(e) Visceral hyperæsthesia is chiefly shown by abnormal consciousness of the presence and action of an organ. Visceral pain usually accompanies this, and is the more prominent symptom.
(f) Increased reflex actions (emotional, motor, vaso-motor, and secretory) rarely fail to accompany hyperæsthesia in its various forms. In the hypnotic exaltation of muscular sense remarkable psychic effects may be induced, partly in a reflex way, but perhaps chiefly through the law of association.
PARÆSTHESIÆ are sensations which arise centrally in nerve-fibres or nervous centres, and are projected outward and referred to the periphery or surface by consciousness, in obedience to the general law of outward projection of sensations in the Ego. They may be produced by external agencies or arise centrally without demonstrable cause. Their number and variety are very great, varying somewhat with the descriptive powers and self-consciousness of the patient, the chief being pain, formication, numbness, coldness and heat, constriction and distension, malposition, imaginary movements, etc. etc.
(a) Pain, the most distinct and frequent of paræsthesiæ, is by most authors classed as a hyperæsthesia, yet a careful analysis will show the difference. Pain and hyperæsthesia often coexist and are inseparable, yet in a large proportion of cases of nervous diseases the former sensation occurs independently, sometimes in regions where absolute anæsthesia exists (anæsthesia dolorosa), and even apparently in lost parts (neuralgia after amputations). We are consequently justified in considering most pains as paræsthesiæ. Pain assumes many forms, some real and typical, others as various as the lively imagination of nervous patients can make them. Thus we have sharp, cutting, darting pains in neuralgia, posterior spinal sclerosis, etc.; aching, throbbing, pounding pains in cephalalgia, inflammatory and traumatic conditions; boring, crushing, distending, constricting, burning pains, etc. etc. In some cases the sensation is only semi-painful, and more akin to paræsthesia (neuritis, parenchymatous lesions).
(b) Numbness, prickling, and formication usually coincide. They may easily be produced experimentally by pressure upon a nerve-trunk or by the exhibition of aconitia, so that any one may study these sensations for himself. By taking one-fiftieth of a grain of Duquesnel's crystallized aconitia the experimenter will soon find himself the possessor of intense subjective sensations of prickling, numbness, vibrations, and cold, lasting several hours. He will be able to satisfy himself that though the finger-tips feel numb, as if there was a coating or layer of something interposed between the skin and objects, he can distinguish tactile perceptions very well. In the more serious experiment of compression of a nerve-trunk a most interesting succession of phenomena will be observed: the first effects of pressure are various paræsthesiæ in the parts supplied by the nerve; then these sensations (prickling, numbness, swelling, vibration, heat, and cold) cease; paralysis and anæsthesia occur. If the compression be now interrupted, after a few moments the paræsthesiæ reappear, more intensely, as a rule, and as they gradually fade a normal state of sensibility is re-established. By making such experiments it is easy to convince one's self that anæsthesia and numbness are different conditions: indeed, during the stage of recovery from nerve-pressure distinct hyperæsthesia may be demonstrated. These results throw much light on the origin and diagnostic value of paræsthesiæ as expressions of irritation, central or neural, of nervous elements. At the same time, in practice, we occasionally meet with slight dulness of tactile sensibility in numb parts. Another point to be remembered is that while patients usually complain loudly of paræsthesiæ, they are sometimes wholly unaware of anæsthesia (hysterical analgesia, for example); therefore sensibility should be tested even if the patient does not mention sensory disturbances.
Numbness, formication, etc. occur in a vast number of nervous affections—in cerebral and spinal organic lesions, in neuritis, in toxic conditions, and in neuroses. The distribution of paræsthesiæ is a valuable index to the seat of the lesion.
(c) Cutaneous itching and prickling may occur independently of any other skin lesion, constituting true or nervous prurigo. This may be universal and last for years.
(d) Paræsthesiæ of pressure are felt either as expansive or constrictive. The part appears swollen to consciousness, or it seems to be tightly compressed. Both these sensations are often felt about the head in a variety of pathological states, and an absurd and dangerous fashion has arisen of looking upon a sense of fulness in the head as indicative of hyperæmia. The sense of constriction may show itself around one toe, a leg, the trunk, around the neck, etc.; it may be narrow, like a cord, or broad and extensive, like a stocking or corset. Sometimes it is localized, and likened to the grasp of a hand or a spot-pressure. Not infrequently, especially in cases of paraplegia, the sensation of pressure is combined with subjective cold, the legs feeling as if tightly encased in ice.
(e) Subjective sensations of heat and cold are often of the strongest kind, and are very distressing. A part whose real objective temperature is normal may appear to the patient's consciousness as icy cold or burning hot, even to the degree of apparent contact of fire (causalgia of Mitchell). We observe such sensations in posterior spinal sclerosis, myelitis, neuritis, injuries to nerves. In some functional cases complaint is made of patches of hot or cold skin, not relieved by cold or heat.
(f) Odd sensations, such as rolling or longitudinal motion of something under the skin, general or local throbbing, coition movements, are described, especially in functional or hysteroid cases.