Spinal Anæsthesia or Analgesia, consists in the production of analgesia in the lower extremities and in the lower part of the trunk by the injection into the subarachnoid space of an anæsthetic drug which blocks the spinal nerves as they enter and leave the spinal cord. The cord ends at the lower border of the first lumbar vertebra and the subarachnoid space at the second sacral vertebra so that there is a considerable area into which the injection may be made without risk of injury to the cord. It is, in reality, a special variety of regional analgesia, the anæsthetic being injected into that part of the subarachnoid space which is occupied by the cauda equina. The subarachnoid space of the medulla spinalis contains the cerebro-spinal fluid and communicates above with the subarachnoid space inside the skull and through the foramen of Magendie, with the ventricular system of the brain. The subdural space of the medulla spinalis is merely a capillary interval. At the upper end of the cauda equina the nerve trunks of the two sides are separated by a median interval—containing only the filum terminale—which has been termed the cysterna terminalis. It is into this median space that the injection is made, in order to avoid wounding the nerve trunks and to procure equal diffusion of the anæsthetic to both sides of the middle line. If the injection is made among the nerve trunks on one side, a unilateral anæsthesia may result, the drug being prevented from diffusing freely to the other side by the presence of the numerous nerves.
The ligamentum denticulatum forms an imperfect scalloped septum between the posterior and the anterior nerve roots, passing from the surface of the cord to the dura mater. The presence of this septum probably explains the fact that the motor nerves are not affected with the same constancy and to the same extent as the sensory roots.
The drug which is most commonly employed in the Edinburgh school is Tropacocaine, and the results of its use with proper technique are eminently satisfactory. The dose of the drug for most purposes is ·07 gramme. Smaller doses are sometimes used but the larger dose gives more constant anæsthesia and appears to be well within the limits of safety. The dose is dissolved in 1 c.cm. of distilled water and sufficient sodium chloride added to make a solution isotonic with the cerebro-spinal fluid. A convenient method of obtaining the drug is in glass ampoules, each ampoule containing one dose, which has been carefully sterilised.
Fig. 53.—Needle and syringe for spinal analgesia. Note the short oblique character of the point of the needle.
The syringe and needle employed are illustrated in Fig. 53.
The point of the needle must be sharp but short. If a needle with a long slender point is employed, only part of the point may enter the membranes; a free flow of cerebro-spinal fluid may then take place, but when the injection is made part of the anæsthetic solution escapes outside the membranes. The needle should he 3½ to 4 inches long and 1 m.m. in diameter. A stylet fits inside the needle and prevents it from becoming blocked during the introduction. To prevent the possibility of rusting, both needle and stylet should consist of hard nickel. The barrel of the syringe must consist of glass so that the appearance of the cerebro-spinal fluid can be seen. The Record type is very satisfactory. The syringe usually supplied for spinal analgesia has a capacity of 2 or 3 c.cm., but one holding 10 c.cm. is more useful. Syringe and needle must be carefully sterilised by boiling in plain water; any trace of soda causes decomposition of the drug. The ampoule containing the tropococaine is sterilised in a strong antiseptic solution so as to avoid the possibility of contamination of the hands when the drug is being transferred to the syringe.
Method of Injection.—The patient should be given a hypodermic injection of ⅛ gr. of morphine and ¹⁄₁₅₀ gr. of scopolamine an hour before the operation. There are a number of minor variations in the method of making the spinal injection, but limitations of space forbid a discussion of theoretical questions and of the relative merits of the different procedures. Only one method, which has been found safe and reliable, will be considered here. The injection is made in the space between the third and fourth lumbar spines, the objective being the mid-line of the subarachnoid space between the two divisions of the cauda equina. The position of the patient is such that the spaces between the lumbar spines are opened up as widely as possible. The most convenient plan is to have the patient sitting on the table with the head and shoulders bent well forward (see Fig. 54.) If the patient is unable to sit up, the injection may be made with him lying on his side, with the knees drawn up and the shoulders bent forward.
Fig. 54. Position for the injection. The cross indicates the point at which the lumbar puncture is made—about half an inch from the median plane and in the space between the third and fourth lumbar spines.
The skin of the back is carefully sterilised; painting with tincture of iodine serves admirably. The ampoule containing the tropacocaine is opened, and the drug sucked into the syringe through a spare cannula. The loaded syringe is then placed on a sterile towel at the back of the patient. With a little practice there is no difficulty in making the lumbar puncture. The fourth lumbar spine is located by noting the level of the highest point on the iliac crest—this may be indicated by an assistant. A line joining the highest points on the two iliac crests will pass through the tip of the fourth lumbar spine. When this process has been carefully identified, the needle is introduced half-an-inch to one side of the median plane and midway between the third and fourth spine. Some surgeons prefer to go in exactly in the middle line to make sure of entering the middle of the subarachnoid space, but in this position the tough supra-spinous and interspinous ligaments are met with, and to avoid the resistance of these it is best to keep a short distance out from the median plane. By carefully noting the direction of the needle, the cysterna terminalis can always be entered. The needle is passed forwards, very slightly upwards, and slightly medially so as to hit off the centre of the subarachnoid space. As the needle passes through the ligamentum flavum, there is a sudden diminution of resistance and immediately afterwards the point of the needle lies in the subarachnoid space. The passage of the needle through the membranes is sometimes accompanied by a slight pricking pain.
The stylet is withdrawn at this stage and the cerebro-spinal fluid usually trickles out drop by drop. The syringe is picked up, carefully emptied of air bubbles, and fitted on to the needle. The piston is withdrawn until the syringe is filled with cerebro-spinal fluid, which mixes freely with the anæsthetic solution, and the contents then slowly injected. The 10 c.cm. syringe is to be preferred for this purpose as it is essential to mix the tropacocaine thoroughly with the cerebro-spinal fluid. If the smaller syringe is used, it should be refilled with cerebro-spinal fluid and emptied a second time so as to ensure thorough diffusion of the drug. The needle is then withdrawn and the puncture sealed with collodion.
The injection should never be made until a free flow of cerebro-spinal fluid is obtained, since this is the only certain indication that the needle has entered the subarachnoid space. If failure is met with in the space between the third and fourth spines, the interspinous space above or below should be tried.
After the injection has been completed the patient is placed flat on his back and then lowered into the Trendelenburg position. Analgesia appears first in the scrotum and perineum, extends down the medial side of the leg to the foot, then appears on the front of the leg, and travels up to the groin and the lower part of the abdomen. The progress of the analgesia is tested from time to time by lightly pinching or pricking the skin, the patient’s eyes being screened. When the analgesia reaches the level of the nipples, the patient is raised into the horizontal position and the the operation may be commenced. Some surgeons object to the lowering of the head as rendering paralysis of the respiratory centre from upward diffusion of the drug more likely. If tropacocaine is used in the dosage indicated and the table elevated when the anæsthesia reaches the nipple line, there seems to be little risk of this complication. If analgesia is only desired in the lower extremity, the lowering of the table may be omitted; but if a good anæsthesia is desired above the level of the groin, it should always be carried out.
Analgesia is complete in five or ten minutes as a rule. The duration varies from three-quarters of an hour to an hour and a half. If a preliminary hypodermic injection of morphine and scopolamine has been given, the patient lies quietly and patiently until the operation is completed. In some cases the patient actually drops off to sleep from the effects of the morphine. It is not uncommon to observe a temporary nausea and faintness about fifteen or twenty minutes after the injection has been made, and it is good practice to give the patient a little brandy and water at this stage.
A great deal has been written in the past with regard to unpleasant results of spinal analgesia, but most of these would appear to have been the result of faulty technique or of the use of an impure or irritating drug. When tropacocaine is used in the manner described, the usual result is that, except for occasional nausea and faintness at the commencement, the patient has a comfortable, painless operation, and a recovery which is unmarred by the sickness and other distressing symptoms which are so common after general anæsthesia.
Deaths have been recorded, and these have been ascribed to the drug having travelled too high and brought about paralysis of the respiratory centre in the medulla oblongata. Too much importance has probably been ascribed to these fatal cases. They have been most common in patients greatly enfeebled by shock, old age, or debilitating illness, who are liable to die during the operation whatever anæsthetic is used. Thousands of cases have been recorded without a death, and in the hands of surgeons of skill and judgment fatal cases are almost unknown.
An occasional complication is severe headache which may persist for a week or longer. Other complications are all exceedingly rare; paralysis of the lateral rectus muscle of the eyeball or of other ocular muscles has been recorded, and is probably due to toxic bye-products which are the result of impurity of the drug. Persistent nausea and paralysis of the bladder and rectum and even of the lower extremities have also been recorded, but are to be regarded as the greatest rareties, and probably due to impurity of the anæsthetic.
Spinal analgesia may be used for any operation at or below the level of the umbilicus. Excellent anæsthesia is obtained for the operation for radical cure of umbilical hernia, but anæsthesia above this level is not so constant, and is regarded by many authorities as unsafe.
The procedure is of special value in cases in which a general anæsthetic is unsafe:—(1) In old enfeebled patients suffering from strangulated hernia, enlarged prostate, disease of the female pelvic organs, and other conditions where anæsthesia is necessary below the umbilicus. (2) In patients who are already suffering, or who are likely to suffer, from severe shock. The drug has the same effect on the nerve trunks of the cauda equina as on the peripheral nerves—it causes blocking of the centripetal sensory impulses which are such a potent factor in the causation of shock.
(3) In diabetic gangrene spinal analgesia is the safest form of anæsthesia to employ.
Children up to the age of fourteen or so are apt to be frightened, and spinal analgesia is better avoided except in special cases. It is contra-indicated also in septic conditions on account of the possibility of septic meningitis resulting from metastasis of the infection, the drug having possibly the action of lowering the vitality of the cord and meninges. In tuberculosis and syphilis it is better avoided for the same reason. It should not be used where organic disease of the spinal cord or brain is already present.
A transient analgesia can be produced by freezing the skin. An ether spray was formerly employed, but was found to be troublesome and inconvenient. The most convenient procedure consists in freezing the part by means of a spray of ethyl chloride. This drug is supplied in a glass cylinder with a very fine outlet so that it breaks up into a fine spray as it escapes. The cylinder is held about 8 or 10 inches from the patient’s skin, and pressure applied with the thumb to a stopcock on the neck of the cylinder. Under the influence of the heat of the hand the liquid escapes in a fine jet which impinges on the patient’s skin. Freezing takes place in a few seconds, the frozen patch becoming hard and white. The freezing can be hastened by blowing on the skin.
This method is only suitable for the opening of small abscesses and other procedures requiring a very short anæsthesia. The anæsthesia is very imperfect, and only lasts for a few seconds. Many patients appear to have as much pain with this form of anæsthesia as without it.