The writing of a word can be accomplished either by the direct lexical method of recalling its spelling or by sounding out its phonemes and transforming them into learned graphemes (motor images), that is, the phonologic method. Some authors state that in agraphia there is a specific difficulty in transforming phonologic information, acquired through the auditory sense, into orthographic forms; others see it as a block between the visual form of phonemes, and the cursive movements of the hand (Basso et al). In support of the latter idea is the fact that reading and writing usually develop together and are long preceded by the development of speech as a means of communication.
Pure agraphia as the initial and sole disturbance of language function is a rarity, but such cases have been described as summarized by Rosati and de Bastiani. Pathologically verified cases are virtually nonexistent, but imaging sometimes discloses a lesion of the posterior perisylvian area. This is in keeping with the observation that a lesion in or near the angular gyrus will occasionally cause a disproportionate disorder of writing as part of the Gerstmann syndrome. As mentioned earlier in the chapter, the notion of specific center for writing in the posterior part of the second frontal convolution (the “Exner writing area”) has been questioned (see Leischner). However, Croisile and associates do cite cases of dysgraphia in which a lesion (in the case they reported, a hematoma) was located in the centrum semiovale beneath the motor parts of the frontal cortex and direct electrical stimulation of the cortex rostral to the primary motor hand area disturbs handwriting without affecting other language or manual tasks according to Roux and colleagues, a veritable apraxia of writing.
Quite apart from these aphasic agraphias, in which spelling and grammatical errors abound, there are special forms of agraphia caused by abnormalities of spatial perception and praxis. Disturbances in the perception of spatial relationships underlie constructional agraphia. In this circumstance, letters and words are formed clearly enough but are wrongly arranged on the page. Words may be superimposed, reversed, written diagonally or in a haphazard arrangement, or from right to left; in the form associated with right parietal lesions, only the right half of the page is used. Usually one finds other constructional difficulties as well, such as inability to copy geometric figures or to make drawings of clocks, flowers, and maps, etc. This is a common feature of developmental dyslexia.
A third group of writing disorder may be called the apraxic agraphias. Here, language formulation is correct and the spatial arrangements of words are respected, but the hand has lost its skill in forming letters and words. Handwriting becomes a scrawl, losing all personal character. There may be an uncertainty as to how the pen should be held and applied to paper; apraxias (ideomotor and ideational) are present in the right hander. As a rule, other learned manual skills are simultaneously disordered. Speculations as to the basic fault here are discussed in Chap. 3, under “Apraxia and Other Nonparalytic Disorders of Motor Functions,” and in Chap. 21, in relation to the functions of the frontal and parietal lobes.
In addition to the neurologic forms of agraphia, described previously, psychologists have defined a group of linguistic agraphias, subdivided into phonologic, lexical, and semantic types. These linguistic models are based on loss of the ability to write (and to spell) particular classes of words. For example, the patient may be unable to spell pronounceable nonsense words, with preserved ability to spell real words (phonologic agraphia); or there may be preserved ability to write nonsense words but not irregular words, such as island (lexical agraphia); patients with semantic agraphia have difficulty incorporating the proper meaning into the written word, for example, “the moon comes out at knight.” For the most part, these linguistic agraphias have no well-established cerebral localization and only tenuous associations with the classic aphasias, for which reason this subject is of greater interest to linguists and psychologists than to neurologists.