Ordinary cases of catarrhal pneumonia, where the inflammation occu pies both lungs, can rarely resemble pleurisy closely enough to be con founded with it. Unless the catarrhal pneumonia be accompanied by plastic pleurisy, the percussion-note is only moderately dull ; the resistance is little increased ; there is usually loud tubular or cavernous breathing at the ex treme base from dilatation of the bronchi ; and the profuse crepitation has a crisp metallic quality which bears little resemblance to the sound pro duced in an inflamed pleura. It is in cases where the catarrhal inflamma tion occurs secondarily in a lung which is already the seat of fibroid indura tion that a real difficulty is found. Here the inflammation is confined to one lung and spreads rapidly, so as to involve the whole thickness of the organ.. Consequently, the lung, already indurated by the fibroid change, gives a character to the percussion-note which is indistinguishable from that produced by pleuritic effusion ; and we find a complete, toneless dul ness with marked sense of resistance all round the affected side—both at the back and front. In the indurated lung, however, the tubular or cav ernous breath-sound is accompanied by a large metallic bubbling rhonchus. In pleurisy the breathing is usually accompanied by no adventitious sound ; but if a little crepitating friction be present, it is much drier in character, and has not the loud ringing resonance which is given to a rhonchus gene rated in a rigid dilated air-tube. In both the vocal resonance may be bronchophonic, but in pneumonia it never has an mgophonic quality.
Collapse of the in exceptional cases may present a very close re semblance to pleurisy ; but the dulness on percussion is rarely so complete, and the sense of resistance seldom so great in collapse as in fluid effusion. The resistance in the latter case to the percussing finger is an element of the utmost importance in the diagnosis, and is only equalled in point of intensity by a fibroid induration of the lung with superadded catarrhal pneumonia, as already described. • With regard to the varieties of pleurisy, it is often very difficult to say whether the fluid is serous or purulent, or, indeed, whether the physical signs are not due to a coating of lymph without liquid effusion at all. If a change in the percussion-note and the character of the physical signs follows a change in the position of the patient, the presence of fluid is placed beyond the possibility of doubt. But if no such characteristic sign of fluid can be discovered, it is no proof that fluid is not present. The effusion may be kept in place by adhesions, or there may be sufficient lymph coating the pleura to produce a dull percussion-note, although fluid be no longer in contact with the wall of the chest at the point of examination.
An Eegophonic resonance of the voice is a certain sign of effusion ; but its absence is by itself no sufficient proof of the absence of fluid. If, however, the outline of the affected side be elliptical and the heart's apex in the natural position ; if the intercostal spaces sink in normally, the percussion note be dull in all changes of position, the respiration be weak over the affected side without blowing quality, and the vocal resonance not at all mgophonic, it is almost certain that no fluid is present. Even here, how ever, no positive conclusion can be arrived at, for with such signs there may be an encysted collection of pus at almost any part of the chest.
The distinction between a serous and a purulent effusion is very diffi cult. No information can be gained from the temperature, for this may be elevated or not without reference to the character of the fluid. It is often high with a serous effusion and perfectly normal with a large purulent collection in the chest. Again, the physical signs are the same whatever be the nature of the pleural contents ; for Bacelli's sign (i.e., the clear and articulate conduction of the whispered voice to the chest-wall as indicative of serous and exclusive of purulent effusion) has not unfortunately the value attributed to it by this physician. The tint of the face, however, if the complexion have assumed the straw yellow hue, although not a decisive proof, is very suggestive of empyema ; and marked clubbing of the finger ends, according to Dr. T. Barlow, is never the consequence of serous effu sion. In every case of doubt an exploratory puncture' with the hypoder mic injection syringe, by withdrawing a specimen of the fluid, will at once decide the question.
Hydrothorax is as a rule readily distinguished from pleurisy by noting the evidences which are always present of interference with the general cir culation. Dropsy of the pleura is almost always a part of general anasarca. There is disease of the heart or kidneys ; the effusion occurs on both sides simultaneously ; and there is also ascites or more or less general cedema.
Prognosis.—In cases of pleurisy the prognosis depends in a great meas ure upon the age and constitution of the child. Under the age of six months the disease is a very serious one, and often ends in death. After that early period the prognosis is good, as a rule, if the child be not the subject of a diathetic taint. The scrofulous habit is, however, a distinctly unfavourable element, for although the disease may eventually end happily, the fluid tends to become quickly purulent ; the febrile excitement is usually great ; interference with nutrition is marked ; and not unfrequently the fluid is continually reproduced as often as it is evacuated.