of intracranial hypertension may develop
by:
Further
increase in resistance at the level of the villi (e.g. caused by steroid
withdrawal, female sex hormone changes, pituitary-adrenal dysfunction,
hypo para-thyroidism, tetracycline or nalidixic acid therapy), leading to
interstitial brain water accumulation and elevation of CSF pressure.
Intracellular
brain water accumulation caused by agents or events interfering with
membrane structure or function and brain water permeability (e.g. head
injury, anemia, vasopressin, or other endocrine dysfunctions).
Both
mechanisms will bring about a rise in intracranial pressure until the CSF
absorptive level for the new equilibrium of CSF dynamics is
reached.
In
some, the BIH syndrome may represent a response to some exogenous factors
and endogenous physiological alterations.
Such
conditions include:
Intracranial
venous drainage occlusion (mastoiditis and lateral sinus obstruction,
paranasal sinus and phayngeal infections, congenital atresia or stenosis
of venous sinuses, extracerebral mass lesions, head injury, polycythemia
vera).
Cervical
or thoracic venous drainage obstruction (intrathoracic mass lesions and
postoperative obstruction of venous return).
Endocrine
dysfunction (pregnancy, menarche, oral contraceptives, obesity, Addison's
disease, hypothyroidism).
Hematological
disorders (acute iron deficiency anemia, pernicious anemia,
thrombocytopenia).
Vitamin
metabolism (hyper/hypo vitaminosis A, vitamin D deficiency).
Reaction
to drugs such as tetracycline, penicilin, sulfamethoxazole, indomethacin,
nalidixic acid, and prophylactic antisera.
Galactosemia,
galactokinase deficiency, sydenham's chorea, sarcoidosis, and Turner's
syndrome are some of the rarer associated conditions.
Symptoms:
Suggested
criteria for BIH include, the symptoms of increased intracranial
pressure (ICP), papilledema and ICP values of more than 20 mmHg, and
absence of focal neurological signs, except those typically for raised
ICP, such as a sixth nerve palsy. There is no symptomatic epilepsy and no
impairment of consciousness; normal composition of cerebrospinal fluid
(CSF); no mass lesion or hydrocephalus on CT scan.
The
typical clinical picture of BIH is that of a young or middle aged obese
woman with headache and sometimes impaired vision. Dizziness, nausea,
vomiting, tinnitus and hearing loss are occasional associated symptoms.
rarely there is CSF rhinorrhoea.
The symptoms of increased intracranial pressure are often intermittent
but usually with a history of less than one year. In contrast to
space-occupying lesions the symptoms of patients with BIH are usually less
pronounced. In addition patients with BIH are characterized by
greater daily variations of their complaints, i.e. every day they feel
different. Signs of lethargy, decreased activity or disturbances of
consciousness are usually missing. Patients with BIH often do not
see the doctor until they suffer from marked visual impairment.
Children
present with diplopia due to 6th nerve paresis which is a secondary sign
due to increased ICP.
In
infants, progressive increase in the size of the head with bulging
fontanels is is presenting symptom.
Diagnostic
evaluation:
BIH
is a diagnostic syndrome and no single test leads to the correct
diagnosis.
Fundi
examination reveals papilledema. Neurological examination usually reveals
no focal neurological signs except those which are typical for raised
intracranial pressure such as sixth nerve palsy (10%), slight gait
disturbance and ataxia mostly due to dizziness. The IIIrd and the
Vth cranial nerve involvement and visual field defect can be observed
rarely.
Chronic
meningoencephalitis, venous sinus thrombophlebitis, congestive
intrathoracic lesions, polyradiculitis and spinal cord tumors may produce
symptoms identical to BIH. These conditions must be ruled out
before the diagnosis of benign intracranial hypertension can be
established.
CT
& MRI:
The
symptoms of increased intracranial hypertension demand a CT or MRI to
exclude a mass lesion or hydrocephalus. The typical finding is a
ventricular system of normal or diminished size (Evans ratio < 0.25
the width of the frontal horns divided by the maximum internal skull
diameter). Enlarged optic nerve sheaths can be observed on the
computed tomograms of the orbit. Attention must be paid to the
patency of the sinus thrombosis and thus symptomatic intracranial
hypertension. In about 10-40% of patients with BIH an empty sella
is diagnosed , and occasionally a neurodegenerative disease can be
revealed. CT or MRI must be performed before a lumbar puncture is
considered.
Lumbar
puncture:
The
examination of CSF is compulsory to exclude symptomatic forms of
intracranial hypertension. In BIH the typical findings are
completely normal values of protein, glucose and cells. The
diagnosis requires the demonstration of significantly increased ICP >
20 mmHg.
Normally
it is sufficient to measure ICP by Lumbar pressure measurement over a
period of 30-120 min. This minimally invasive procedure enables one
to recognize steady state increased ICP and abnormal pressure
waves. In BIH lumbar pressure correlates very well with pressure
values recorded on other sites, provided that a strictly flat lateral
recumbent position is maintained. If there is any doubt of the
quality of the recorded pressure, other forms of pressure measurements
with overnight recording of ICP must be used.
Visual
assessment:
Visual
fields, intraocular pressure measurement, fundus photographs and visual
acuity are performed, since the most serious sequel of BIH is permanent
visual loss due to the chronic papilledema. BIH is a cause of
progressive visual loss in children and young adults.
Papilledema
(95% <10% unilateral, dioptres mostly ranging from 1.0 to 4.0), visual
obscurations or blurring of vision (80%), double vision (20%), partial
visual field loss (20%), enlarged blind spots (90%) and loss of central
vision are common. The results of these investigations determine the
therapeutic procedure and the urgency of treatment .
Measurement
of CSF outflow resistance (R):
Provides
specific diagnostic information in BIH. It can be performed by means of
lumbar infusion. Computerized infusion tests should be preferred to
reduce duration and plateau pressure as well as to increase
accuracy. R is found to be increased up to 50 mmHG/min/ml.
Mean R ranges are 12 to 35 mmHg/min/ml. However, there is no
correlation between ICP and R.
ICP
recording:
ICP
overnight recording reveals an increased intracranial pressure (>15
mmHg) with plateau-waves in 30% of patients and B-waves in 90% of
patients. There is also a suggestion of transient episodes of raised ICP
due to nocturnal hypoxia and hypercarbia in some patients which may
contribute to symptoms such as papilledema and visual failure.
Management:
BIH
is self limiting with no sequelae in most.
Management
of patients with BIH should be based on the presence and progression of
visual loss.
BIH
must be treated, if rapid or progressive visual impairment occurs. Hence,
treatment and follow-up must be performed together with an
ophthalmologist.
Treatment
is directed at the associated cause, if any (such as steroids in
Addison's, and iron in anemia). Elimination of exogenous
factors and endogenous physiological alterations, promptly gives relief.
Acetazolamide
and/or Frusemide is given for a period of 6-24 months to lower CSF
production.
Steroids
for about 2.4 weeks may be effective, but the bodyweight of obese young
women is frequently increasing, so that it should not be recommended as a
therapeutic measure of first choice. Many physicians question the role of
steroids.
Drug
therapy provides temporary improvement but fails in approximately one
third of cases. Despite all the measures above, impress upon the
patient the necessity to lose weight.
Lumbar
puncture is also only a temporary measure, but it can easily be performed
to relieve the symptoms in the acute phase. Furthermore it is
compulsory for diagnostic evaluation, i.e. to exclude pathological
composition of CSF as a possible cause of intracranial hypertension and
to measure outflow resistance.
Shunting
provides long lasting CSF pressure reduction. It can be carried out
as a lumboperitoneal shunt, or ventriculo-peritoneal/atrial shunt.
Decompressive
procedures to prevent further visual loss are frequently needed.
Presently,
optic nerve sheath decompression is the treatment of choice in patients
with visual loss, but this does not alleviate the increased ICP or its
other symptoms of increased ICP. However it improves visual function
in about 70% of all operated eyes in acute papilledema and 30% in chronic
papilledema. Repeated optic nerve sheath decompression or optic nerve
fenestration should be considered in cases of persistent recurring
symptoms.
CSF
draining into the orbit has been hypothesized as its mechanism of action.
But orbit can not accommodate all the CSF. pulse pressure damping may
have a role. resultant scarring around the nerve may protect the nerve;
but this can not explain the improvement of the opposite nerve. Sub
temporal decompression is less commonly performed nowadays. Bilateral
decompression may be required. It does not always prevent further visual
loss, and may result in contra-lateral partial seizures, mostly beginning
6-12 months post-operatively.
Follow-up
assessment and prognosis
BIH
is a self limiting disorder with a course of less than 12 months in most
cases. It is characterized by a high spontaneous remission rate and
a very good long term prognosis. After 1 to 4 years only 20% of all
patients formerly presenting with symptoms of elevated ICP are still
suffering from headache. The other signs usually disappear
completely. Sometimes however it runs a protracted course, with
recurring symptoms.
In
less than 50% the papilledema resolves and the majority develops chronic
disc changes, e.g. about 50% develop optic disc gliosis. Rarely,
permanent visual loss is seen in patients with BIH.
It
is of clinical importance to emphasize that after the disappearance of
the symptoms the physiological abnormalities remain. The main
parameters (ICP, R) tend toward the normal values but do not reach
them. A sustained increased ICP is seen in about 60% of cases (i.e.
> 15 mmHg) despite the well-being of the patients. Even more
astonishing is the fact that measurements of R up to 48 months after the
time of chief complaints continue to show increased values; in 80% of
cases the R was > 12mmHg/min/ml. In follow up assessment,
frusemide and acetazolmide must be tapered off 2 weeks before infusion tests
to obtain correct results in ICP and R measurements.
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