Surgical removal of epileptogenic brain tissue is indicated for treatment of many medically refractory focal seizure disorders, epilepsy being by far the most common. One of the important factors in providing good results from such surgery is the degree of accuracy in identifying epileptogenic foci. Various approaches have been used in attempting to determine epileptogenic foci, one of them being the employment of subdural electrodes. Of course, all such approaches involve sensing of cortical electrical activity using electrical contacts or disks as parts of electrodes applied in various ways.
Intracranial surgery for the treatment of epilepsy was undertaken at least as early as about 1927. Such surgery is known to have involved the use of some sort of electrode in some way. Subdural electrodes have been used to help identify epileptogenic foci since at least as early as 1954. Stated another way, neurosurgeons have had a concern about the mechanical and electrical integrity of subdural electrodes--and especially about the integrity of connections within the electrode itself--for more than 35 years.
Intracranial recording techniques have used either of two different types of electrodes--intracortical depth electrodes or subdural electrodes. The relative safety of subdural electrodes is due to the fact that, unlike depth electrodes, they are not invasive of brain tissue. Depth electrodes are narrow, typically cylindrical dielectric structures which are inserted into the brain in order to establish good electrical contact with different portions of the rain. On the other hand, subdural electrodes are flat strips which support contacts or disks spaced thereon. An earlier type of such electrode used as an electrical contact a small metallic ball formed on the end of a lead wire.
Subdural electrodes are inserted between the dura and the brain, along the surface of and in contact with the brain, but not within the brain. Insertion is by incising the patient's scalp and spreading it with retractors, drilling a burr hole in the skull and incising the dura across the diameter of the burr hole. Tack-up sutures are placed in both dural margins for retraction and the electrode grasped with forceps and inserted under the dural edge, all in a known manner. Typically, four electrodes are used per burr hole and four burr holes in the patient's skull are used for more accurate location of the foci.
The lead wires for all electrodes are brought to the exterior and following temporary sutured closure of the dura and the scalp, cortical electrical activity is monitored. Typically, such monitoring occurs over a period of one to three weeks. At the end of the monitoring period, the electrodes are, of course, removed. With the exception of electrodes made in accordance with U.S. Pat. No. 4,735,208, such removal often requires major general surgery in the operating room, using a general anesthetic. This is not only expensive but it is also very difficult for the patient.
Also of the foregoing activity (and the anticipation of the procedure) can be very traumatizing to the patient, both physically and phychologically. The psychological effect may be rather profound, notwithstanding the relative safety of the procedure. Clearly, any inventive advancement which not only facilitates successful treatment of certain seizure disorders (which are sometimes a source of acute embarrassment to the patient) but which also helps avoid unnecessary major surgery would be welcomed by patient and physician alike.
Subdural electrodes occur in at least two embodiments, namely strip electrodes and grid electrodes. A strip electrode is shown in U.S. Pat. No. 4,735,208 and such electrodes use upper and lower elongated flexible dielectric layers to form the strip. Confined between the layers is a plurality of spaced, aligned flat contact disks together with their electrical lead wires.
Subdural grid electrodes use an array of spaced flat disks similarly confined between two dielectric layers. An example of such a grid electrode is shown in U.S. Pat. No. 4,869,255. The aforementioned patents are assigned to the same assignee as this invention and are incorporated herein by reference.
Irrespective of whether a subdural electrode is of the strip or grid type, such electrodes include an imperforate upper dielectric layer which is that layer which contacts the dura tissue. Such electrodes also have a lower dielectric layer which is in direct contact with the brain when the electrode has been inserted between the brain and the surrounding dura tissue. This lower layer has at least one and preferably a plurality of openings therethrough. A plurality of electrode disks is interposed and confined between the layers, there being one disk for each opening in the lower dielectric layer. Each disk is positioned adjacent its associated opening and is connected to a separate, dedicated electrical lead wire.
Typically, such lead wires have a bare portion which is connected to the disk and an insulated portion which extends from the disk to an exit point on the electrode for connection to a diagnostic instrument. Such disks and lead wires are typically extremely small. For example, preferred disks have a diameter in the range of 0.100 inches to 0.250 inches with a highly preferred diameter being 0.156 inches. The associated lead wires preferably range in size from 36 gauge to 50 gauge and a highly preferred wire size is 40 gauge. Such 40 gauge wire has a diameter of about 0.004", only slightly larger than that of a human hair. Such disks and lead wires are commonly made of stainless steel, platinum or other metals suitable for cranial implantation.
Various means for making electrical connections have been used in other environments. Soldering is one such approach but is not practical for intracranial electrodes since parts to be soldered (disks and wires) must have flux applied thereto. Flux residue must be thoroughly removed before the parts are used and there is always some change of incomplete cleaning. Stated another way, the use of flux risks the introduction of chemical contaminants into the cranial cavity.
Another approach to making electrical connections generally involves the use of recently-introduced conductive adhesives. However, such adhesives often contain silver which has a tendency to introduce a level of toxicity in intracranial applications.
Heretofore, connection of a disk and its lead wire has been by welding, such delicate welding being accomplished by the careful application of heat from an external source. Attachment by welding apparently tends to cause certain changes in the molecular structure of the metal at the site of the weld. Specifically, such molecular structure changes seem to impart a degree of brittleness to that short length of lead wire which is connected to a given disk. The result is that a phenomenon in the nature of a "boundary" is formed between the more brittle end of the wire connected to the disk and the more ductile remainder of the wire strand which leads from the disk.
Such apparent change in molecular structure and the formation of any boundary tends to make the lead wire more susceptible to breakage at the point at which the wire connects to the disk. Breakage is undesirable for several reasons. Among the most important is the resulting inability to derive an electrical signal from a particular disk. The absence of a particular signal will either impair the accuracy of the resulting diagnosis or, assuming the breakage is detected, will require removal of the defective electrode. As explained below, electrode replacement cannot occur for some time following such an event.
In addition, breakage of the electrical lead wire may have a tendency to permit the disk to migrate from the electrode. In such a case, there is a change that the loss of a disk would go undetected. Clearly, the presence of such foreign matter would be adverse to the welfare of the patient.
It is apparent that subdural electrodes perform their function by permitting the detection and analysis of electrical signals. Such signals are produced by electrical currents that are extremely minute. Therefore, even if breakage of an electrical lead does not occur (and such breakage is relatively rare), slight variations in electrical resistances at the points of welding may have a tendency to distort or otherwise impair the electrical signals emanating from the electrode. Therefore, the quality of the signals--and of the diagnosis and any resulting brain surgery based upon them--may be impaired.
An understanding of the effect of a failure of a connection between a disk and its lead wire is particularly important. In the event of such failure, the electrode is unusable and must be removed. Further, a new electrode cannot be re-inserted for a period of several months, perhaps up to a year. This is so since such repeated insertions which are too closely spaced in time tend to result in scarring of the very delicate cortex tissue. In fact, some researchers believe that such scarring itself may cause or contribute to seizures.
Since a good diagnosis of foci cannot proceed in the absence of even one electrode and since immediate re-insertion is not advised, all electrodes must be removed from all burr holes. Then the dura, skull and scalp openings must be closed and further diagnostic effort delayed for an extended period. One can only imagine the level of discouragement and anxiety which may result in a patient who experiences such an aborted diagnostic attempt and subsequent delay.
A subdural electrode which has disks and lead wires attached to one another using means other than the application of heat and where the means of attachment enhances the electrical and mechanical integrity of the connection would be an important advance in the art.