Most cases of shunt malfunctions are due to occlusions (otherwise known as a blockage of the proximal ventricular catheter) In these instances pumping of the shunt will show a valve that is slow to refill, or does not refill at all. An imaging (CT or MRI) scan which shows ventricles large enough so that if the shunt were working properly, the valve should have refilled promptly.
In most cases of shunt malfunction, the diagnosis is obvious because of the obvious signs of elevated intracranial pressure, including headaches, vomiting and lethargy. In about 30% there will be only subtle signs of deterioration, with neuropsychologic, cognitive and behavioral symptoms heralding their shunt dysfunction.

Shunt blockage can also have much more subtle consequences and the headaches may be infrequent, the main problem being behavioural deterioration. In older children this might take the form of increased irritability, 'laziness', poor or disruptive school performance or even more antisocial activity. This may be very difficult to distinguish from the usual teenage angst but, if there is any reason to suspect that the deterioration in behaviour is not 'normal', assessment must be carried out by an experienced educational psychologist with a knowledge of hydrocephalus. If the shunt is to blame, a dramatic improvement can result from appropriate treatment, though this form of shunt problem is particularly difficult to diagnose. It may be necessary to monitor CSF pressure, often over 24 hours. This can be done using a pressure monitor in the scalp connected to a recorder. In this way pressure can be recorded during sleep and changes in posture. Scans to show the size of the ventricles are particularly useful if they can be compared to previous scans, though in someone with clear symptoms of either high or low CSF pressure they may also serve to support the diagnosis.

In a majority of cases,
infection is the cause of shunt malfunctions when a distal malfunction is suspected. The rate of shunt infection is about 10-15%, and 95% of infections will occur in the first 5 days after surgery. Overall, between 5 and 15% of shunts can be expected to become infected over the life of the shunt. Of these infections, 70% are diagnosed within one month after surgery and close to 90% by six months. There are, however, late shunt infections which can occur after six months of a shunt procedure. A preoperative CSF specimen from a shunt tap should be obtained to exclude this possibility. The more proximal system can be tested by insuring free flow of CSF, whereas the distal system can be tested by runoff using a manometer.The signs of shunt infection may include fever, neck stiffness, light sensitivity (also called photophobia), headaches, or signs of shunt malfunction. Shunt infections can present with signs of meningitis and ventriculitis. In addition, signs of septicemia or peritonitis can be seen, depending on the type of shunt. The shunt may be reddened along its course under the skin, or the wounds may be reddened and/or draining pus. Most commonly the bacteria responsible are those that reside normally in the skin of the patient - Staphylococcus species, distal shunt malfunctions frequently accompany shunt infections. In VA shunts blockage due to infection is rare, and many months or years can go by before the infection becomes apparent. During this time there will be tiredness, irritability, poor appetite, various aches and pains, skin rashes and other signs but all of these can be due to common disorders. A blood test will usually reveal anaemia and this is an important though, on its own, not a specific indication of infection. Blood cultures and even CSF cultures can be negative. Later, blood may appear in the urine due to secondary kidney damage.

Over-drainage is a difficult problem,a change of valve to a higher pressure cannot be relied upon to cure it, though it appears to do so in some cases. Studies have shown that the use of an 'antisyphon device', a small button inserted into the shunt tubing, will often solve the problem, but this does not always work. Some shunts have these built-in, but neurosurgical opinion varies as to whether they should be used. To change a valve pressure it is necessary to remove the valve and insert another.

Over-drainage may lead to a variety of problems such as subdural hematoma, post-shunt craniosynostosis and slit ventricle syndrome. As postural changes and patient height are partially related to the overdrainage, these problems, with the exception of craniosynostosis, are seen mostly in children and adolescents rather than infants. In the case of over-drainage, the shunt allows CSF to drain from the ventricles more quickly than it is produced. If this happens suddenly, usually soon after the shunt is inserted, then the ventricles in the brain collapse, tearing delicate blood vessels on the outside of the brain and causing a haemorrhage ('subdural haematoma'). This can be trivial or it can cause symptoms similar to those of a stroke. The blood may have to be removed, and in some cases if this is not done it may be a cause of epilepsy later. If the overdrainage is more gradual, the ventricles collapse gradually to become slit-like ('slit ventricles'). This often interferes with shunt function causing the opposite problem, high CSF pressure, to reappear, but unfortunately the slit ventricles do not always increase in size again, producing the situation where there is very high CSF pressure with headache, vomiting etc but very small ventricles on scan.

Under-drainage is where the fluid is not removed quickly enough and the symptoms of hydrocephalus return, is one of the most common problems. It is usually due to blockage of the upper or lower tubes of the shunt tissue, though it can be due to the shunt breaking or its parts becoming disconnected from each other. It is rarely due to the valve itself, which usually continues to function in the same way for years. Pressure may sometimes build up rapidly, resulting in loss of consciousness, and treatment is required as an emergency. However, in most cases the onset is more gradual, and can follow a minor illness such as a cold. Headaches increase in frequency and severity, often worse on waking in the morning. Vomiting and dizziness also occur, and sometimes other symptoms which vary from patient to patient.

The symptoms of overdrainage can be very similar to those of under-drainage though there are important differences. Headaches, dizziness and fainting occur and are often worse after getting up from lying down are the symptoms of under-drainage, whereas the headaches caused by high CSF pressure are often worse on waking, before rising in the morning. However, the best way to diagnose the problem, having recognized that one exists, is to monitor your sypmtoms and have your neurologist or neurosurgeon evaluate the patient. This can also be checked by measuring the CSF pressure over 24 hours.

Slit ventricle syndrome has been described in hydrocephalus patients with small ventricles on imaging who continue to have shunt malfunction–like symptoms even in the presence of a functioning shunt. They are sometimes so small that they are barely visible on CT scan or MRI. Symptoms usually present years after shunt placement or revision. Adults do not have slit ventricle syndrome unless it started in childhood. Symptoms can include headache, varying degrees of lethargy, with or without nausea, and vomiting. The symptoms may be intermittent. Slit ventricle syndrome is a constellation of symptoms which, for example, can appear in patients with a functioning shunt and in whom the brain has lost part of its elasticity. The symptoms consist of headaches, vomiting, drowsiness etc. Infuriatingly enough, these symptoms resemble those of shunt malformation. The appearance of the symptoms is very cyclical, often with the regularity of a clock for example, the patient is well for three weeks, then violently ill and sleepy for 24 hours and then well again. There is more often than not no cause for the symptoms, although a minor viral illness can kick off the symptoms. Often the patient is taken to hospital and imaging is done which shows the ventricles to be small, or even unchanged, compared to previous imaging.To increase the frustration, the symptoms disappear spontaneously. Until the next time, that is. In ordinary circumstances, the shunt drains the cerebro-spinal fluid (CSF) from the ventricles to whichever cavity of the body. This causes, in certain patients, the ventricles to collapse. The ventricle closes on the ventricular catheter that drains the ventricles and blocks off the outflow of CSF. That's when the symptoms appear. The pressure in the brain rises very quickly and the patient becomes ill. Because the brain has lost some of its elasticity due to the initial disease which caused the hydrocephalus or the hydrocephalus itself, the ventricles do not blow up rapidly and the symptoms can persist. After a while, the ventricles get slightly bigger and the normal CSF drainage resumes. All these changes, in shape and size, may not be easy to detect and often the illness goes unexplained.